Commit Graph

15 Commits

Author SHA1 Message Date
Denys Vlasenko
3e1aa7cb59 x86/asm: Optimize unnecessarily wide TEST instructions
By the nature of the TEST operation, it is often possible to test
a narrower part of the operand:

    "testl $3,  mem"  ->  "testb $3, mem",
    "testq $3, %rcx"  ->  "testb $3, %cl"

This results in shorter instructions, because the TEST instruction
has no sign-entending byte-immediate forms unlike other ALU ops.

Note that this change does not create any LCP (Length-Changing Prefix)
stalls, which happen when adding a 0x66 prefix, which happens when
16-bit immediates are used, which changes such TEST instructions:

  [test_opcode] [modrm] [imm32]

to:

  [0x66] [test_opcode] [modrm] [imm16]

where [imm16] has a *different length* now: 2 bytes instead of 4.
This confuses the decoder and slows down execution.

REX prefixes were carefully designed to almost never hit this case:
adding REX prefix does not change instruction length except MOVABS
and MOV [addr],RAX instruction.

This patch does not add instructions which would use a 0x66 prefix,
code changes in assembly are:

    -48 f7 07 01 00 00 00 	testq  $0x1,(%rdi)
    +f6 07 01             	testb  $0x1,(%rdi)
    -48 f7 c1 01 00 00 00 	test   $0x1,%rcx
    +f6 c1 01             	test   $0x1,%cl
    -48 f7 c1 02 00 00 00 	test   $0x2,%rcx
    +f6 c1 02             	test   $0x2,%cl
    -41 f7 c2 01 00 00 00 	test   $0x1,%r10d
    +41 f6 c2 01          	test   $0x1,%r10b
    -48 f7 c1 04 00 00 00 	test   $0x4,%rcx
    +f6 c1 04             	test   $0x4,%cl
    -48 f7 c1 08 00 00 00 	test   $0x8,%rcx
    +f6 c1 08             	test   $0x8,%cl

Linus further notes:

   "There are no stalls from using 8-bit instruction forms.

    Now, changing from 64-bit or 32-bit 'test' instructions to 8-bit ones
    *could* cause problems if it ends up having forwarding issues, so that
    instead of just forwarding the result, you end up having to wait for
    it to be stable in the L1 cache (or possibly the register file). The
    forwarding from the store buffer is simplest and most reliable if the
    read is done at the exact same address and the exact same size as the
    write that gets forwarded.

    But that's true only if:

     (a) the write was very recent and is still in the write queue. I'm
         not sure that's the case here anyway.

     (b) on at least most Intel microarchitectures, you have to test a
         different byte than the lowest one (so forwarding a 64-bit write
         to a 8-bit read ends up working fine, as long as the 8-bit read
         is of the low 8 bits of the written data).

    A very similar issue *might* show up for registers too, not just
    memory writes, if you use 'testb' with a high-byte register (where
    instead of forwarding the value from the original producer it needs to
    go through the register file and then shifted). But it's mainly a
    problem for store buffers.

    But afaik, the way Denys changed the test instructions, neither of the
    above issues should be true.

    The real problem for store buffer forwarding tends to be "write 8
    bits, read 32 bits". That can be really surprisingly expensive,
    because the read ends up having to wait until the write has hit the
    cacheline, and we might talk tens of cycles of latency here. But
    "write 32 bits, read the low 8 bits" *should* be fast on pretty much
    all x86 chips, afaik."

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1425675332-31576-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-03-07 11:12:43 +01:00
H. Peter Anvin
a3d7b7dddc x86, asm, cleanup: Replace open-coded control register values with symbolic
Clean up an unnecessary open-coded control register values.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/n/tip-um7za1nzf6brb17o0h4om6e3@git.kernel.org
2013-06-25 16:26:06 -07:00
Huang Ying
050438ed5a kexec, x86: Fix incorrect jump back address if not preserving context
In kexec jump support, jump back address passed to the kexeced
kernel via function calling ABI, that is, the function call
return address is the jump back entry.

Furthermore, jump back entry == 0 should be used to signal that
the jump back or preserve context is not enabled in the original
kernel.

But in the current implementation the stack position used for
function call return address is not cleared context
preservation is disabled. The patch fixes this bug.

