lkml.org 
[lkml]   [2020]   [Apr]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC PATCH v2 11/13] powerpc/syscall: Avoid stack frame in likely part of syscall_call_exception()
From
Date


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@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

\
 
 \ /
  Last update: 2020-04-06 20:19    [W:0.075 / U:4.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site