Re: [PATCH 2/9] perf thread-stack: Report branch stack hw_idx as not available

On 11/08/2026 18:58, Adrian Hunter wrote:

On 03/08/2026 12:06, Amir Ayupov wrote:

quoted

thread_stack__br_sample() and thread_stack__br_sample_late() fill a
caller-supplied branch_stack that is typically allocated with zalloc(),
leaving hw_idx as 0. 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 <redacted>

Fixes tag?

Otherwise:

Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>

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?

quoted

---
 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;