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

[Christophe Leroy]

Le 06/04/2020 Ã 03:29, Nicholas Piggin a ÃcritÂ:
> 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@xxxxxx>
> > ---
> >   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