Re: [PATCH v3 3/3] powerpc/perf: Fix the check for SIAR value

On 8/18/21 6:58 PM, Christophe Leroy wrote:

Le 18/08/2021 à 15:19, Kajol Jain a écrit :

quoted

Incase of random sampling, there can be scenarios where
Sample Instruction Address Register(SIAR) may not latch
to the sampled instruction and could result in
the value of 0. In these scenarios it is preferred to
return regs->nip. These corner cases are seen in the
previous generation (p9) also.

Patch adds the check for SIAR value along with use_siar and
siar_valid checks so that the function will return regs->nip
incase SIAR is zero.

Patch drops the code under PPMU_P10_DD1 flag check
which handles SIAR 0 case only for Power10 DD1.

Fixes: 2ca13a4cc56c9 ("powerpc/perf: Use regs->nip when SIAR is zero")
Signed-off-by: Kajol Jain <redacted>
---

Changelog:
- Drop adding new ternary condition to check siar value.
- Remove siar check specific for PPMU_P10_DD1 and add
   it along with common checks as suggested by Christophe Leroy
   and Michael Ellermen

  arch/powerpc/perf/core-book3s.c | 7 +------
  1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 23ec89a59893..55efbba7572b 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c

@@ -2254,12 +2254,7 @@ unsigned long perf_instruction_pointer(struct pt_regs *regs)

      bool use_siar = regs_use_siar(regs);
      unsigned long siar = mfspr(SPRN_SIAR);
  -    if (ppmu && (ppmu->flags & PPMU_P10_DD1)) {
-        if (siar)
-            return siar;
-        else
-            return regs->nip;
-    } else if (use_siar && siar_valid(regs))
+    if (use_siar && siar_valid(regs) && siar)

You can probably now do

+    if (regs_use_siar(regs) && siar_valid(regs) && siar)

and remove use_siar

Hi Christophe,
     I will update it. Thanks for pointing it.

Thanks,
Kajol Jain

quoted

          return siar + perf_ip_adjust(regs);
      else
          return regs->nip;