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2143 lines
78 KiB
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2143 lines
78 KiB
Plaintext
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Debugging on Linux for s/390 & z/Architecture
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by
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Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com)
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Copyright (C) 2000-2001 IBM Deutschland Entwicklung GmbH, IBM Corporation
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Best viewed with fixed width fonts
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Overview of Document:
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=====================
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This document is intended to give a good overview of how to debug Linux for
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s/390 and z/Architecture. It is not intended as a complete reference and not a
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tutorial on the fundamentals of C & assembly. It doesn't go into
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390 IO in any detail. It is intended to complement the documents in the
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reference section below & any other worthwhile references you get.
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It is intended like the Enterprise Systems Architecture/390 Reference Summary
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to be printed out & used as a quick cheat sheet self help style reference when
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problems occur.
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Contents
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========
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Register Set
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Address Spaces on Intel Linux
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Address Spaces on Linux for s/390 & z/Architecture
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The Linux for s/390 & z/Architecture Kernel Task Structure
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Register Usage & Stackframes on Linux for s/390 & z/Architecture
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A sample program with comments
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Compiling programs for debugging on Linux for s/390 & z/Architecture
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Debugging under VM
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s/390 & z/Architecture IO Overview
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Debugging IO on s/390 & z/Architecture under VM
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GDB on s/390 & z/Architecture
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Stack chaining in gdb by hand
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Examining core dumps
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ldd
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Debugging modules
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The proc file system
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SysRq
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References
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Special Thanks
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Register Set
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============
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The current architectures have the following registers.
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16 General propose registers, 32 bit on s/390 and 64 bit on z/Architecture,
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r0-r15 (or gpr0-gpr15), used for arithmetic and addressing.
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16 Control registers, 32 bit on s/390 and 64 bit on z/Architecture, cr0-cr15,
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kernel usage only, used for memory management, interrupt control, debugging
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control etc.
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16 Access registers (ar0-ar15), 32 bit on both s/390 and z/Architecture,
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normally not used by normal programs but potentially could be used as
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temporary storage. These registers have a 1:1 association with general
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purpose registers and are designed to be used in the so-called access
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register mode to select different address spaces.
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Access register 0 (and access register 1 on z/Architecture, which needs a
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64 bit pointer) is currently used by the pthread library as a pointer to
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the current running threads private area.
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16 64 bit floating point registers (fp0-fp15 ) IEEE & HFP floating
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point format compliant on G5 upwards & a Floating point control reg (FPC)
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4 64 bit registers (fp0,fp2,fp4 & fp6) HFP only on older machines.
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Note:
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Linux (currently) always uses IEEE & emulates G5 IEEE format on older machines,
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( provided the kernel is configured for this ).
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The PSW is the most important register on the machine it
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is 64 bit on s/390 & 128 bit on z/Architecture & serves the roles of
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a program counter (pc), condition code register,memory space designator.
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In IBM standard notation I am counting bit 0 as the MSB.
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It has several advantages over a normal program counter
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in that you can change address translation & program counter
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in a single instruction. To change address translation,
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e.g. switching address translation off requires that you
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have a logical=physical mapping for the address you are
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currently running at.
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Bit Value
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s/390 z/Architecture
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0 0 Reserved ( must be 0 ) otherwise specification exception occurs.
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1 1 Program Event Recording 1 PER enabled,
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PER is used to facilitate debugging e.g. single stepping.
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2-4 2-4 Reserved ( must be 0 ).
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5 5 Dynamic address translation 1=DAT on.
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6 6 Input/Output interrupt Mask
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7 7 External interrupt Mask used primarily for interprocessor
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signalling and clock interrupts.
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8-11 8-11 PSW Key used for complex memory protection mechanism
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(not used under linux)
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12 12 1 on s/390 0 on z/Architecture
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13 13 Machine Check Mask 1=enable machine check interrupts
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14 14 Wait State. Set this to 1 to stop the processor except for
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interrupts and give time to other LPARS. Used in CPU idle in
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the kernel to increase overall usage of processor resources.
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15 15 Problem state ( if set to 1 certain instructions are disabled )
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all linux user programs run with this bit 1
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( useful info for debugging under VM ).
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16-17 16-17 Address Space Control
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00 Primary Space Mode:
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The register CR1 contains the primary address-space control ele-
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ment (PASCE), which points to the primary space region/segment
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table origin.
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01 Access register mode
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10 Secondary Space Mode:
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The register CR7 contains the secondary address-space control
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element (SASCE), which points to the secondary space region or
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segment table origin.
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11 Home Space Mode:
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The register CR13 contains the home space address-space control
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element (HASCE), which points to the home space region/segment
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table origin.
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See "Address Spaces on Linux for s/390 & z/Architecture" below
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for more information about address space usage in Linux.
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18-19 18-19 Condition codes (CC)
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20 20 Fixed point overflow mask if 1=FPU exceptions for this event
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occur ( normally 0 )
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21 21 Decimal overflow mask if 1=FPU exceptions for this event occur
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( normally 0 )
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22 22 Exponent underflow mask if 1=FPU exceptions for this event occur
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( normally 0 )
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23 23 Significance Mask if 1=FPU exceptions for this event occur
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( normally 0 )
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24-31 24-30 Reserved Must be 0.
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31 Extended Addressing Mode
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32 Basic Addressing Mode
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Used to set addressing mode
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PSW 31 PSW 32
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0 0 24 bit
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0 1 31 bit
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1 1 64 bit
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32 1=31 bit addressing mode 0=24 bit addressing mode (for backward
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compatibility), linux always runs with this bit set to 1
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33-64 Instruction address.
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33-63 Reserved must be 0
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64-127 Address
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In 24 bits mode bits 64-103=0 bits 104-127 Address
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In 31 bits mode bits 64-96=0 bits 97-127 Address
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Note: unlike 31 bit mode on s/390 bit 96 must be zero
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when loading the address with LPSWE otherwise a
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specification exception occurs, LPSW is fully backward
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compatible.
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Prefix Page(s)
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--------------
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This per cpu memory area is too intimately tied to the processor not to mention.
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It exists between the real addresses 0-4096 on s/390 and between 0-8192 on
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z/Architecture and is exchanged with one page on s/390 or two pages on
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z/Architecture in absolute storage by the set prefix instruction during Linux
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startup.
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This page is mapped to a different prefix for each processor in an SMP
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configuration (assuming the OS designer is sane of course).
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Bytes 0-512 (200 hex) on s/390 and 0-512, 4096-4544, 4604-5119 currently on
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z/Architecture are used by the processor itself for holding such information
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as exception indications and entry points for exceptions.
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Bytes after 0xc00 hex are used by linux for per processor globals on s/390 and
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z/Architecture (there is a gap on z/Architecture currently between 0xc00 and
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0x1000, too, which is used by Linux).
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The closest thing to this on traditional architectures is the interrupt
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vector table. This is a good thing & does simplify some of the kernel coding
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however it means that we now cannot catch stray NULL pointers in the
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kernel without hard coded checks.
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Address Spaces on Intel Linux
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=============================
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The traditional Intel Linux is approximately mapped as follows forgive
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the ascii art.
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0xFFFFFFFF 4GB Himem *****************
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* *
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* Kernel Space *
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* *
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***************** ****************
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User Space Himem * User Stack * * *
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(typically 0xC0000000 3GB ) ***************** * *
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* Shared Libs * * Next Process *
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***************** * to *
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* * <== * Run * <==
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* User Program * * *
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* Data BSS * * *
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* Text * * *
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* Sections * * *
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0x00000000 ***************** ****************
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Now it is easy to see that on Intel it is quite easy to recognise a kernel
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address as being one greater than user space himem (in this case 0xC0000000),
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and addresses of less than this are the ones in the current running program on
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this processor (if an smp box).
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If using the virtual machine ( VM ) as a debugger it is quite difficult to
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know which user process is running as the address space you are looking at
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could be from any process in the run queue.
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The limitation of Intels addressing technique is that the linux
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kernel uses a very simple real address to virtual addressing technique
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of Real Address=Virtual Address-User Space Himem.
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This means that on Intel the kernel linux can typically only address
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Himem=0xFFFFFFFF-0xC0000000=1GB & this is all the RAM these machines
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can typically use.
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They can lower User Himem to 2GB or lower & thus be
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able to use 2GB of RAM however this shrinks the maximum size
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of User Space from 3GB to 2GB they have a no win limit of 4GB unless
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they go to 64 Bit.
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On 390 our limitations & strengths make us slightly different.
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For backward compatibility we are only allowed use 31 bits (2GB)
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of our 32 bit addresses, however, we use entirely separate address
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spaces for the user & kernel.
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This means we can support 2GB of non Extended RAM on s/390, & more
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with the Extended memory management swap device &
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currently 4TB of physical memory currently on z/Architecture.
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Address Spaces on Linux for s/390 & z/Architecture
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==================================================
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Our addressing scheme is basically as follows:
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Primary Space Home Space
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Himem 0x7fffffff 2GB on s/390 ***************** ****************
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currently 0x3ffffffffff (2^42)-1 * User Stack * * *
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on z/Architecture. ***************** * *
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* Shared Libs * * *
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***************** * *
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* * * Kernel *
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* User Program * * *
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* Data BSS * * *
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* Text * * *
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* Sections * * *
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0x00000000 ***************** ****************
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This also means that we need to look at the PSW problem state bit and the
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addressing mode to decide whether we are looking at user or kernel space.
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User space runs in primary address mode (or access register mode within
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the vdso code).
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The kernel usually also runs in home space mode, however when accessing
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user space the kernel switches to primary or secondary address mode if
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the mvcos instruction is not available or if a compare-and-swap (futex)
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instruction on a user space address is performed.
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When also looking at the ASCE control registers, this means:
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User space:
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- runs in primary or access register mode
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- cr1 contains the user asce
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- cr7 contains the user asce
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- cr13 contains the kernel asce
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Kernel space:
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- runs in home space mode
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- cr1 contains the user or kernel asce
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-> the kernel asce is loaded when a uaccess requires primary or
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secondary address mode
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- cr7 contains the user or kernel asce, (changed with set_fs())
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- cr13 contains the kernel asce
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In case of uaccess the kernel changes to:
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- primary space mode in case of a uaccess (copy_to_user) and uses
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e.g. the mvcp instruction to access user space. However the kernel
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will stay in home space mode if the mvcos instruction is available
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- secondary space mode in case of futex atomic operations, so that the
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instructions come from primary address space and data from secondary
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space
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In case of KVM, the kernel runs in home space mode, but cr1 gets switched
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to contain the gmap asce before the SIE instruction gets executed. When
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the SIE instruction is finished, cr1 will be switched back to contain the
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user asce.
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Virtual Addresses on s/390 & z/Architecture
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===========================================
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A virtual address on s/390 is made up of 3 parts
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The SX (segment index, roughly corresponding to the PGD & PMD in Linux
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terminology) being bits 1-11.
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The PX (page index, corresponding to the page table entry (pte) in Linux
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terminology) being bits 12-19.
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The remaining bits BX (the byte index are the offset in the page )
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i.e. bits 20 to 31.
