[RFC PATCH 2/2] powerpc/64s: system call support for scv/rfscv instructions

[Nicholas Piggin]

Excerpts from Matheus Castanho's message of May 14, 2020 6:55 am:
> Hi Nicholas,
> 
> Small comment below:
> 
> On 4/30/20 1:02 AM, Nicholas Piggin wrote:
>> Add support for the scv instruction on POWER9 and later CPUs.
>> 
>> For now this implements the zeroth scv vector 'scv 0', as identical
>> to 'sc' system calls, with the exception that lr is not preserved, and
>> it is 64-bit only. There may yet be changes made to this ABI, so it's
>> for testing only.
>> 
>> rfscv is implemented to return from scv type system calls. It can not
>> be used to return from sc system calls because those are defined to
>> preserve lr.
>> 
>> In a comparison of getpid syscall, the test program had scv taking
>> about 3 more cycles in user mode (92 vs 89 for sc), due to lr handling.
>> getpid syscall throughput on POWER9 is improved by 33%, mostly due to
>> reducing mtmsr and mtspr.
>> 
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>> ---
>>  Documentation/powerpc/syscall64-abi.rst   |  42 ++++--
> 
> [...]
> 
>> +Return value
>> +------------
>> +- For the sc instruction, 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.
>> +
>> +- For the scv 0 instruction, there is a return value indicates failure if it
>> +  is >= -MAX_ERRNO (-4095) as an unsigned comparison, in which case it is the
>> +  negated return error code. Otherwise it is the successful return value.
> 
> I believe this last paragraph is a bit confusing (didn't quite get the
> unsigned comparison with negative values). So instead of cr0.SO to
> indicate failure, scv returns the negated error code, and positive in
> case of success?

Yes, it will be like other major architectures and return values from
-4095..-1 indicate an error with error value equal to -return value.

I will try to make it a bit clearer.

Thanks,
Nick