[PATCH 5/5] powerpc/perf: use regs->nip when siar is zero

[Madhavan Srinivasan]

On 10/22/20 6:55 AM, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy at csgroup.eu> writes:
>> Le 21/10/2020 à 10:53, Madhavan Srinivasan a écrit :
>>> In power10 DD1, there is an issue where the
>>> Sampled Instruction Address Register (SIAR)
>>> not latching to the sampled address during
>>> random sampling. This results in value of 0s
>>> in the SIAR. Patch adds a check to use regs->nip
>>> when SIAR is zero.
>> Why not use regs->nip at all time in that case, and not read SPRN_SIAR at all ?
> Yeah that's a reasonable question.
>
> I can't really find anywhere in the ISA that explains it.
>
> I believe the main (or only?) reason is that interrupts might be
> disabled when the PMU samples the instruction. So in that case the SIAR
> will point at an instruction somewhere in interrupts-off code, whereas
> the NIP will point to the location where we re-enabled interrupts and
> took the PMU interrupt.

sorry for the delayed response, was out.

thats correct and also we see SIAR zeroing only for some of
the events. We still want to use the SIAR address for
the sample creation and use nip only if it  zero.

Maddy


>
> cheers