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On z/Architecture in linux we currently make up an address from 4 parts.
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The region index bits (RX) 0-32 we currently use bits 22-32
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The segment index (SX) being bits 33-43
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The page index (PX) being bits 44-51
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The byte index (BX) being bits 52-63
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Notes:
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1) s/390 has no PMD so the PMD is really the PGD also.
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A lot of this stuff is defined in pgtable.h.
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2) Also seeing as s/390's page indexes are only 1k in size
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(bits 12-19 x 4 bytes per pte ) we use 1 ( page 4k )
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to make the best use of memory by updating 4 segment indices
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entries each time we mess with a PMD & use offsets
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0,1024,2048 & 3072 in this page as for our segment indexes.
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On z/Architecture our page indexes are now 2k in size
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( bits 12-19 x 8 bytes per pte ) we do a similar trick
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but only mess with 2 segment indices each time we mess with
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a PMD.
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3) As z/Architecture supports up to a massive 5-level page table lookup we
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can only use 3 currently on Linux ( as this is all the generic kernel
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currently supports ) however this may change in future
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this allows us to access ( according to my sums )
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4TB of virtual storage per process i.e.
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4096*512(PTES)*1024(PMDS)*2048(PGD) = 4398046511104 bytes,
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enough for another 2 or 3 of years I think :-).
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to do this we use a region-third-table designation type in
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our address space control registers.
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The Linux for s/390 & z/Architecture Kernel Task Structure
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==========================================================
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Each process/thread under Linux for S390 has its own kernel task_struct
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defined in linux/include/linux/sched.h
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The S390 on initialisation & resuming of a process on a cpu sets
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the __LC_KERNEL_STACK variable in the spare prefix area for this cpu
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(which we use for per-processor globals).
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The kernel stack pointer is intimately tied with the task structure for
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each processor as follows.
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s/390
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************************
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* 1 page kernel stack *
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* ( 4K ) *
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************************
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* 1 page task_struct *
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* ( 4K ) *
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8K aligned ************************
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z/Architecture
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************************
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* 2 page kernel stack *
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* ( 8K ) *
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************************
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* 2 page task_struct *
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* ( 8K ) *
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16K aligned ************************
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What this means is that we don't need to dedicate any register or global
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variable to point to the current running process & can retrieve it with the
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following very simple construct for s/390 & one very similar for z/Architecture.
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static inline struct task_struct * get_current(void)
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{
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struct task_struct *current;
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__asm__("lhi %0,-8192\n\t"
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"nr %0,15"
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: "=r" (current) );
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return current;
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}
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i.e. just anding the current kernel stack pointer with the mask -8192.
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Thankfully because Linux doesn't have support for nested IO interrupts
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& our devices have large buffers can survive interrupts being shut for
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short amounts of time we don't need a separate stack for interrupts.
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Register Usage & Stackframes on Linux for s/390 & z/Architecture
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=================================================================
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Overview:
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---------
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This is the code that gcc produces at the top & the bottom of
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each function. It usually is fairly consistent & similar from
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function to function & if you know its layout you can probably
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make some headway in finding the ultimate cause of a problem
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after a crash without a source level debugger.
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Note: To follow stackframes requires a knowledge of C or Pascal &
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limited knowledge of one assembly language.
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It should be noted that there are some differences between the
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s/390 and z/Architecture stack layouts as the z/Architecture stack layout
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didn't have to maintain compatibility with older linkage formats.
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Glossary:
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---------
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alloca:
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This is a built in compiler function for runtime allocation
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of extra space on the callers stack which is obviously freed
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up on function exit ( e.g. the caller may choose to allocate nothing
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of a buffer of 4k if required for temporary purposes ), it generates
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very efficient code ( a few cycles ) when compared to alternatives
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like malloc.
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automatics: These are local variables on the stack,
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i.e they aren't in registers & they aren't static.
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back-chain:
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This is a pointer to the stack pointer before entering a
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framed functions ( see frameless function ) prologue got by
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dereferencing the address of the current stack pointer,
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i.e. got by accessing the 32 bit value at the stack pointers
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current location.
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base-pointer:
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This is a pointer to the back of the literal pool which
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is an area just behind each procedure used to store constants
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in each function.
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call-clobbered: The caller probably needs to save these registers if there
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is something of value in them, on the stack or elsewhere before making a
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call to another procedure so that it can restore it later.
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epilogue:
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The code generated by the compiler to return to the caller.
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frameless-function
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A frameless function in Linux for s390 & z/Architecture is one which doesn't
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need more than the register save area (96 bytes on s/390, 160 on z/Architecture)
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given to it by the caller.
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A frameless function never:
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1) Sets up a back chain.
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2) Calls alloca.
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3) Calls other normal functions
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4) Has automatics.
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GOT-pointer:
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This is a pointer to the global-offset-table in ELF
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( Executable Linkable Format, Linux'es most common executable format ),
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all globals & shared library objects are found using this pointer.
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lazy-binding
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ELF shared libraries are typically only loaded when routines in the shared
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library are actually first called at runtime. This is lazy binding.
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procedure-linkage-table
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This is a table found from the GOT which contains pointers to routines
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in other shared libraries which can't be called to by easier means.
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prologue:
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The code generated by the compiler to set up the stack frame.
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outgoing-args:
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This is extra area allocated on the stack of the calling function if the
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parameters for the callee's cannot all be put in registers, the same
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area can be reused by each function the caller calls.
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routine-descriptor:
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A COFF executable format based concept of a procedure reference
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actually being 8 bytes or more as opposed to a simple pointer to the routine.
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This is typically defined as follows
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Routine Descriptor offset 0=Pointer to Function
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Routine Descriptor offset 4=Pointer to Table of Contents
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The table of contents/TOC is roughly equivalent to a GOT pointer.
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& it means that shared libraries etc. can be shared between several
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environments each with their own TOC.
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|
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static-chain: This is used in nested functions a concept adopted from pascal
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by gcc not used in ansi C or C++ ( although quite useful ), basically it
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|
is a pointer used to reference local variables of enclosing functions.
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|
You might come across this stuff once or twice in your lifetime.
|
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e.g.
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The function below should return 11 though gcc may get upset & toss warnings
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|
about unused variables.
|
|
int FunctionA(int a)
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{
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int b;
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FunctionC(int c)
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{
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b=c+1;
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}
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FunctionC(10);
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return(b);
|
|
}
|
|
|
|
|
|
s/390 & z/Architecture Register usage
|
|
=====================================
|
|
r0 used by syscalls/assembly call-clobbered
|
|
r1 used by syscalls/assembly call-clobbered
|
|
r2 argument 0 / return value 0 call-clobbered
|
|
r3 argument 1 / return value 1 (if long long) call-clobbered
|
|
r4 argument 2 call-clobbered
|
|
r5 argument 3 call-clobbered
|
|
r6 argument 4 saved
|
|
r7 pointer-to arguments 5 to ... saved
|
|
r8 this & that saved
|
|
r9 this & that saved
|
|
r10 static-chain ( if nested function ) saved
|
|
r11 frame-pointer ( if function used alloca ) saved
|
|
r12 got-pointer saved
|
|
r13 base-pointer saved
|
|
r14 return-address saved
|
|
r15 stack-pointer saved
|
|
|
|
f0 argument 0 / return value ( float/double ) call-clobbered
|
|
f2 argument 1 call-clobbered
|
|
f4 z/Architecture argument 2 saved
|
|
f6 z/Architecture argument 3 saved
|
|
The remaining floating points
|
|
f1,f3,f5 f7-f15 are call-clobbered.
|
|
|
|
Notes:
|
|
------
|
|
1) The only requirement is that registers which are used
|
|
by the callee are saved, e.g. the compiler is perfectly
|
|
capable of using r11 for purposes other than a frame a
|
|
frame pointer if a frame pointer is not needed.
|
|
2) In functions with variable arguments e.g. printf the calling procedure
|
|
is identical to one without variable arguments & the same number of
|
|
parameters. However, the prologue of this function is somewhat more
|
|
hairy owing to it having to move these parameters to the stack to
|
|
get va_start, va_arg & va_end to work.
|
|
3) Access registers are currently unused by gcc but are used in
|
|
the kernel. Possibilities exist to use them at the moment for
|
|
temporary storage but it isn't recommended.
|
|
4) Only 4 of the floating point registers are used for
|
|
parameter passing as older machines such as G3 only have only 4
|
|
& it keeps the stack frame compatible with other compilers.
|
|
However with IEEE floating point emulation under linux on the
|
|
older machines you are free to use the other 12.
|
|
5) A long long or double parameter cannot be have the
|
|
first 4 bytes in a register & the second four bytes in the
|
|
outgoing args area. It must be purely in the outgoing args
|
|
area if crossing this boundary.
|
|
6) Floating point parameters are mixed with outgoing args
|
|
on the outgoing args area in the order the are passed in as parameters.
|
|
7) Floating point arguments 2 & 3 are saved in the outgoing args area for
|
|
z/Architecture
|
|
|
|
|
|
Stack Frame Layout
|
|
------------------
|
|
s/390 z/Architecture
|
|
0 0 back chain ( a 0 here signifies end of back chain )
|
|
4 8 eos ( end of stack, not used on Linux for S390 used in other linkage formats )
|
|
8 16 glue used in other s/390 linkage formats for saved routine descriptors etc.
|
|
12 24 glue used in other s/390 linkage formats for saved routine descriptors etc.
|
|
16 32 scratch area
|
|
20 40 scratch area
|
|
24 48 saved r6 of caller function
|
|
28 56 saved r7 of caller function
|
|
32 64 saved r8 of caller function
|
|
36 72 saved r9 of caller function
|
|
40 80 saved r10 of caller function
|
|
44 88 saved r11 of caller function
|
|
48 96 saved r12 of caller function
|
|
52 104 saved r13 of caller function
|
|
56 112 saved r14 of caller function
|
|
60 120 saved r15 of caller function
|
|
64 128 saved f4 of caller function
|
|
72 132 saved f6 of caller function
|
|
80 undefined
|
|
96 160 outgoing args passed from caller to callee
|
|
96+x 160+x possible stack alignment ( 8 bytes desirable )
|
|
96+x+y 160+x+y alloca space of caller ( if used )
|
|
96+x+y+z 160+x+y+z automatics of caller ( if used )
|
|
0 back-chain
|
|
|
|
A sample program with comments.
|
|
===============================
|
|
|
|
Comments on the function test
|
|
-----------------------------
|
|
1) It didn't need to set up a pointer to the constant pool gpr13 as it is not
|
|
used ( :-( ).
|
|
2) This is a frameless function & no stack is bought.
|
|
3) The compiler was clever enough to recognise that it could return the
|
|
value in r2 as well as use it for the passed in parameter ( :-) ).
