mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-21 11:44:01 +08:00
5665bc35c1
The sc and scv 0 system calls have different ABI conventions, and
ptracers need to know which system call type is being used if they want
to look at the syscall registers.
Document that pt_regs.trap can be used for this, and fix one in-tree user
to work with scv 0 syscalls.
Fixes: 7fa95f9ada ("powerpc/64s: system call support for scv/rfscv instructions")
Cc: stable@vger.kernel.org # v5.9+
Reported-by: "Dmitry V. Levin" <ldv@altlinux.org>
Suggested-by: "Dmitry V. Levin" <ldv@altlinux.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210520111931.2597127-1-npiggin@gmail.com
154 lines
6.4 KiB
ReStructuredText
154 lines
6.4 KiB
ReStructuredText
===============================================
|
|
Power Architecture 64-bit Linux system call ABI
|
|
===============================================
|
|
|
|
syscall
|
|
=======
|
|
|
|
Invocation
|
|
----------
|
|
The syscall is made with the sc instruction, and returns with execution
|
|
continuing at the instruction following the sc instruction.
|
|
|
|
If PPC_FEATURE2_SCV appears in the AT_HWCAP2 ELF auxiliary vector, the
|
|
scv 0 instruction is an alternative that may provide better performance,
|
|
with some differences to calling sequence.
|
|
|
|
syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI
|
|
specification C function calling sequence, including register preservation
|
|
rules, with the following differences.
|
|
|
|
.. [1] Some syscalls (typically low-level management functions) may have
|
|
different calling sequences (e.g., rt_sigreturn).
|
|
|
|
Parameters
|
|
----------
|
|
The system call number is specified in r0.
|
|
|
|
There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.
|
|
|
|
Return value
|
|
------------
|
|
- For the sc instruction, both a value and an error condition are returned.
|
|
cr0.SO is the error condition, and r3 is the return value. When cr0.SO is
|
|
clear, the syscall succeeded and r3 is the return value. When cr0.SO is set,
|
|
the syscall failed and r3 is the error value (that normally corresponds to
|
|
errno).
|
|
|
|
- For the scv 0 instruction, the return value indicates failure if it is
|
|
-4095..-1 (i.e., it is >= -MAX_ERRNO (-4095) as an unsigned comparison),
|
|
in which case the error value is the negated return value.
|
|
|
|
Stack
|
|
-----
|
|
System calls do not modify the caller's stack frame. For example, the caller's
|
|
stack frame LR and CR save fields are not used.
|
|
|
|
Register preservation rules
|
|
---------------------------
|
|
Register preservation rules match the ELF ABI calling sequence with some
|
|
differences.
|
|
|
|
For the sc instruction, the differences from the ELF ABI are as follows:
|
|
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| Register | Preservation Rules | Purpose |
|
|
+==============+====================+=========================================+
|
|
| r0 | Volatile | (System call number.) |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| r3 | Volatile | (Parameter 1, and return value.) |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| r4-r8 | Volatile | (Parameters 2-6.) |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| cr0 | Volatile | (cr0.SO is the return error condition.) |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| cr1, cr5-7 | Nonvolatile | |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| lr | Nonvolatile | |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
|
|
For the scv 0 instruction, the differences from the ELF ABI are as follows:
|
|
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| Register | Preservation Rules | Purpose |
|
|
+==============+====================+=========================================+
|
|
| r0 | Volatile | (System call number.) |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| r3 | Volatile | (Parameter 1, and return value.) |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
| r4-r8 | Volatile | (Parameters 2-6.) |
|
|
+--------------+--------------------+-----------------------------------------+
|
|
|
|
All floating point and vector data registers as well as control and status
|
|
registers are nonvolatile.
|
|
|
|
Transactional Memory
|
|
--------------------
|
|
Syscall behavior can change if the processor is in transactional or suspended
|
|
transaction state, and the syscall can affect the behavior of the transaction.
|
|
|
|
If the processor is in suspended state when a syscall is made, the syscall
|
|
will be performed as normal, and will return as normal. The syscall will be
|
|
performed in suspended state, so its side effects will be persistent according
|
|
to the usual transactional memory semantics. A syscall may or may not result
|
|
in the transaction being doomed by hardware.
|
|
|
|
If the processor is in transactional state when a syscall is made, then the
|
|
behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF
|
|
auxiliary vector.
|
|
|
|
- If present, which is the case for newer kernels, then the syscall will not
|
|
be performed and the transaction will be doomed by the kernel with the
|
|
failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR.
|
|
|
|
- If not present (older kernels), then the kernel will suspend the
|
|
transactional state and the syscall will proceed as in the case of a
|
|
suspended state syscall, and will resume the transactional state before
|
|
returning to the caller. This case is not well defined or supported, so this
|
|
behavior should not be relied upon.
|
|
|
|
scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC.
|
|
|
|
ptrace
|
|
------
|
|
When ptracing system calls (PTRACE_SYSCALL), the pt_regs.trap value contains
|
|
the system call type that can be used to distinguish between sc and scv 0
|
|
system calls, and the different register conventions can be accounted for.
|
|
|
|
If the value of (pt_regs.trap & 0xfff0) is 0xc00 then the system call was
|
|
performed with the sc instruction, if it is 0x3000 then the system call was
|
|
performed with the scv 0 instruction.
|
|
|
|
vsyscall
|
|
========
|
|
|
|
vsyscall calling sequence matches the syscall calling sequence, with the
|
|
following differences. Some vsyscalls may have different calling sequences.
|
|
|
|
Parameters and return value
|
|
---------------------------
|
|
r0 is not used as an input. The vsyscall is selected by its address.
|
|
|
|
Stack
|
|
-----
|
|
The vsyscall may or may not use the caller's stack frame save areas.
|
|
|
|
Register preservation rules
|
|
---------------------------
|
|
|
|
=========== ========
|
|
r0 Volatile
|
|
cr1, cr5-7 Volatile
|
|
lr Volatile
|
|
=========== ========
|
|
|
|
Invocation
|
|
----------
|
|
The vsyscall is performed with a branch-with-link instruction to the vsyscall
|
|
function address.
|
|
|
|
Transactional Memory
|
|
--------------------
|
|
vsyscalls will run in the same transactional state as the caller. A vsyscall
|
|
may or may not result in the transaction being doomed by hardware.
|