Re: [PATCH] powerpc/64: Avoid link stack corruption in kexec_wait()

[Christophe Leroy]

Le 31/08/2021 à 08:17, Daniel Axtens a écrit :
> Hi Christophe,
>
> > Use bcl 20,31,+4 instead of bl in order to preserve link stack.
> >
> > See commit c974809a26a1 ("powerpc/vdso: Avoid link stack corruption
> > in __get_datapage()") for details.
>
>  From my understanding of that commit message, the change helps to keep
> the link stack correctly balanced which is helpful for performance,
> rather than for correctness. If I understand correctly, kexec_wait is
> not in a hot path - rather it is where CPUs spin while waiting for
> kexec. Is there any benefit in using the more complicated opcode in this
> situation?

AFAICS the main benefit is to keep things consistent over the kernel and not have to wonder "is it a 
hot path or not ? If it is I use bcl 20,31, if it is not I use bl". The best way to keep things in 
order is to always use the right instruction.

> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> >   arch/powerpc/kernel/misc_64.S | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/kernel/misc_64.S b/arch/powerpc/kernel/misc_64.S
> > index 4b761a18a74d..613509907166 100644
> > --- a/arch/powerpc/kernel/misc_64.S
> > +++ b/arch/powerpc/kernel/misc_64.S
> > @@ -255,7 +255,7 @@ _GLOBAL(scom970_write)
> >    * Physical (hardware) cpu id should be in r3.
> >    */
> >   _GLOBAL(kexec_wait)
> > -	bl	1f
> > +	bcl	20,31,1f
> >   1:	mflr	r5
>
> Would it be better to create a macro of some sort to wrap this unusual
> special form so that the meaning is more clear?

Not sure, I think people working with assembly will easily recognise that form whereas an obscure 
macro is always puzzling.

I like macros when they allow you to not repeat again and again the same sequence of several 
instructions, but here it is a single quite simple instruction which is not worth a macro in my mind.

Christophe