[PATCH v2 2/2] powerpc/perf: Return regs->nip as instruction pointer value when SIAR is 0

[Christophe Leroy]

Le 13/08/2021 à 10:24, Kajol Jain a écrit :
> Incase of random sampling, there can be scenarios where SIAR is not
> latching sample address and results in 0 value. Since current code
> directly returning the siar value, we could see multiple instruction
> pointer values as 0 in perf report.
> Patch resolves this issue by adding a ternary condition to return
> regs->nip incase SIAR is 0.
> 
> Fixes: 75382aa72f06 ("powerpc/perf: Move code to select SIAR or pt_regs
> into perf_read_regs")
> Signed-off-by: Kajol Jain <kjain at linux.ibm.com>
> ---
>   arch/powerpc/perf/core-book3s.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
> index 1b464aad29c4..aeecaaf6810f 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -2260,7 +2260,7 @@ unsigned long perf_instruction_pointer(struct pt_regs *regs)
>   		else
>   			return regs->nip;
>   	} else if (use_siar && siar_valid(regs))
> -		return siar + perf_ip_adjust(regs);
> +		return siar ? siar + perf_ip_adjust(regs) : regs->nip;

Why bother about returning SIAR at all if regs->nip is ok ? Why not just return regs->nip all the time ?

>   	else if (use_siar)
>   		return 0;		// no valid instruction pointer
>   	else
>