Re: [PATCH v2] powerpc/32: Add support for out-of-line static calls

Christophe Leroy
2022-03-11T14:57:03Z

Hi Peter,

Le 31/08/2021 à 16:00, Peter Zijlstra a écrit :
> On Tue, Aug 31, 2021 at 01:12:26PM +0000, Christophe Leroy wrote:
>
> Yes, this should work nicely!
>
> Since you have the two nop's at the end, you could frob in an
> optimization for __static_call_return0 without too much issue.
>
> Replace the two nops with (excuse my ppc asm):
>
> li r3, 0
> blr
>
> and augment arch_static_call_transform() with something like:
>
> if (func == &__static_call_return0)
> err = patch_branch(tramp, tramp+24, 0);

I just discovered that we likely have an issue with the implementation
of that RET0 static call.

Looking at System.map I have:

c0004fc0 t __static_call_return0
c0011518 t __static_call_return0
c00d8160 t __static_call_return0

So when we do:

if (func == &__static_call_return0)

It is unlikely that we'll get the expected one.

I see __static_call_return0 is defined as 'static inline' in
include/linux/static_call.h

Any reason for not having it as a single global symbol instead ?

Thanks
Christophe