|
|
4) The basr ( branch relative & save ) trick works as follows the instruction
|
|
has a special case with r0,r0 with some instruction operands is understood as
|
|
the literal value 0, some risc architectures also do this ). So now
|
|
we are branching to the next address & the address new program counter is
|
|
in r13,so now we subtract the size of the function prologue we have executed
|
|
+ the size of the literal pool to get to the top of the literal pool
|
|
0040037c int test(int b)
|
|
{ # Function prologue below
|
|
40037c: 90 de f0 34 stm %r13,%r14,52(%r15) # Save registers r13 & r14
|
|
400380: 0d d0 basr %r13,%r0 # Set up pointer to constant pool using
|
|
400382: a7 da ff fa ahi %r13,-6 # basr trick
|
|
return(5+b);
|
|
# Huge main program
|
|
400386: a7 2a 00 05 ahi %r2,5 # add 5 to r2
|
|
|
|
# Function epilogue below
|
|
40038a: 98 de f0 34 lm %r13,%r14,52(%r15) # restore registers r13 & 14
|
|
40038e: 07 fe br %r14 # return
|
|
}
|
|
|
|
Comments on the function main
|
|
-----------------------------
|
|
1) The compiler did this function optimally ( 8-) )
|
|
|
|
Literal pool for main.
|
|
400390: ff ff ff ec .long 0xffffffec
|
|
main(int argc,char *argv[])
|
|
{ # Function prologue below
|
|
400394: 90 bf f0 2c stm %r11,%r15,44(%r15) # Save necessary registers
|
|
400398: 18 0f lr %r0,%r15 # copy stack pointer to r0
|
|
40039a: a7 fa ff a0 ahi %r15,-96 # Make area for callee saving
|
|
40039e: 0d d0 basr %r13,%r0 # Set up r13 to point to
|
|
4003a0: a7 da ff f0 ahi %r13,-16 # literal pool
|
|
4003a4: 50 00 f0 00 st %r0,0(%r15) # Save backchain
|
|
|
|
return(test(5)); # Main Program Below
|
|
4003a8: 58 e0 d0 00 l %r14,0(%r13) # load relative address of test from
|
|
# literal pool
|
|
4003ac: a7 28 00 05 lhi %r2,5 # Set first parameter to 5
|
|
4003b0: 4d ee d0 00 bas %r14,0(%r14,%r13) # jump to test setting r14 as return
|
|
# address using branch & save instruction.
|
|
|
|
# Function Epilogue below
|
|
4003b4: 98 bf f0 8c lm %r11,%r15,140(%r15)# Restore necessary registers.
|
|
4003b8: 07 fe br %r14 # return to do program exit
|
|
}
|
|
|
|
|
|
Compiler updates
|
|
----------------
|
|
|
|
main(int argc,char *argv[])
|
|
{
|
|
4004fc: 90 7f f0 1c stm %r7,%r15,28(%r15)
|
|
400500: a7 d5 00 04 bras %r13,400508 <main+0xc>
|
|
400504: 00 40 04 f4 .long 0x004004f4
|
|
# compiler now puts constant pool in code to so it saves an instruction
|
|
400508: 18 0f lr %r0,%r15
|
|
40050a: a7 fa ff a0 ahi %r15,-96
|
|
40050e: 50 00 f0 00 st %r0,0(%r15)
|
|
return(test(5));
|
|
400512: 58 10 d0 00 l %r1,0(%r13)
|
|
400516: a7 28 00 05 lhi %r2,5
|
|
40051a: 0d e1 basr %r14,%r1
|
|
# compiler adds 1 extra instruction to epilogue this is done to
|
|
# avoid processor pipeline stalls owing to data dependencies on g5 &
|
|
# above as register 14 in the old code was needed directly after being loaded
|
|
# by the lm %r11,%r15,140(%r15) for the br %14.
|
|
40051c: 58 40 f0 98 l %r4,152(%r15)
|
|
400520: 98 7f f0 7c lm %r7,%r15,124(%r15)
|
|
400524: 07 f4 br %r4
|
|
}
|
|
|
|
|
|
Hartmut ( our compiler developer ) also has been threatening to take out the
|
|
stack backchain in optimised code as this also causes pipeline stalls, you
|
|
have been warned.
|
|
|
|
64 bit z/Architecture code disassembly
|
|
--------------------------------------
|
|
|
|
If you understand the stuff above you'll understand the stuff
|
|
below too so I'll avoid repeating myself & just say that
|
|
some of the instructions have g's on the end of them to indicate
|
|
they are 64 bit & the stack offsets are a bigger,
|
|
the only other difference you'll find between 32 & 64 bit is that
|
|
we now use f4 & f6 for floating point arguments on 64 bit.
|
|
00000000800005b0 <test>:
|
|
int test(int b)
|
|
{
|
|
return(5+b);
|
|
800005b0: a7 2a 00 05 ahi %r2,5
|
|
800005b4: b9 14 00 22 lgfr %r2,%r2 # downcast to integer
|
|
800005b8: 07 fe br %r14
|
|
800005ba: 07 07 bcr 0,%r7
|
|
|
|
|
|
}
|
|
|
|
00000000800005bc <main>:
|
|
main(int argc,char *argv[])
|
|
{
|
|
800005bc: eb bf f0 58 00 24 stmg %r11,%r15,88(%r15)
|
|
800005c2: b9 04 00 1f lgr %r1,%r15
|
|
800005c6: a7 fb ff 60 aghi %r15,-160
|
|
800005ca: e3 10 f0 00 00 24 stg %r1,0(%r15)
|
|
return(test(5));
|
|
800005d0: a7 29 00 05 lghi %r2,5
|
|
# brasl allows jumps > 64k & is overkill here bras would do fune
|
|
800005d4: c0 e5 ff ff ff ee brasl %r14,800005b0 <test>
|
|
800005da: e3 40 f1 10 00 04 lg %r4,272(%r15)
|
|
800005e0: eb bf f0 f8 00 04 lmg %r11,%r15,248(%r15)
|
|
800005e6: 07 f4 br %r4
|
|
}
|
|
|
|
|
|
|
|
Compiling programs for debugging on Linux for s/390 & z/Architecture
|
|
====================================================================
|
|
-gdwarf-2 now works it should be considered the default debugging
|
|
format for s/390 & z/Architecture as it is more reliable for debugging
|
|
shared libraries, normal -g debugging works much better now
|
|
Thanks to the IBM java compiler developers bug reports.
|
|
|
|
This is typically done adding/appending the flags -g or -gdwarf-2 to the
|
|
CFLAGS & LDFLAGS variables Makefile of the program concerned.
|
|
|
|
If using gdb & you would like accurate displays of registers &
|
|
stack traces compile without optimisation i.e make sure
|
|
that there is no -O2 or similar on the CFLAGS line of the Makefile &
|
|
the emitted gcc commands, obviously this will produce worse code
|
|
( not advisable for shipment ) but it is an aid to the debugging process.
|
|
|
|
This aids debugging because the compiler will copy parameters passed in
|
|
in registers onto the stack so backtracing & looking at passed in
|
|
parameters will work, however some larger programs which use inline functions
|
|
will not compile without optimisation.
|
|
|
|
Debugging with optimisation has since much improved after fixing
|
|
some bugs, please make sure you are using gdb-5.0 or later developed
|
|
after Nov'2000.
|
|
|
|
|
|
|
|
Debugging under VM
|
|
==================
|
|
|
|
Notes
|
|
-----
|
|
Addresses & values in the VM debugger are always hex never decimal
|
|
Address ranges are of the format <HexValue1>-<HexValue2> or
|
|
<HexValue1>.<HexValue2>
|
|
For example, the address range 0x2000 to 0x3000 can be described as 2000-3000
|
|
or 2000.1000
|
|
|
|
The VM Debugger is case insensitive.
|
|
|
|
VM's strengths are usually other debuggers weaknesses you can get at any
|
|
resource no matter how sensitive e.g. memory management resources, change
|
|
address translation in the PSW. For kernel hacking you will reap dividends if
|
|
you get good at it.
|
|
|
|
The VM Debugger displays operators but not operands, and also the debugger
|
|
displays useful information on the same line as the author of the code probably
|
|
felt that it was a good idea not to go over the 80 columns on the screen.
|
|
This isn't as unintuitive as it may seem as the s/390 instructions are easy to
|
|
decode mentally and you can make a good guess at a lot of them as all the
|
|
operands are nibble (half byte aligned).
|
|
So if you have an objdump listing by hand, it is quite easy to follow, and if
|
|
you don't have an objdump listing keep a copy of the s/390 Reference Summary
|
|
or alternatively the s/390 principles of operation next to you.
|
|
e.g. even I can guess that
|
|
0001AFF8' LR 180F CC 0
|
|
is a ( load register ) lr r0,r15
|
|
|
|
Also it is very easy to tell the length of a 390 instruction from the 2 most
|
|
significant bits in the instruction (not that this info is really useful except
|
|
if you are trying to make sense of a hexdump of code).
|
|
Here is a table
|
|
Bits Instruction Length
|
|
------------------------------------------
|
|
00 2 Bytes
|
|
01 4 Bytes
|
|
10 4 Bytes
|
|
11 6 Bytes
|
|
|
|
The debugger also displays other useful info on the same line such as the
|
|
addresses being operated on destination addresses of branches & condition codes.
|
|
e.g.
|
|
00019736' AHI A7DAFF0E CC 1
|
|
000198BA' BRC A7840004 -> 000198C2' CC 0
|
|
000198CE' STM 900EF068 >> 0FA95E78 CC 2
|
|
|
|
|
|
|
|
Useful VM debugger commands
|
|
---------------------------
|
|
|
|
I suppose I'd better mention this before I start
|
|
to list the current active traces do
|
|
Q TR
|
|
there can be a maximum of 255 of these per set
|
|
( more about trace sets later ).
|
|
To stop traces issue a
|
|
TR END.
|
|
To delete a particular breakpoint issue
|
|
TR DEL <breakpoint number>
|
|
|
|
The PA1 key drops to CP mode so you can issue debugger commands,
|
|
Doing alt c (on my 3270 console at least ) clears the screen.
|
|
hitting b <enter> comes back to the running operating system
|
|
from cp mode ( in our case linux ).
|
|
It is typically useful to add shortcuts to your profile.exec file
|
|
if you have one ( this is roughly equivalent to autoexec.bat in DOS ).
|
|
file here are a few from mine.
|
|
/* this gives me command history on issuing f12 */
|
|
set pf12 retrieve
|
|
/* this continues */
|
|
set pf8 imm b
|
|
/* goes to trace set a */
|
|
set pf1 imm tr goto a
|
|
/* goes to trace set b */
|
|
set pf2 imm tr goto b
|
|
/* goes to trace set c */
|
|
set pf3 imm tr goto c
|
|
|
|
|
|
|
|
Instruction Tracing
|
|
-------------------
|
|
Setting a simple breakpoint
|
|
TR I PSWA <address>
|
|
To debug a particular function try
|
|
TR I R <function address range>
|
|
TR I on its own will single step.
|
|
TR I DATA <MNEMONIC> <OPTIONAL RANGE> will trace for particular mnemonics
|
|
e.g.
|
|
TR I DATA 4D R 0197BC.4000
|
|
will trace for BAS'es ( opcode 4D ) in the range 0197BC.4000
|
|
if you were inclined you could add traces for all branch instructions &
|
|
suffix them with the run prefix so you would have a backtrace on screen
|
|
when a program crashes.
