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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Wed, Aug 12, 2026 at 06:48:24PM +0300, Adrian Hunter wrote:
> On 11/08/2026 18:58, Adrian Hunter wrote:
> > On 03/08/2026 12:06, 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.
> >>
> >> Signed-off-by: Amir Ayupov <aaupov@fb.com>
> > 
> > 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.

This is one of those options that few people use as its so specialized,
do you think we could auto-enable it if we notice it is a good idea for
some specific machine and request from the user? I.e. user requests
callchains, unlimited or with a limit that is more than what we can do
without stitching: we auto stich?

- Arnaldo
 
> 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;
> >