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

Daniel Axtens
2021-08-31T06:17:57Z

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?

> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> 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?

Kind regards,
Daniel

> addi r5,r5,kexec_flag-1b
>
> --
> 2.25.0