|
|
TR BR <INTO OR FROM> will trace branches into or out of an address.
|
|
e.g.
|
|
TR BR INTO 0 is often quite useful if a program is getting awkward & deciding
|
|
to branch to 0 & crashing as this will stop at the address before in jumps to 0.
|
|
TR I R <address range> RUN cmd d g
|
|
single steps a range of addresses but stays running &
|
|
displays the gprs on each step.
|
|
|
|
|
|
|
|
Displaying & modifying Registers
|
|
--------------------------------
|
|
D G will display all the gprs
|
|
Adding a extra G to all the commands is necessary to access the full 64 bit
|
|
content in VM on z/Architecture. Obviously this isn't required for access
|
|
registers as these are still 32 bit.
|
|
e.g. DGG instead of DG
|
|
D X will display all the control registers
|
|
D AR will display all the access registers
|
|
D AR4-7 will display access registers 4 to 7
|
|
CPU ALL D G will display the GRPS of all CPUS in the configuration
|
|
D PSW will display the current PSW
|
|
st PSW 2000 will put the value 2000 into the PSW &
|
|
cause crash your machine.
|
|
D PREFIX displays the prefix offset
|
|
|
|
|
|
Displaying Memory
|
|
-----------------
|
|
To display memory mapped using the current PSW's mapping try
|
|
D <range>
|
|
To make VM display a message each time it hits a particular address and
|
|
continue try
|
|
D I<range> will disassemble/display a range of instructions.
|
|
ST addr 32 bit word will store a 32 bit aligned address
|
|
D T<range> will display the EBCDIC in an address (if you are that way inclined)
|
|
D R<range> will display real addresses ( without DAT ) but with prefixing.
|
|
There are other complex options to display if you need to get at say home space
|
|
but are in primary space the easiest thing to do is to temporarily
|
|
modify the PSW to the other addressing mode, display the stuff & then
|
|
restore it.
|
|
|
|
|
|
|
|
Hints
|
|
-----
|
|
If you want to issue a debugger command without halting your virtual machine
|
|
with the PA1 key try prefixing the command with #CP e.g.
|
|
#cp tr i pswa 2000
|
|
also suffixing most debugger commands with RUN will cause them not
|
|
to stop just display the mnemonic at the current instruction on the console.
|
|
If you have several breakpoints you want to put into your program &
|
|
you get fed up of cross referencing with System.map
|
|
you can do the following trick for several symbols.
|
|
grep do_signal System.map
|
|
which emits the following among other things
|
|
0001f4e0 T do_signal
|
|
now you can do
|
|
|
|
TR I PSWA 0001f4e0 cmd msg * do_signal
|
|
This sends a message to your own console each time do_signal is entered.
|
|
( As an aside I wrote a perl script once which automatically generated a REXX
|
|
script with breakpoints on every kernel procedure, this isn't a good idea
|
|
because there are thousands of these routines & VM can only set 255 breakpoints
|
|
at a time so you nearly had to spend as long pruning the file down as you would
|
|
entering the msgs by hand), however, the trick might be useful for a single
|
|
object file. In the 3270 terminal emulator x3270 there is a very useful option
|
|
in the file menu called "Save Screen In File" - this is very good for keeping a
|
|
copy of traces.
|
|
|
|
From CMS help <command name> will give you online help on a particular command.
|
|
e.g.
|
|
HELP DISPLAY
|
|
|
|
Also CP has a file called profile.exec which automatically gets called
|
|
on startup of CMS ( like autoexec.bat ), keeping on a DOS analogy session
|
|
CP has a feature similar to doskey, it may be useful for you to
|
|
use profile.exec to define some keystrokes.
|
|
e.g.
|
|
SET PF9 IMM B
|
|
This does a single step in VM on pressing F8.
|
|
SET PF10 ^
|
|
This sets up the ^ key.
|
|
which can be used for ^c (ctrl-c),^z (ctrl-z) which can't be typed directly
|
|
into some 3270 consoles.
|
|
SET PF11 ^-
|
|
This types the starting keystrokes for a sysrq see SysRq below.
|
|
SET PF12 RETRIEVE
|
|
This retrieves command history on pressing F12.
|
|
|
|
|
|
Sometimes in VM the display is set up to scroll automatically this
|
|
can be very annoying if there are messages you wish to look at
|
|
to stop this do
|
|
TERM MORE 255 255
|
|
This will nearly stop automatic screen updates, however it will
|
|
cause a denial of service if lots of messages go to the 3270 console,
|
|
so it would be foolish to use this as the default on a production machine.
|
|
|
|
|
|
Tracing particular processes
|
|
----------------------------
|
|
The kernel's text segment is intentionally at an address in memory that it will
|
|
very seldom collide with text segments of user programs ( thanks Martin ),
|
|
this simplifies debugging the kernel.
|
|
However it is quite common for user processes to have addresses which collide
|
|
this can make debugging a particular process under VM painful under normal
|
|
circumstances as the process may change when doing a
|
|
TR I R <address range>.
|
|
Thankfully after reading VM's online help I figured out how to debug
|
|
I particular process.
|
|
|
|
Your first problem is to find the STD ( segment table designation )
|
|
of the program you wish to debug.
|
|
There are several ways you can do this here are a few
|
|
1) objdump --syms <program to be debugged> | grep main
|
|
To get the address of main in the program.
|
|
tr i pswa <address of main>
|
|
Start the program, if VM drops to CP on what looks like the entry
|
|
point of the main function this is most likely the process you wish to debug.
|
|
Now do a D X13 or D XG13 on z/Architecture.
|
|
On 31 bit the STD is bits 1-19 ( the STO segment table origin )
|
|
& 25-31 ( the STL segment table length ) of CR13.
|
|
now type
|
|
TR I R STD <CR13's value> 0.7fffffff
|
|
e.g.
|
|
TR I R STD 8F32E1FF 0.7fffffff
|
|
Another very useful variation is
|
|
TR STORE INTO STD <CR13's value> <address range>
|
|
for finding out when a particular variable changes.
|
|
|
|
An alternative way of finding the STD of a currently running process
|
|
is to do the following, ( this method is more complex but
|
|
could be quite convenient if you aren't updating the kernel much &
|
|
so your kernel structures will stay constant for a reasonable period of
|
|
time ).
|
|
|
|
grep task /proc/<pid>/status
|
|
from this you should see something like
|
|
task: 0f160000 ksp: 0f161de8 pt_regs: 0f161f68
|
|
This now gives you a pointer to the task structure.
|
|
Now make CC:="s390-gcc -g" kernel/sched.s
|
|
To get the task_struct stabinfo.
|
|
( task_struct is defined in include/linux/sched.h ).
|
|
Now we want to look at
|
|
task->active_mm->pgd
|
|
on my machine the active_mm in the task structure stab is
|
|
active_mm:(4,12),672,32
|
|
its offset is 672/8=84=0x54
|
|
the pgd member in the mm_struct stab is
|
|
pgd:(4,6)=*(29,5),96,32
|
|
so its offset is 96/8=12=0xc
|
|
|
|
so we'll
|
|
hexdump -s 0xf160054 /dev/mem | more
|
|
i.e. task_struct+active_mm offset
|
|
to look at the active_mm member
|
|
f160054 0fee cc60 0019 e334 0000 0000 0000 0011
|
|
hexdump -s 0x0feecc6c /dev/mem | more
|
|
i.e. active_mm+pgd offset
|
|
feecc6c 0f2c 0000 0000 0001 0000 0001 0000 0010
|
|
we get something like
|
|
now do
|
|
TR I R STD <pgd|0x7f> 0.7fffffff
|
|
i.e. the 0x7f is added because the pgd only
|
|
gives the page table origin & we need to set the low bits
|
|
to the maximum possible segment table length.
|
|
TR I R STD 0f2c007f 0.7fffffff
|
|
on z/Architecture you'll probably need to do
|
|
TR I R STD <pgd|0x7> 0.ffffffffffffffff
|
|
to set the TableType to 0x1 & the Table length to 3.
|
|
|
|
|
|
|
|
Tracing Program Exceptions
|
|
--------------------------
|
|
If you get a crash which says something like
|
|
illegal operation or specification exception followed by a register dump
|
|
You can restart linux & trace these using the tr prog <range or value> trace
|
|
option.
|
|
|
|
|
|
The most common ones you will normally be tracing for is
|
|
1=operation exception
|
|
2=privileged operation exception
|
|
4=protection exception
|
|
5=addressing exception
|
|
6=specification exception
|
|
10=segment translation exception
|
|
11=page translation exception
|
|
|
|
The full list of these is on page 22 of the current s/390 Reference Summary.
|
|
e.g.
|
|
tr prog 10 will trace segment translation exceptions.
|
|
tr prog on its own will trace all program interruption codes.
|
|
|
|
Trace Sets
|
|
----------
|
|
On starting VM you are initially in the INITIAL trace set.
|
|
You can do a Q TR to verify this.
|
|
If you have a complex tracing situation where you wish to wait for instance
|
|
till a driver is open before you start tracing IO, but know in your
|
|
heart that you are going to have to make several runs through the code till you
|
|
have a clue whats going on.
|
|
|
|
What you can do is
|
|
TR I PSWA <Driver open address>
|
|
hit b to continue till breakpoint
|
|
reach the breakpoint
|
|
now do your
|
|
TR GOTO B
|
|
TR IO 7c08-7c09 inst int run
|
|
or whatever the IO channels you wish to trace are & hit b
|
|
|
|
To got back to the initial trace set do
|
|
TR GOTO INITIAL
|
|
& the TR I PSWA <Driver open address> will be the only active breakpoint again.
|
|
|
|
|
|
Tracing linux syscalls under VM
|
|
-------------------------------
|
|
Syscalls are implemented on Linux for S390 by the Supervisor call instruction
|
|
(SVC). There 256 possibilities of these as the instruction is made up of a 0xA
|
|
opcode and the second byte being the syscall number. They are traced using the
|
|
simple command:
|
|
TR SVC <Optional value or range>
|
|
the syscalls are defined in linux/arch/s390/include/asm/unistd.h
|
|
e.g. to trace all file opens just do
|
|
TR SVC 5 ( as this is the syscall number of open )
|
|
|
|
|
|
SMP Specific commands
|
|
---------------------
|
|
To find out how many cpus you have
|
|
Q CPUS displays all the CPU's available to your virtual machine
|
|
To find the cpu that the current cpu VM debugger commands are being directed at
|
|
do Q CPU to change the current cpu VM debugger commands are being directed at do
|
|
CPU <desired cpu no>
|
|
|
|
On a SMP guest issue a command to all CPUs try prefixing the command with cpu
|
|
all. To issue a command to a particular cpu try cpu <cpu number> e.g.
|
|
CPU 01 TR I R 2000.3000
|
|
If you are running on a guest with several cpus & you have a IO related problem
|
|
& cannot follow the flow of code but you know it isn't smp related.
|
|
from the bash prompt issue
|
|
shutdown -h now or halt.
|
|
do a Q CPUS to find out how many cpus you have
|
|
detach each one of them from cp except cpu 0
|
|
by issuing a
|
|
DETACH CPU 01-(number of cpus in configuration)
|
|
& boot linux again.
