Messages in this thread | | | From | Daniel Axtens <> | Subject | Re: [PATCH] powerpc/64: Avoid link stack corruption in kexec_wait() | Date | Tue, 31 Aug 2021 16:17:52 +1000 |
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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
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