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authorMichael Ellerman <mpe@ellerman.id.au>2020-07-23 17:43:44 +1000
committerMichael Ellerman <mpe@ellerman.id.au>2020-07-23 17:43:44 +1000
commit335aca5f65f1a39670944930131da5f2276f888f (patch)
treec8f41223681fc064f558083a2d46f712e43d8b66 /Documentation/powerpc
parent8ac9b9d61f0eceba6ce571e7527798465ae9a7c5 (diff)
parent7fa95f9adaee7e5cbb195d3359741120829e488b (diff)
Merge branch 'scv' support into next
From Nick's cover letter: Linux powerpc new system call instruction and ABI System Call Vectored (scv) ABI ============================== The scv instruction is introduced with POWER9 / ISA3, it comes with an rfscv counter-part. The benefit of these instructions is performance (trading slower SRR0/1 with faster LR/CTR registers, and entering the kernel with MSR[EE] and MSR[RI] left enabled, which can reduce MSR updates. The scv instruction has 128 levels (not enough to cover the Linux system call space). Assignment and advertisement ---------------------------- The proposal is to assign scv levels conservatively, and advertise them with HWCAP feature bits as we add support for more. Linux has not enabled FSCR[SCV] yet, so executing the scv instruction will cause the kernel to log a "SCV facility unavilable" message, and deliver a SIGILL with ILL_ILLOPC to the process. Linux has defined a HWCAP2 bit PPC_FEATURE2_SCV for SCV support, but does not set it. This change allocates the zero level ('scv 0'), advertised with PPC_FEATURE2_SCV, which will be used to provide normal Linux system calls (equivalent to 'sc'). Attempting to execute scv with other levels will cause a SIGILL to be delivered the same as before, but will not log a "SCV facility unavailable" message (because the processor facility is enabled). Calling convention ------------------ The proposal is for scv 0 to provide the standard Linux system call ABI with the following differences from sc convention[1]: - LR is to be volatile across scv calls. This is necessary because the scv instruction clobbers LR. From previous discussion, this should be possible to deal with in GCC clobbers and CFI. - cr1 and cr5-cr7 are volatile. This matches the C ABI and would allow the kernel system call exit to avoid restoring the volatile cr registers (although we probably still would anyway to avoid information leaks). - Error handling: The consensus among kernel, glibc, and musl is to move to using negative return values in r3 rather than CR0[SO]=1 to indicate error, which matches most other architectures, and is closer to a function call. Notes ----- - r0,r4-r8 are documented as volatile in the ABI, but the kernel patch as submitted currently preserves them. This is to leave room for deciding which way to go with these. Some small benefit was found by preserving them[1] but I'm not convinced it's worth deviating from the C function call ABI just for this. Release code should follow the ABI. Previous discussions: https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/208691.html https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/209268.html [1] https://github.com/torvalds/linux/blob/master/Documentation/powerpc/syscall64-abi.rst [2] https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/209263.html
Diffstat (limited to 'Documentation/powerpc')
-rw-r--r--Documentation/powerpc/syscall64-abi.rst42
1 files changed, 30 insertions, 12 deletions
diff --git a/Documentation/powerpc/syscall64-abi.rst b/Documentation/powerpc/syscall64-abi.rst
index e49f69f941b9..46caaadbb029 100644
--- a/Documentation/powerpc/syscall64-abi.rst
+++ b/Documentation/powerpc/syscall64-abi.rst
@@ -5,6 +5,15 @@ 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.
@@ -12,16 +21,23 @@ rules, with the following differences.
.. [1] Some syscalls (typically low-level management functions) may have
different calling sequences (e.g., rt_sigreturn).
-Parameters and return value
----------------------------
+Parameters
+----------
The system call number is specified in r0.
There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.
-Both a return value and a return error code are returned. cr0.SO is the return
-error code, and r3 is the return value or error code. 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 code that generally corresponds to errno.
+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
-----
@@ -34,22 +50,23 @@ Register preservation rules match the ELF ABI calling sequence with the
following differences:
=========== ============= ========================================
+--- For the sc instruction, differences with the ELF ABI ---
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)
+cr0 Volatile (cr0.SO is the return error condition.)
cr1, cr5-7 Nonvolatile
lr Nonvolatile
+
+--- For the scv 0 instruction, differences with the ELF ABI ---
+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.
-Invocation
-----------
-The syscall is performed with the sc instruction, and returns with execution
-continuing at the instruction following the sc instruction.
-
Transactional Memory
--------------------
Syscall behavior can change if the processor is in transactional or suspended
@@ -75,6 +92,7 @@ auxiliary vector.
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.
vsyscall
========