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

Christophe Leroy [off-list ref] writes:

Le 13/08/2021 à 10:24, Kajol Jain a écrit :

quoted

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 <redacted>
---
  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 ?

Same answer as last time :)

https://lore.kernel.org/linuxppc-dev/87r1prxd9e.fsf@mpe.ellerman.id.au/ (local)

ie. SIAR can point into interrupts-off code, whereas regs->nip will
point to where we re-enabled interrupts.

cheers