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

[James Clark <james.clark@linaro.org>]

On 03/08/2026 10:06, Amir Ayupov wrote:
> 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.
> 

This fix makes sense in case someone doesn't overwrite it, but I think 
"matches what cs-etm already puts in the branch stacks" isn't quite 
right. Based on the hw_id docs, cs-etm should be setting 0 shouldn't it?

  * For the architectures whose raw branch records are
  * already stored in age order, the hw_idx should be 0.

-1 "unknown" isn't accurate as we do know what the order is because we 
generate them in order. If anyone is reading the field 0 is much more 
useful than -1.

Can we change cs_etm__synth_branch_sample() to 0 and update this commit 
message to not say that -1 is right for cs-etm?

> Signed-off-by: Amir Ayupov <aaupov@fb.com>
> ---
>   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;