It's incorrect to assume that LBR can/should only be used with sampling events. BPF subsystem provides bpf_get_branch_snapshot() BPF helper, which expects a properly setup and activated perf event which allows kernel to capture LBR data. For instance, retsnoop tool ([0]) makes an extensive use of this functionality and sets up perf event as follows: struct perf_event_attr attr; memset(&attr, 0, sizeof(attr)); attr.size = sizeof(attr); attr.type = PERF_TYPE_HARDWARE; attr.config = PERF_COUNT_HW_CPU_CYCLES; attr.sample_type = PERF_SAMPLE_BRANCH_STACK; attr.branch_sample_type = PERF_SAMPLE_BRANCH_KERNEL; Commit referenced in Fixes tag broke this setup by making invalid assumption that LBR is useful only for sampling events. Remove that assumption. Note, earlier we removed a similar assumption on AMD side of LBR support, see [1] for details. [0] https://github.com/anakryiko/retsnoop [1] 9794563d4d05 ("perf/x86/amd: Don't reject non-sampling events with configured LBR") Cc: stable@xxxxxxxxxxxxxxx # 6.8+ Fixes: 85846b27072d ("perf/x86: Add PERF_X86_EVENT_NEEDS_BRANCH_STACK flag") Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> --- arch/x86/events/intel/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c index 9e519d8a810a..f82a342b8852 100644 --- a/arch/x86/events/intel/core.c +++ b/arch/x86/events/intel/core.c @@ -3972,7 +3972,7 @@ static int intel_pmu_hw_config(struct perf_event *event) x86_pmu.pebs_aliases(event); } - if (needs_branch_stack(event) && is_sampling_event(event)) + if (needs_branch_stack(event)) event->hw.flags |= PERF_X86_EVENT_NEEDS_BRANCH_STACK; if (branch_sample_counters(event)) { -- 2.43.5