|
|
TR SIGP will trace inter processor signal processor instructions.
|
|
DEFINE CPU 01-(number in configuration)
|
|
will get your guests cpus back.
|
|
|
|
|
|
Help for displaying ascii textstrings
|
|
-------------------------------------
|
|
On the very latest VM Nucleus'es VM can now display ascii
|
|
( thanks Neale for the hint ) by doing
|
|
D TX<lowaddr>.<len>
|
|
e.g.
|
|
D TX0.100
|
|
|
|
Alternatively
|
|
=============
|
|
Under older VM debuggers (I love EBDIC too) you can use following little
|
|
program which converts a command line of hex digits to ascii text. It can be
|
|
compiled under linux and you can copy the hex digits from your x3270 terminal
|
|
to your xterm if you are debugging from a linuxbox.
|
|
|
|
This is quite useful when looking at a parameter passed in as a text string
|
|
under VM ( unless you are good at decoding ASCII in your head ).
|
|
|
|
e.g. consider tracing an open syscall
|
|
TR SVC 5
|
|
We have stopped at a breakpoint
|
|
000151B0' SVC 0A05 -> 0001909A' CC 0
|
|
|
|
D 20.8 to check the SVC old psw in the prefix area and see was it from userspace
|
|
(for the layout of the prefix area consult the "Fixed Storage Locations"
|
|
chapter of the s/390 Reference Summary if you have it available).
|
|
V00000020 070C2000 800151B2
|
|
The problem state bit wasn't set & it's also too early in the boot sequence
|
|
for it to be a userspace SVC if it was we would have to temporarily switch the
|
|
psw to user space addressing so we could get at the first parameter of the open
|
|
in gpr2.
|
|
Next do a
|
|
D G2
|
|
GPR 2 = 00014CB4
|
|
Now display what gpr2 is pointing to
|
|
D 00014CB4.20
|
|
V00014CB4 2F646576 2F636F6E 736F6C65 00001BF5
|
|
V00014CC4 FC00014C B4001001 E0001000 B8070707
|
|
Now copy the text till the first 00 hex ( which is the end of the string
|
|
to an xterm & do hex2ascii on it.
|
|
hex2ascii 2F646576 2F636F6E 736F6C65 00
|
|
outputs
|
|
Decoded Hex:=/ d e v / c o n s o l e 0x00
|
|
We were opening the console device,
|
|
|
|
You can compile the code below yourself for practice :-),
|
|
/*
|
|
* hex2ascii.c
|
|
* a useful little tool for converting a hexadecimal command line to ascii
|
|
*
|
|
* Author(s): Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com)
|
|
* (C) 2000 IBM Deutschland Entwicklung GmbH, IBM Corporation.
|
|
*/
|
|
#include <stdio.h>
|
|
|
|
int main(int argc,char *argv[])
|
|
{
|
|
int cnt1,cnt2,len,toggle=0;
|
|
int startcnt=1;
|
|
unsigned char c,hex;
|
|
|
|
if(argc>1&&(strcmp(argv[1],"-a")==0))
|
|
startcnt=2;
|
|
printf("Decoded Hex:=");
|
|
for(cnt1=startcnt;cnt1<argc;cnt1++)
|
|
{
|
|
len=strlen(argv[cnt1]);
|
|
for(cnt2=0;cnt2<len;cnt2++)
|
|
{
|
|
c=argv[cnt1][cnt2];
|
|
if(c>='0'&&c<='9')
|
|
c=c-'0';
|
|
if(c>='A'&&c<='F')
|
|
c=c-'A'+10;
|
|
if(c>='a'&&c<='f')
|
|
c=c-'a'+10;
|
|
switch(toggle)
|
|
{
|
|
case 0:
|
|
hex=c<<4;
|
|
toggle=1;
|
|
break;
|
|
case 1:
|
|
hex+=c;
|
|
if(hex<32||hex>127)
|
|
{
|
|
if(startcnt==1)
|
|
printf("0x%02X ",(int)hex);
|
|
else
|
|
printf(".");
|
|
}
|
|
else
|
|
{
|
|
printf("%c",hex);
|
|
if(startcnt==1)
|
|
printf(" ");
|
|
}
|
|
toggle=0;
|
|
break;
|
|
}
|
|
}
|
|
}
|
|
printf("\n");
|
|
}
|
|
|
|
|
|
|
|
|
|
Stack tracing under VM
|
|
----------------------
|
|
A basic backtrace
|
|
-----------------
|
|
|
|
Here are the tricks I use 9 out of 10 times it works pretty well,
|
|
|
|
When your backchain reaches a dead end
|
|
--------------------------------------
|
|
This can happen when an exception happens in the kernel and the kernel is
|
|
entered twice. If you reach the NULL pointer at the end of the back chain you
|
|
should be able to sniff further back if you follow the following tricks.
|
|
1) A kernel address should be easy to recognise since it is in
|
|
primary space & the problem state bit isn't set & also
|
|
The Hi bit of the address is set.
|
|
2) Another backchain should also be easy to recognise since it is an
|
|
address pointing to another address approximately 100 bytes or 0x70 hex
|
|
behind the current stackpointer.
|
|
|
|
|
|
Here is some practice.
|
|
boot the kernel & hit PA1 at some random time
|
|
d g to display the gprs, this should display something like
|
|
GPR 0 = 00000001 00156018 0014359C 00000000
|
|
GPR 4 = 00000001 001B8888 000003E0 00000000
|
|
GPR 8 = 00100080 00100084 00000000 000FE000
|
|
GPR 12 = 00010400 8001B2DC 8001B36A 000FFED8
|
|
Note that GPR14 is a return address but as we are real men we are going to
|
|
trace the stack.
|
|
display 0x40 bytes after the stack pointer.
|
|
|
|
V000FFED8 000FFF38 8001B838 80014C8E 000FFF38
|
|
V000FFEE8 00000000 00000000 000003E0 00000000
|
|
V000FFEF8 00100080 00100084 00000000 000FE000
|
|
V000FFF08 00010400 8001B2DC 8001B36A 000FFED8
|
|
|
|
|
|
Ah now look at whats in sp+56 (sp+0x38) this is 8001B36A our saved r14 if
|
|
you look above at our stackframe & also agrees with GPR14.
|
|
|
|
now backchain
|
|
d 000FFF38.40
|
|
we now are taking the contents of SP to get our first backchain.
|
|
|
|
V000FFF38 000FFFA0 00000000 00014995 00147094
|
|
V000FFF48 00147090 001470A0 000003E0 00000000
|
|
V000FFF58 00100080 00100084 00000000 001BF1D0
|
|
V000FFF68 00010400 800149BA 80014CA6 000FFF38
|
|
|
|
This displays a 2nd return address of 80014CA6
|
|
|
|
now do d 000FFFA0.40 for our 3rd backchain
|
|
|
|
V000FFFA0 04B52002 0001107F 00000000 00000000
|
|
V000FFFB0 00000000 00000000 FF000000 0001107F
|
|
V000FFFC0 00000000 00000000 00000000 00000000
|
|
V000FFFD0 00010400 80010802 8001085A 000FFFA0
|
|
|
|
|
|
our 3rd return address is 8001085A
|
|
|
|
as the 04B52002 looks suspiciously like rubbish it is fair to assume that the
|
|
kernel entry routines for the sake of optimisation don't set up a backchain.
|
|
|
|
now look at System.map to see if the addresses make any sense.
|
|
|
|
grep -i 0001b3 System.map
|
|
outputs among other things
|
|
0001b304 T cpu_idle
|
|
so 8001B36A
|
|
is cpu_idle+0x66 ( quiet the cpu is asleep, don't wake it )
|
|
|
|
|
|
grep -i 00014 System.map
|
|
produces among other things
|
|
00014a78 T start_kernel
|
|
so 0014CA6 is start_kernel+some hex number I can't add in my head.
|
|
|
|
grep -i 00108 System.map
|
|
this produces
|
|
00010800 T _stext
|
|
so 8001085A is _stext+0x5a
|
|
|
|
Congrats you've done your first backchain.
|
|
|
|
|
|
|
|
s/390 & z/Architecture IO Overview
|
|
==================================
|
|
|
|
I am not going to give a course in 390 IO architecture as this would take me
|
|
quite a while and I'm no expert. Instead I'll give a 390 IO architecture
|
|
summary for Dummies. If you have the s/390 principles of operation available
|
|
read this instead. If nothing else you may find a few useful keywords in here
|
|
and be able to use them on a web search engine to find more useful information.
|
|
|
|
Unlike other bus architectures modern 390 systems do their IO using mostly
|
|
fibre optics and devices such as tapes and disks can be shared between several
|
|
mainframes. Also S390 can support up to 65536 devices while a high end PC based
|
|
system might be choking with around 64.
|
|
|
|
Here is some of the common IO terminology:
|
|
|
|
Subchannel:
|
|
This is the logical number most IO commands use to talk to an IO device. There
|
|
can be up to 0x10000 (65536) of these in a configuration, typically there are a
|
|
few hundred. Under VM for simplicity they are allocated contiguously, however
|
|
on the native hardware they are not. They typically stay consistent between
|
|
boots provided no new hardware is inserted or removed.
|
|
Under Linux for s390 we use these as IRQ's and also when issuing an IO command
|
|
(CLEAR SUBCHANNEL, HALT SUBCHANNEL, MODIFY SUBCHANNEL, RESUME SUBCHANNEL,
|
|
START SUBCHANNEL, STORE SUBCHANNEL and TEST SUBCHANNEL). We use this as the ID
|
|
of the device we wish to talk to. The most important of these instructions are
|
|
START SUBCHANNEL (to start IO), TEST SUBCHANNEL (to check whether the IO
|
|
completed successfully) and HALT SUBCHANNEL (to kill IO). A subchannel can have
|
|
up to 8 channel paths to a device, this offers redundancy if one is not
|
|
available.
|
|
|
|
Device Number:
|
|
This number remains static and is closely tied to the hardware. There are 65536
|
|
of these, made up of a CHPID (Channel Path ID, the most significant 8 bits) and
|
|
another lsb 8 bits. These remain static even if more devices are inserted or
|
|
removed from the hardware. There is a 1 to 1 mapping between subchannels and
|
|
device numbers, provided devices aren't inserted or removed.
|
|
|
|
Channel Control Words:
|
|
CCWs are linked lists of instructions initially pointed to by an operation
|
|
request block (ORB), which is initially given to Start Subchannel (SSCH)
|
|
command along with the subchannel number for the IO subsystem to process
|
|
while the CPU continues executing normal code.
|
|
CCWs come in two flavours, Format 0 (24 bit for backward compatibility) and
|
|
Format 1 (31 bit). These are typically used to issue read and write (and many
|
|
other) instructions. They consist of a length field and an absolute address
|
|
field.
|
|
Each IO typically gets 1 or 2 interrupts, one for channel end (primary status)
|
|
when the channel is idle, and the second for device end (secondary status).
