From: Al Grant <redacted>

Commit 42bbabed09ce ("perf tools: Add hw_idx in struct branch_stack")
changed the format of branch stacks in perf samples. When samples use
this new format, a flag must be set in the corresponding event.
Synthesized branch stacks generated from CoreSight ETM trace were using
the new format, but not setting the event attribute, leading to
consumers seeing corrupt data. This patch fixes the issue by setting the
event attribute to indicate use of the new format.

Fixes: 42bbabed09ce ("perf tools: Add hw_idx in struct branch_stack")
Signed-off-by: Al Grant <redacted>
Reviewed-by: Andrea Brunato <redacted>
Signed-off-by: Leo Yan <redacted>
---
 tools/perf/util/cs-etm.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index c283223fb31f..a2a369e2fbb6 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c

@@ -1344,8 +1344,15 @@ static int cs_etm__synth_events(struct cs_etm_auxtrace *etm,
 		attr.sample_type &= ~(u64)PERF_SAMPLE_ADDR;
 	}
 
-	if (etm->synth_opts.last_branch)
+	if (etm->synth_opts.last_branch) {
 		attr.sample_type |= PERF_SAMPLE_BRANCH_STACK;
+		/*
+		 * We don't use the hardware index, but the sample generation
+		 * code uses the new format branch_stack with this field,
+		 * so the event attributes must indicate that it's present.
+		 */
+		attr.branch_sample_type |= PERF_SAMPLE_BRANCH_HW_INDEX;
+	}