Reported-and-tested-by: Yin Kangkai <kangkai.yin@intel.com>
Signed-off-by: Huang Ying <ying.huang@intel.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: <stable@kernel.org>
Link: http://lkml.kernel.org/r/1310607277-25029-1-git-send-email-ying.huang@intel.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2011-07-21 11:19:28 +02:00
Huang Ying
fef3a7a174 x86, kexec: fix kexec x86 coding style
Impact: Cleanup

Fix some coding style issue for kexec x86.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2009-03-10 18:13:25 -07:00
Jeremy Fitzhardinge
0341c14da4 x86: use _types.h headers in asm where available
In general, the only definitions that assembly files can use
are in _types.S headers (where available), so convert them.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
2009-02-13 11:35:01 -08:00
Huang Ying
9868ee63b8 kexec/i386: setup kexec page table in C
Impact: change the kexec bootstrap code implementation from assembly to C

This patch transforms the kexec page tables setup code from assembler
code to C code in machine_kexec_prepare. This improves readability and
reduces code line number.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-10-31 10:01:57 +01:00
Huang Ying
31498a0149 kexec/i386: remove PAGE_SIZE alignment from relocate_kernel
Impact: save kernel .text by loosening kexec page alignment

This patch removes PAGE_SIZE alignment from relocate_kernel(). Before
kexec jump patches are merged, control page is mapped to
relocate_kernel in kexec page tables, so relocate_kernel must be
PAGE_SIZE aligned. Now, control page is mapped to identity mapped
address, so relocate_kernel need not to be PAGE_SIZE aligned any
more. This can reduce a few KB from kernel text segement.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-10-31 10:01:55 +01:00
Huang Ying
fb45daa69d kexec jump: check code size in control page
Kexec/Kexec-jump require code size in control page is less than
PAGE_SIZE/2.  This patch add link-time checking for this.

ASSERT() of ld link script is used as the link-time checking mechanism.

[akpm@linux-foundation.org: build fix]
Signed-off-by: Huang Ying <ying.huang@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-08-15 08:35:42 -07:00
Huang Ying
3ab8352137 kexec jump
This patch provides an enhancement to kexec/kdump.  It implements the
following features:

- Backup/restore memory used by the original kernel before/after
  kexec.

- Save/restore CPU state before/after kexec.

The features of this patch can be used as a general method to call program in
physical mode (paging turning off).  This can be used to call BIOS code under
Linux.

kexec-tools needs to be patched to support kexec jump. The patches and
the precompiled kexec can be download from the following URL:

       source: http://khibernation.sourceforge.net/download/release_v10/kexec-tools/kexec-tools-src_git_kh10.tar.bz2
       patches: http://khibernation.sourceforge.net/download/release_v10/kexec-tools/kexec-tools-patches_git_kh10.tar.bz2
       binary: http://khibernation.sourceforge.net/download/release_v10/kexec-tools/kexec_git_kh10

Usage example of calling some physical mode code and return:

1. Compile and install patched kernel with following options selected:

CONFIG_X86_32=y
CONFIG_KEXEC=y
CONFIG_PM=y
CONFIG_KEXEC_JUMP=y

2. Build patched kexec-tool or download the pre-built one.

3. Build some physical mode executable named such as "phy_mode"

4. Boot kernel compiled in step 1.

5. Load physical mode executable with /sbin/kexec. The shell command
   line can be as follow:

   /sbin/kexec --load-preserve-context --args-none phy_mode

6. Call physical mode executable with following shell command line:

   /sbin/kexec -e

Implementation point:

To support jumping without reserving memory.  One shadow backup page (source
page) is allocated for each page used by kexeced code image (destination
page).  When do kexec_load, the image of kexeced code is loaded into source
pages, and before executing, the destination pages and the source pages are
swapped, so the contents of destination pages are backupped.  Before jumping
to the kexeced code image and after jumping back to the original kernel, the
destination pages and the source pages are swapped too.

C ABI (calling convention) is used as communication protocol between
kernel and called code.

A flag named KEXEC_PRESERVE_CONTEXT for sys_kexec_load is added to
indicate that the loaded kernel image is used for jumping back.

Now, only the i386 architecture is supported.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Nigel Cunningham <nigel@nigel.suspend2.net>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-07-26 12:00:04 -07:00
gorcunov@gmail.com
366932deb3 x86: relocate_kernel - use predefined macroses for page attributes
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-04-17 17:41:29 +02:00
gorcunov@gmail.com
fd3af53122 x86: relocate_kernel - use predefined macroses for processor state
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-04-17 17:41:29 +02:00
gorcunov@gmail.com
a7bba17bf0 x86: relocate_kernel - use PAGE_SIZE instead of numeric constant
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-04-17 17:41:29 +02:00
gorcunov@gmail.com
4039ae5380 x86: relocate_kernel_32.S - clear register in more elegant way
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-04-17 17:41:29 +02:00
Cyrill Gorcunov
288621e32a x86: relocate_kernel - use predefined PAGE_SIZE instead of own alias
This patch does clean up relocate_kernel_(32|64).S a bit by getting rid
of local PAGE_ALIGNED macro. We should use well-known PAGE_SIZE instead

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-04-17 17:41:21 +02:00
Thomas Gleixner
9a163ed8e0 i386: move kernel
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
2007-10-11 11:17:01 +02:00