|
|
Sometimes you get both concurrently. You check how the IO went on by issuing a
|
|
TEST SUBCHANNEL at each interrupt, from which you receive an Interruption
|
|
response block (IRB). If you get channel and device end status in the IRB
|
|
without channel checks etc. your IO probably went okay. If you didn't you
|
|
probably need to examine the IRB, extended status word etc.
|
|
If an error occurs, more sophisticated control units have a facility known as
|
|
concurrent sense. This means that if an error occurs Extended sense information
|
|
will be presented in the Extended status word in the IRB. If not you have to
|
|
issue a subsequent SENSE CCW command after the test subchannel.
|
|
|
|
|
|
TPI (Test pending interrupt) can also be used for polled IO, but in
|
|
multitasking multiprocessor systems it isn't recommended except for
|
|
checking special cases (i.e. non looping checks for pending IO etc.).
|
|
|
|
Store Subchannel and Modify Subchannel can be used to examine and modify
|
|
operating characteristics of a subchannel (e.g. channel paths).
|
|
|
|
Other IO related Terms:
|
|
Sysplex: S390's Clustering Technology
|
|
QDIO: S390's new high speed IO architecture to support devices such as gigabit
|
|
ethernet, this architecture is also designed to be forward compatible with
|
|
upcoming 64 bit machines.
|
|
|
|
|
|
General Concepts
|
|
|
|
Input Output Processors (IOP's) are responsible for communicating between
|
|
the mainframe CPU's & the channel & relieve the mainframe CPU's from the
|
|
burden of communicating with IO devices directly, this allows the CPU's to
|
|
concentrate on data processing.
|
|
|
|
IOP's can use one or more links ( known as channel paths ) to talk to each
|
|
IO device. It first checks for path availability & chooses an available one,
|
|
then starts ( & sometimes terminates IO ).
|
|
There are two types of channel path: ESCON & the Parallel IO interface.
|
|
|
|
IO devices are attached to control units, control units provide the
|
|
logic to interface the channel paths & channel path IO protocols to
|
|
the IO devices, they can be integrated with the devices or housed separately
|
|
& often talk to several similar devices ( typical examples would be raid
|
|
controllers or a control unit which connects to 1000 3270 terminals ).
|
|
|
|
|
|
+---------------------------------------------------------------+
|
|
| +-----+ +-----+ +-----+ +-----+ +----------+ +----------+ |
|
|
| | CPU | | CPU | | CPU | | CPU | | Main | | Expanded | |
|
|
| | | | | | | | | | Memory | | Storage | |
|
|
| +-----+ +-----+ +-----+ +-----+ +----------+ +----------+ |
|
|
|---------------------------------------------------------------+
|
|
| IOP | IOP | IOP |
|
|
|---------------------------------------------------------------
|
|
| C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C |
|
|
----------------------------------------------------------------
|
|
|| ||
|
|
|| Bus & Tag Channel Path || ESCON
|
|
|| ====================== || Channel
|
|
|| || || || Path
|
|
+----------+ +----------+ +----------+
|
|
| | | | | |
|
|
| CU | | CU | | CU |
|
|
| | | | | |
|
|
+----------+ +----------+ +----------+
|
|
| | | | |
|
|
+----------+ +----------+ +----------+ +----------+ +----------+
|
|
|I/O Device| |I/O Device| |I/O Device| |I/O Device| |I/O Device|
|
|
+----------+ +----------+ +----------+ +----------+ +----------+
|
|
CPU = Central Processing Unit
|
|
C = Channel
|
|
IOP = IP Processor
|
|
CU = Control Unit
|
|
|
|
The 390 IO systems come in 2 flavours the current 390 machines support both
|
|
|
|
The Older 360 & 370 Interface,sometimes called the Parallel I/O interface,
|
|
sometimes called Bus-and Tag & sometimes Original Equipment Manufacturers
|
|
Interface (OEMI).
|
|
|
|
This byte wide Parallel channel path/bus has parity & data on the "Bus" cable
|
|
and control lines on the "Tag" cable. These can operate in byte multiplex mode
|
|
for sharing between several slow devices or burst mode and monopolize the
|
|
channel for the whole burst. Up to 256 devices can be addressed on one of these
|
|
cables. These cables are about one inch in diameter. The maximum unextended
|
|
length supported by these cables is 125 Meters but this can be extended up to
|
|
2km with a fibre optic channel extended such as a 3044. The maximum burst speed
|
|
supported is 4.5 megabytes per second. However, some really old processors
|
|
support only transfer rates of 3.0, 2.0 & 1.0 MB/sec.
|
|
One of these paths can be daisy chained to up to 8 control units.
|
|
|
|
|
|
ESCON if fibre optic it is also called FICON
|
|
Was introduced by IBM in 1990. Has 2 fibre optic cables and uses either leds or
|
|
lasers for communication at a signaling rate of up to 200 megabits/sec. As
|
|
10bits are transferred for every 8 bits info this drops to 160 megabits/sec
|
|
and to 18.6 Megabytes/sec once control info and CRC are added. ESCON only
|
|
operates in burst mode.
|
|
|
|
ESCONs typical max cable length is 3km for the led version and 20km for the
|
|
laser version known as XDF (extended distance facility). This can be further
|
|
extended by using an ESCON director which triples the above mentioned ranges.
|
|
Unlike Bus & Tag as ESCON is serial it uses a packet switching architecture,
|
|
the standard Bus & Tag control protocol is however present within the packets.
|
|
Up to 256 devices can be attached to each control unit that uses one of these
|
|
interfaces.
|
|
|
|
Common 390 Devices include:
|
|
Network adapters typically OSA2,3172's,2116's & OSA-E gigabit ethernet adapters,
|
|
Consoles 3270 & 3215 (a teletype emulated under linux for a line mode console).
|
|
DASD's direct access storage devices ( otherwise known as hard disks ).
|
|
Tape Drives.
|
|
CTC ( Channel to Channel Adapters ),
|
|
ESCON or Parallel Cables used as a very high speed serial link
|
|
between 2 machines.
|
|
|
|
|
|
Debugging IO on s/390 & z/Architecture under VM
|
|
===============================================
|
|
|
|
Now we are ready to go on with IO tracing commands under VM
|
|
|
|
A few self explanatory queries:
|
|
Q OSA
|
|
Q CTC
|
|
Q DISK ( This command is CMS specific )
|
|
Q DASD
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Q OSA on my machine returns
|
|
OSA 7C08 ON OSA 7C08 SUBCHANNEL = 0000
|
|
OSA 7C09 ON OSA 7C09 SUBCHANNEL = 0001
|
|
OSA 7C14 ON OSA 7C14 SUBCHANNEL = 0002
|
|
OSA 7C15 ON OSA 7C15 SUBCHANNEL = 0003
|
|
|
|
If you have a guest with certain privileges you may be able to see devices
|
|
which don't belong to you. To avoid this, add the option V.
|
|
e.g.
|
|
Q V OSA
|
|
|
|
Now using the device numbers returned by this command we will
|
|
Trace the io starting up on the first device 7c08 & 7c09
|
|
In our simplest case we can trace the
|
|
start subchannels
|
|
like TR SSCH 7C08-7C09
|
|
or the halt subchannels
|
|
or TR HSCH 7C08-7C09
|
|
MSCH's ,STSCH's I think you can guess the rest
|
|
|
|
A good trick is tracing all the IO's and CCWS and spooling them into the reader
|
|
of another VM guest so he can ftp the logfile back to his own machine. I'll do
|
|
a small bit of this and give you a look at the output.
|
|
|
|
1) Spool stdout to VM reader
|
|
SP PRT TO (another vm guest ) or * for the local vm guest
|
|
2) Fill the reader with the trace
|
|
TR IO 7c08-7c09 INST INT CCW PRT RUN
|
|
3) Start up linux
|
|
i 00c
|
|
4) Finish the trace
|
|
TR END
|
|
5) close the reader
|
|
C PRT
|
|
6) list reader contents
|
|
RDRLIST
|
|
7) copy it to linux4's minidisk
|
|
RECEIVE / LOG TXT A1 ( replace
|
|
8)
|
|
filel & press F11 to look at it
|
|
You should see something like:
|
|
|
|
00020942' SSCH B2334000 0048813C CC 0 SCH 0000 DEV 7C08
|
|
CPA 000FFDF0 PARM 00E2C9C4 KEY 0 FPI C0 LPM 80
|
|
CCW 000FFDF0 E4200100 00487FE8 0000 E4240100 ........
|
|
IDAL 43D8AFE8
|
|
IDAL 0FB76000
|
|
00020B0A' I/O DEV 7C08 -> 000197BC' SCH 0000 PARM 00E2C9C4
|
|
00021628' TSCH B2354000 >> 00488164 CC 0 SCH 0000 DEV 7C08
|
|
CCWA 000FFDF8 DEV STS 0C SCH STS 00 CNT 00EC
|
|
KEY 0 FPI C0 CC 0 CTLS 4007
|
|
00022238' STSCH B2344000 >> 00488108 CC 0 SCH 0000 DEV 7C08
|
|
|
|
If you don't like messing up your readed ( because you possibly booted from it )
|
|
you can alternatively spool it to another readers guest.
|
|
|
|
|
|
Other common VM device related commands
|
|
---------------------------------------------
|
|
These commands are listed only because they have
|
|
been of use to me in the past & may be of use to
|
|
you too. For more complete info on each of the commands
|
|
use type HELP <command> from CMS.
|
|
detaching devices
|
|
DET <devno range>
|
|
ATT <devno range> <guest>
|
|
attach a device to guest * for your own guest
|
|
READY <devno> cause VM to issue a fake interrupt.
|
|
|
|
The VARY command is normally only available to VM administrators.
|
|
VARY ON PATH <path> TO <devno range>
|
|
VARY OFF PATH <PATH> FROM <devno range>
|
|
This is used to switch on or off channel paths to devices.
|
|
|
|
Q CHPID <channel path ID>
|
|
This displays state of devices using this channel path
|
|
D SCHIB <subchannel>
|
|
This displays the subchannel information SCHIB block for the device.
|
|
this I believe is also only available to administrators.
|
|
DEFINE CTC <devno>
|
|
defines a virtual CTC channel to channel connection
|
|
2 need to be defined on each guest for the CTC driver to use.
|
|
COUPLE devno userid remote devno
|
|
Joins a local virtual device to a remote virtual device
|
|
( commonly used for the CTC driver ).
|
|
|
|
Building a VM ramdisk under CMS which linux can use
|
|
def vfb-<blocksize> <subchannel> <number blocks>
|
|
blocksize is commonly 4096 for linux.
|
|
Formatting it
|
|
format <subchannel> <driver letter e.g. x> (blksize <blocksize>
|
|
|
|
Sharing a disk between multiple guests
|
|
LINK userid devno1 devno2 mode password
|
|
|
|
|
|
|
|
GDB on S390
|
|
===========
|
|
N.B. if compiling for debugging gdb works better without optimisation
|
|
( see Compiling programs for debugging )
|
|
|
|
invocation
|
|
----------
|
|
gdb <victim program> <optional corefile>
|
|
|
|
Online help
|
|
-----------
|
|
help: gives help on commands
|
|
e.g.
