linux-kernel - [PATCH] perf/x86: fix wrong assumption that LBR is only useful for sampling events

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Message-ID: <20240905180055.1221620-1-andrii@kernel.org>
Date: Thu,  5 Sep 2024 11:00:55 -0700
From: Andrii Nakryiko <andrii@...nel.org>
To: linux-perf-users@...r.kernel.org,
	peterz@...radead.org,
	kan.liang@...ux.intel.com
Cc: x86@...nel.org,
	mingo@...hat.com,
	linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org,
	acme@...nel.org,
	kernel-team@...a.com,
	Andrii Nakryiko <andrii@...nel.org>,
	stable@...r.kernel.org
Subject: [PATCH] perf/x86: fix wrong assumption that LBR is only useful for sampling events

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@...r.kernel.org # 6.8+
Fixes: 85846b27072d ("perf/x86: Add PERF_X86_EVENT_NEEDS_BRANCH_STACK flag")
Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
---
 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