[RFC PATCH v2 11/13] powerpc/syscall: Avoid stack frame in likely part of syscall_call_exception()

[Christophe Leroy]

Le 06/04/2020 à 03:29, Nicholas Piggin a écrit :
> Christophe Leroy's on April 6, 2020 3:44 am:
>> When r3 is not modified, reload it from regs->orig_r3 to free
>> volatile registers. This avoids a stack frame for the likely part
>> of syscall_call_exception()
>>
>> Before : 353 cycles on null_syscall
>> After  : 347 cycles on null_syscall
>>

[...]

>>
>> Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
>> ---
>>   arch/powerpc/kernel/syscall.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
>> index 69d75fc4a5eb..630c423e089a 100644
>> --- a/arch/powerpc/kernel/syscall.c
>> +++ b/arch/powerpc/kernel/syscall.c
>> @@ -91,6 +91,8 @@ notrace long system_call_exception(long r3, long r4, long r5,
>>   
>>   	} else if (unlikely(r0 >= NR_syscalls)) {
>>   		return -ENOSYS;
>> +	} else {
>> +		r3 = regs->orig_gpr3;
>>   	}
> 
> So this just gives enough volatiles to avoid spilling to stack? I wonder
> about other various options here if they would cause a spill anyway.
> 
> Interesting optimisation, it would definitely need a comment. Would be
> nice if we had a way to tell the compiler that a local can be reloaded
> from a particular address.

Ok, comment added.

Christophe