|
|
help
|
|
help display
|
|
Note gdb's online help is very good use it.
|
|
|
|
|
|
Assembly
|
|
--------
|
|
info registers: displays registers other than floating point.
|
|
info all-registers: displays floating points as well.
|
|
disassemble: disassembles
|
|
e.g.
|
|
disassemble without parameters will disassemble the current function
|
|
disassemble $pc $pc+10
|
|
|
|
Viewing & modifying variables
|
|
-----------------------------
|
|
print or p: displays variable or register
|
|
e.g. p/x $sp will display the stack pointer
|
|
|
|
display: prints variable or register each time program stops
|
|
e.g.
|
|
display/x $pc will display the program counter
|
|
display argc
|
|
|
|
undisplay : undo's display's
|
|
|
|
info breakpoints: shows all current breakpoints
|
|
|
|
info stack: shows stack back trace (if this doesn't work too well, I'll show
|
|
you the stacktrace by hand below).
|
|
|
|
info locals: displays local variables.
|
|
|
|
info args: display current procedure arguments.
|
|
|
|
set args: will set argc & argv each time the victim program is invoked.
|
|
|
|
set <variable>=value
|
|
set argc=100
|
|
set $pc=0
|
|
|
|
|
|
|
|
Modifying execution
|
|
-------------------
|
|
step: steps n lines of sourcecode
|
|
step steps 1 line.
|
|
step 100 steps 100 lines of code.
|
|
|
|
next: like step except this will not step into subroutines
|
|
|
|
stepi: steps a single machine code instruction.
|
|
e.g. stepi 100
|
|
|
|
nexti: steps a single machine code instruction but will not step into
|
|
subroutines.
|
|
|
|
finish: will run until exit of the current routine
|
|
|
|
run: (re)starts a program
|
|
|
|
cont: continues a program
|
|
|
|
quit: exits gdb.
|
|
|
|
|
|
breakpoints
|
|
------------
|
|
|
|
break
|
|
sets a breakpoint
|
|
e.g.
|
|
|
|
break main
|
|
|
|
break *$pc
|
|
|
|
break *0x400618
|
|
|
|
Here's a really useful one for large programs
|
|
rbr
|
|
Set a breakpoint for all functions matching REGEXP
|
|
e.g.
|
|
rbr 390
|
|
will set a breakpoint with all functions with 390 in their name.
|
|
|
|
info breakpoints
|
|
lists all breakpoints
|
|
|
|
delete: delete breakpoint by number or delete them all
|
|
e.g.
|
|
delete 1 will delete the first breakpoint
|
|
delete will delete them all
|
|
|
|
watch: This will set a watchpoint ( usually hardware assisted ),
|
|
This will watch a variable till it changes
|
|
e.g.
|
|
watch cnt, will watch the variable cnt till it changes.
|
|
As an aside unfortunately gdb's, architecture independent watchpoint code
|
|
is inconsistent & not very good, watchpoints usually work but not always.
|
|
|
|
info watchpoints: Display currently active watchpoints
|
|
|
|
condition: ( another useful one )
|
|
Specify breakpoint number N to break only if COND is true.
|
|
Usage is `condition N COND', where N is an integer and COND is an
|
|
expression to be evaluated whenever breakpoint N is reached.
|
|
|
|
|
|
|
|
User defined functions/macros
|
|
-----------------------------
|
|
define: ( Note this is very very useful,simple & powerful )
|
|
usage define <name> <list of commands> end
|
|
|
|
examples which you should consider putting into .gdbinit in your home directory
|
|
define d
|
|
stepi
|
|
disassemble $pc $pc+10
|
|
end
|
|
|
|
define e
|
|
nexti
|
|
disassemble $pc $pc+10
|
|
end
|
|
|
|
|
|
Other hard to classify stuff
|
|
----------------------------
|
|
signal n:
|
|
sends the victim program a signal.
|
|
e.g. signal 3 will send a SIGQUIT.
|
|
|
|
info signals:
|
|
what gdb does when the victim receives certain signals.
|
|
|
|
list:
|
|
e.g.
|
|
list lists current function source
|
|
list 1,10 list first 10 lines of current file.
|
|
list test.c:1,10
|
|
|
|
|
|
directory:
|
|
Adds directories to be searched for source if gdb cannot find the source.
|
|
(note it is a bit sensitive about slashes)
|
|
e.g. To add the root of the filesystem to the searchpath do
|
|
directory //
|
|
|
|
|
|
call <function>
|
|
This calls a function in the victim program, this is pretty powerful
|
|
e.g.
|
|
(gdb) call printf("hello world")
|
|
outputs:
|
|
$1 = 11
|
|
|
|
You might now be thinking that the line above didn't work, something extra had
|
|
to be done.
|
|
(gdb) call fflush(stdout)
|
|
hello world$2 = 0
|
|
As an aside the debugger also calls malloc & free under the hood
|
|
to make space for the "hello world" string.
|
|
|
|
|
|
|
|
hints
|
|
-----
|
|
1) command completion works just like bash
|
|
( if you are a bad typist like me this really helps )
|
|
e.g. hit br <TAB> & cursor up & down :-).
|
|
|
|
2) if you have a debugging problem that takes a few steps to recreate
|
|
put the steps into a file called .gdbinit in your current working directory
|
|
if you have defined a few extra useful user defined commands put these in
|
|
your home directory & they will be read each time gdb is launched.
|
|
|
|
A typical .gdbinit file might be.
|
|
break main
|
|
run
|
|
break runtime_exception
|
|
cont
|
|
|
|
|
|
stack chaining in gdb by hand
|
|
-----------------------------
|
|
This is done using a the same trick described for VM
|
|
p/x (*($sp+56))&0x7fffffff get the first backchain.
|
|
|
|
For z/Architecture
|
|
Replace 56 with 112 & ignore the &0x7fffffff
|
|
in the macros below & do nasty casts to longs like the following
|
|
as gdb unfortunately deals with printed arguments as ints which
|
|
messes up everything.
|
|
i.e. here is a 3rd backchain dereference
|
|
p/x *(long *)(***(long ***)$sp+112)
|
|
|
|
|
|
this outputs
|
|
$5 = 0x528f18
|
|
on my machine.
|
|
Now you can use
|
|
info symbol (*($sp+56))&0x7fffffff
|
|
you might see something like.
|
|
rl_getc + 36 in section .text telling you what is located at address 0x528f18
|
|
Now do.
|
|
p/x (*(*$sp+56))&0x7fffffff
|
|
This outputs
|
|
$6 = 0x528ed0
|
|
Now do.
|
|
info symbol (*(*$sp+56))&0x7fffffff
|
|
rl_read_key + 180 in section .text
|
|
now do
|
|
p/x (*(**$sp+56))&0x7fffffff
|
|
& so on.
|
|
|
|
Disassembling instructions without debug info
|
|
---------------------------------------------
|
|
gdb typically complains if there is a lack of debugging
|
|
symbols in the disassemble command with
|
|
"No function contains specified address." To get around
|
|
this do
|
|
x/<number lines to disassemble>xi <address>
|
|
e.g.
|
|
x/20xi 0x400730
|
|
|
|
|
|
|
|
Note: Remember gdb has history just like bash you don't need to retype the
|
|
whole line just use the up & down arrows.
|
|
|
|
|
|
|
|
For more info
|
|
-------------
|
|
From your linuxbox do
|
|
man gdb or info gdb.
|
|
|
|
core dumps
|
|
----------
|
|
What a core dump ?,
|
|
A core dump is a file generated by the kernel (if allowed) which contains the
|
|
registers and all active pages of the program which has crashed.
|
|
From this file gdb will allow you to look at the registers, stack trace and
|
|
memory of the program as if it just crashed on your system. It is usually
|
|
called core and created in the current working directory.
|
|
This is very useful in that a customer can mail a core dump to a technical
|
|
support department and the technical support department can reconstruct what
|
|
happened. Provided they have an identical copy of this program with debugging
|
|
symbols compiled in and the source base of this build is available.
|
|
In short it is far more useful than something like a crash log could ever hope
|
|
to be.
|
|
|
|
Why have I never seen one ?.
|
|
Probably because you haven't used the command
|
|
ulimit -c unlimited in bash
|
|
to allow core dumps, now do
|
|
ulimit -a
|
|
to verify that the limit was accepted.
|
|
|
|
A sample core dump
|
|
To create this I'm going to do
|
|
ulimit -c unlimited
|
|
gdb
|
|
to launch gdb (my victim app. ) now be bad & do the following from another
|
|
telnet/xterm session to the same machine
|
|
ps -aux | grep gdb
|
|
kill -SIGSEGV <gdb's pid>
|
|
or alternatively use killall -SIGSEGV gdb if you have the killall command.
|
|
Now look at the core dump.
|
|
./gdb core
|
|
Displays the following
|
|
GNU gdb 4.18
|
|
Copyright 1998 Free Software Foundation, Inc.
|
|
GDB is free software, covered by the GNU General Public License, and you are
|
|
welcome to change it and/or distribute copies of it under certain conditions.
|
|
Type "show copying" to see the conditions.
|
|
There is absolutely no warranty for GDB. Type "show warranty" for details.
|
|
This GDB was configured as "s390-ibm-linux"...
|
|
Core was generated by `./gdb'.
|
|
Program terminated with signal 11, Segmentation fault.
|
|
Reading symbols from /usr/lib/libncurses.so.4...done.
|
|
Reading symbols from /lib/libm.so.6...done.
|
|
Reading symbols from /lib/libc.so.6...done.
|
|
Reading symbols from /lib/ld-linux.so.2...done.
|
|
#0 0x40126d1a in read () from /lib/libc.so.6
|
|
Setting up the environment for debugging gdb.
|
|
Breakpoint 1 at 0x4dc6f8: file utils.c, line 471.
|
|
Breakpoint 2 at 0x4d87a4: file top.c, line 2609.
|
|
(top-gdb) info stack
|
|
#0 0x40126d1a in read () from /lib/libc.so.6
|
|
#1 0x528f26 in rl_getc (stream=0x7ffffde8) at input.c:402
|
|
#2 0x528ed0 in rl_read_key () at input.c:381
|
|
#3 0x5167e6 in readline_internal_char () at readline.c:454
|
|
#4 0x5168ee in readline_internal_charloop () at readline.c:507
|
|
#5 0x51692c in readline_internal () at readline.c:521
|
|
#6 0x5164fe in readline (prompt=0x7ffff810)
|
|
at readline.c:349
|
|
#7 0x4d7a8a in command_line_input (prompt=0x564420 "(gdb) ", repeat=1,
|
|
annotation_suffix=0x4d6b44 "prompt") at top.c:2091
|
|
#8 0x4d6cf0 in command_loop () at top.c:1345
|
|
#9 0x4e25bc in main (argc=1, argv=0x7ffffdf4) at main.c:635
|
|
|
|
|
|
LDD
|
|
===
|
|
This is a program which lists the shared libraries which a library needs,
|
|
Note you also get the relocations of the shared library text segments which
|
|
help when using objdump --source.
|
|
e.g.
