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Wed, 12 Aug 2026 15:48:31 +0000 Message-ID: <83378be0-1e89-4e31-94a4-ba083f0f7c8c@intel.com> Date: Wed, 12 Aug 2026 18:48:24 +0300 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/9] perf thread-stack: Report branch stack hw_idx as not available From: Adrian Hunter To: Amir Ayupov , , , , "Suzuki K Poulose" , James Clark , "Leo Yan" , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , "Alexander Shishkin" , Jiri Olsa , Ian Rogers , John Garry , "Will Deacon" CC: , Mike Leach , "Jonathan Corbet" , Shuah Khan , "Swapnil Sapkal" References: <20260803090640.2412336-1-aaupov@fb.com> <20260803090640.2412336-2-aaupov@fb.com> <544211a6-29c5-49ab-b9f9-543f199b9533@intel.com> Content-Language: en-US Organization: Intel Finland Oy, Registered Address: c/o Alberga Business Park, 6 krs, Bertel Jungin Aukio 5, 02600 Espoo, Business Identity Code: 0357606 - 4, Domiciled in Helsinki In-Reply-To: <544211a6-29c5-49ab-b9f9-543f199b9533@intel.com> Content-Type: text/plain; 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Zero is a valid hardware index, so consumers that >> honour PERF_SAMPLE_BRANCH_HW_INDEX see a reconstructed branch stack >> claiming to start at LBR TOS entry 0. >> >> These branch stacks are reconstructed from instruction trace and have no >> hardware index at all. Set hw_idx to -1ULL, which is the established way >> to say "not available" and matches what intel-pt and cs-etm already put >> in the branch stacks they synthesise directly. >> >> Signed-off-by: Amir Ayupov > > Fixes tag? > > Otherwise: > > Reviewed-by: Adrian Hunter On second thoughts, it seems that hw_idx is only used for stitching LBRs which is anyway disabled by default and only enabled by --stitch-lbr. Setting -1ULL will prevent has_stitched_lbr() making a match, but we can rely on the user to decide that for themselves via --stitch-lbr. So, in fact, it doesn't look like this change should be needed? > >> --- >> tools/perf/util/thread-stack.c | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/tools/perf/util/thread-stack.c b/tools/perf/util/thread-stack.c >> index c5ce741b07446..1a3dffa83bde2 100644 >> --- a/tools/perf/util/thread-stack.c >> +++ b/tools/perf/util/thread-stack.c >> @@ -624,6 +624,7 @@ void thread_stack__br_sample(struct thread *thread, int cpu, >> unsigned int nr; >> >> dst->nr = 0; >> + dst->hw_idx = -1ULL; >> >> if (!ts) >> return; >> @@ -686,6 +687,7 @@ void thread_stack__br_sample_late(struct thread *thread, int cpu, >> bool start = false; >> >> dst->nr = 0; >> + dst->hw_idx = -1ULL; >> >> if (!ts) >> return; >