|
|
ldd ./gdb
|
|
outputs
|
|
libncurses.so.4 => /usr/lib/libncurses.so.4 (0x40018000)
|
|
libm.so.6 => /lib/libm.so.6 (0x4005e000)
|
|
libc.so.6 => /lib/libc.so.6 (0x40084000)
|
|
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
|
|
|
|
|
|
Debugging shared libraries
|
|
==========================
|
|
Most programs use shared libraries, however it can be very painful
|
|
when you single step instruction into a function like printf for the
|
|
first time & you end up in functions like _dl_runtime_resolve this is
|
|
the ld.so doing lazy binding, lazy binding is a concept in ELF where
|
|
shared library functions are not loaded into memory unless they are
|
|
actually used, great for saving memory but a pain to debug.
|
|
To get around this either relink the program -static or exit gdb type
|
|
export LD_BIND_NOW=true this will stop lazy binding & restart the gdb'ing
|
|
the program in question.
|
|
|
|
|
|
|
|
Debugging modules
|
|
=================
|
|
As modules are dynamically loaded into the kernel their address can be
|
|
anywhere to get around this use the -m option with insmod to emit a load
|
|
map which can be piped into a file if required.
|
|
|
|
The proc file system
|
|
====================
|
|
What is it ?.
|
|
It is a filesystem created by the kernel with files which are created on demand
|
|
by the kernel if read, or can be used to modify kernel parameters,
|
|
it is a powerful concept.
|
|
|
|
e.g.
|
|
|
|
cat /proc/sys/net/ipv4/ip_forward
|
|
On my machine outputs
|
|
0
|
|
telling me ip_forwarding is not on to switch it on I can do
|
|
echo 1 > /proc/sys/net/ipv4/ip_forward
|
|
cat it again
|
|
cat /proc/sys/net/ipv4/ip_forward
|
|
On my machine now outputs
|
|
1
|
|
IP forwarding is on.
|
|
There is a lot of useful info in here best found by going in and having a look
|
|
around, so I'll take you through some entries I consider important.
|
|
|
|
All the processes running on the machine have their own entry defined by
|
|
/proc/<pid>
|
|
So lets have a look at the init process
|
|
cd /proc/1
|
|
|
|
cat cmdline
|
|
emits
|
|
init [2]
|
|
|
|
cd /proc/1/fd
|
|
This contains numerical entries of all the open files,
|
|
some of these you can cat e.g. stdout (2)
|
|
|
|
cat /proc/29/maps
|
|
on my machine emits
|
|
|
|
00400000-00478000 r-xp 00000000 5f:00 4103 /bin/bash
|
|
00478000-0047e000 rw-p 00077000 5f:00 4103 /bin/bash
|
|
0047e000-00492000 rwxp 00000000 00:00 0
|
|
40000000-40015000 r-xp 00000000 5f:00 14382 /lib/ld-2.1.2.so
|
|
40015000-40016000 rw-p 00014000 5f:00 14382 /lib/ld-2.1.2.so
|
|
40016000-40017000 rwxp 00000000 00:00 0
|
|
40017000-40018000 rw-p 00000000 00:00 0
|
|
40018000-4001b000 r-xp 00000000 5f:00 14435 /lib/libtermcap.so.2.0.8
|
|
4001b000-4001c000 rw-p 00002000 5f:00 14435 /lib/libtermcap.so.2.0.8
|
|
4001c000-4010d000 r-xp 00000000 5f:00 14387 /lib/libc-2.1.2.so
|
|
4010d000-40111000 rw-p 000f0000 5f:00 14387 /lib/libc-2.1.2.so
|
|
40111000-40114000 rw-p 00000000 00:00 0
|
|
40114000-4011e000 r-xp 00000000 5f:00 14408 /lib/libnss_files-2.1.2.so
|
|
4011e000-4011f000 rw-p 00009000 5f:00 14408 /lib/libnss_files-2.1.2.so
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7fffd000-80000000 rwxp ffffe000 00:00 0
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|
|
|
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Showing us the shared libraries init uses where they are in memory
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|
& memory access permissions for each virtual memory area.
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|
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/proc/1/cwd is a softlink to the current working directory.
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|
/proc/1/root is the root of the filesystem for this process.
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|
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/proc/1/mem is the current running processes memory which you
|
|
can read & write to like a file.
|
|
strace uses this sometimes as it is a bit faster than the
|
|
rather inefficient ptrace interface for peeking at DATA.
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|
|
|
|
|
cat status
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|
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Name: init
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|
State: S (sleeping)
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|
Pid: 1
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|
PPid: 0
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|
Uid: 0 0 0 0
|
|
Gid: 0 0 0 0
|
|
Groups:
|
|
VmSize: 408 kB
|
|
VmLck: 0 kB
|
|
VmRSS: 208 kB
|
|
VmData: 24 kB
|
|
VmStk: 8 kB
|
|
VmExe: 368 kB
|
|
VmLib: 0 kB
|
|
SigPnd: 0000000000000000
|
|
SigBlk: 0000000000000000
|
|
SigIgn: 7fffffffd7f0d8fc
|
|
SigCgt: 00000000280b2603
|
|
CapInh: 00000000fffffeff
|
|
CapPrm: 00000000ffffffff
|
|
CapEff: 00000000fffffeff
|
|
|
|
User PSW: 070de000 80414146
|
|
task: 004b6000 tss: 004b62d8 ksp: 004b7ca8 pt_regs: 004b7f68
|
|
User GPRS:
|
|
00000400 00000000 0000000b 7ffffa90
|
|
00000000 00000000 00000000 0045d9f4
|
|
0045cafc 7ffffa90 7fffff18 0045cb08
|
|
00010400 804039e8 80403af8 7ffff8b0
|
|
User ACRS:
|
|
00000000 00000000 00000000 00000000
|
|
00000001 00000000 00000000 00000000
|
|
00000000 00000000 00000000 00000000
|
|
00000000 00000000 00000000 00000000
|
|
Kernel BackChain CallChain BackChain CallChain
|
|
004b7ca8 8002bd0c 004b7d18 8002b92c
|
|
004b7db8 8005cd50 004b7e38 8005d12a
|
|
004b7f08 80019114
|
|
Showing among other things memory usage & status of some signals &
|
|
the processes'es registers from the kernel task_structure
|
|
as well as a backchain which may be useful if a process crashes
|
|
in the kernel for some unknown reason.
|
|
|
|
Some driver debugging techniques
|
|
================================
|
|
debug feature
|
|
-------------
|
|
Some of our drivers now support a "debug feature" in
|
|
/proc/s390dbf see s390dbf.txt in the linux/Documentation directory
|
|
for more info.
|
|
e.g.
|
|
to switch on the lcs "debug feature"
|
|
echo 5 > /proc/s390dbf/lcs/level
|
|
& then after the error occurred.
|
|
cat /proc/s390dbf/lcs/sprintf >/logfile
|
|
the logfile now contains some information which may help
|
|
tech support resolve a problem in the field.
|
|
|
|
|
|
|
|
high level debugging network drivers
|
|
------------------------------------
|
|
ifconfig is a quite useful command
|
|
it gives the current state of network drivers.
|
|
|
|
If you suspect your network device driver is dead
|
|
one way to check is type
|
|
ifconfig <network device>
|
|
e.g. tr0
|
|
You should see something like
|
|
tr0 Link encap:16/4 Mbps Token Ring (New) HWaddr 00:04:AC:20:8E:48
|
|
inet addr:9.164.185.132 Bcast:9.164.191.255 Mask:255.255.224.0
|
|
UP BROADCAST RUNNING MULTICAST MTU:2000 Metric:1
|
|
RX packets:246134 errors:0 dropped:0 overruns:0 frame:0
|
|
TX packets:5 errors:0 dropped:0 overruns:0 carrier:0
|
|
collisions:0 txqueuelen:100
|
|
|
|
if the device doesn't say up
|
|
try
|
|
/etc/rc.d/init.d/network start
|
|
( this starts the network stack & hopefully calls ifconfig tr0 up ).
|
|
ifconfig looks at the output of /proc/net/dev and presents it in a more
|
|
presentable form.
|
|
Now ping the device from a machine in the same subnet.
|
|
if the RX packets count & TX packets counts don't increment you probably
|
|
have problems.
|
|
next
|
|
cat /proc/net/arp
|
|
Do you see any hardware addresses in the cache if not you may have problems.
|
|
Next try
|
|
ping -c 5 <broadcast_addr> i.e. the Bcast field above in the output of
|
|
ifconfig. Do you see any replies from machines other than the local machine
|
|
if not you may have problems. also if the TX packets count in ifconfig
|
|
hasn't incremented either you have serious problems in your driver
|
|
(e.g. the txbusy field of the network device being stuck on )
|
|
or you may have multiple network devices connected.
|
|
|
|
|
|
chandev
|
|
-------
|
|
There is a new device layer for channel devices, some
|
|
drivers e.g. lcs are registered with this layer.
|
|
If the device uses the channel device layer you'll be
|
|
able to find what interrupts it uses & the current state
|
|
of the device.
|
|
See the manpage chandev.8 &type cat /proc/chandev for more info.
|
|
|
|
|
|
SysRq
|
|
=====
|
|
This is now supported by linux for s/390 & z/Architecture.
|
|
To enable it do compile the kernel with
|
|
Kernel Hacking -> Magic SysRq Key Enabled
|
|
echo "1" > /proc/sys/kernel/sysrq
|
|
also type
|
|
echo "8" >/proc/sys/kernel/printk
|
|
To make printk output go to console.
|
|
On 390 all commands are prefixed with
|
|
^-
|
|
e.g.
|
|
^-t will show tasks.
|
|
^-? or some unknown command will display help.
|
|
The sysrq key reading is very picky ( I have to type the keys in an
|
|
xterm session & paste them into the x3270 console )
|
|
& it may be wise to predefine the keys as described in the VM hints above
|
|
|
|
This is particularly useful for syncing disks unmounting & rebooting
|
|
if the machine gets partially hung.
|
|
|
|
Read Documentation/admin-guide/sysrq.rst for more info
|
|
|
|
References:
|
|
===========
|
|
Enterprise Systems Architecture Reference Summary
|
|
Enterprise Systems Architecture Principles of Operation
|
|
Hartmut Penners s390 stack frame sheet.
|
|
IBM Mainframe Channel Attachment a technology brief from a CISCO webpage
|
|
Various bits of man & info pages of Linux.
|
|
Linux & GDB source.
|
|
Various info & man pages.
|
|
CMS Help on tracing commands.
|
|
Linux for s/390 Elf Application Binary Interface
|
|
Linux for z/Series Elf Application Binary Interface ( Both Highly Recommended )
|
|
z/Architecture Principles of Operation SA22-7832-00
|
|
Enterprise Systems Architecture/390 Reference Summary SA22-7209-01 & the
|
|
Enterprise Systems Architecture/390 Principles of Operation SA22-7201-05
|
|
|
|
Special Thanks
|
|
==============
|
|
Special thanks to Neale Ferguson who maintains a much
|
|
prettier HTML version of this page at
|
|
http://linuxvm.org/penguinvm/
|
|
Bob Grainger Stefan Bader & others for reporting bugs
|