[Patch v5 13/18] perf/x86/intel: Refresh LBR stack for non-PEBS LBR events if needed

[Dapeng Mi]

PEBS and non-PEBS events currently share the same LBR stack,
cpuc->lbr_stack. In the PMI handler, the non-PEBS path refreshes the
LBR state before the PEBS buffer is drained.

When PEBS and non-PEBS LBR events overflow in the same PMI, draining the
PEBS buffer can overwrite the shared LBR stack with the PEBS snapshot.
This leaves stale LBR data from the PEBS record in place, and userspace
may observe the wrong branch stack instead of the live LBR state at PMI
entry.

Add a lbr_need_update flag to indicate when the LBR state must be
refreshed. If PEBS events and non-PEBS LBR events overflow in the same
PMI, set this flag so the non-PEBS LBR event refreshes the live LBR
state. Since the LBR state is frozen for the duration of PMI handling, it
only needs to be read once even when multiple non-PEBS LBR events
overflow in the same PMI.

Fixes: c22497f5838c ("perf/x86/intel: Support adaptive PEBS v4")
Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
---
 arch/x86/events/intel/core.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 110fff65bd50..37f304a1b48c 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3863,8 +3863,9 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
 {
 	struct perf_sample_data data;
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
-	int bit;
+	bool lbr_need_update = false;
 	int handled = 0;
+	int bit;
 
 	inc_perf_irq_stat();
 
@@ -3929,6 +3930,8 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
 		if (cpuc->events[INTEL_PMC_IDX_FIXED_SLOTS] &&
 		    is_pebs_counter_event_group(cpuc->events[INTEL_PMC_IDX_FIXED_SLOTS]))
 			status &= ~GLOBAL_STATUS_PERF_METRICS_OVF_BIT;
+
+		lbr_need_update = true;
 	}
 
 	/*
@@ -3944,6 +3947,8 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
 		if (cpuc->events[INTEL_PMC_IDX_FIXED_SLOTS] &&
 		    is_pebs_counter_event_group(cpuc->events[INTEL_PMC_IDX_FIXED_SLOTS]))
 			status &= ~GLOBAL_STATUS_PERF_METRICS_OVF_BIT;
+
+		lbr_need_update = true;
 	}
 
 	/*
@@ -4002,8 +4007,14 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
 		 *
 		 * The PEBS buffer has to be drained before handling the A-PMI
 		 */
-		if (is_pebs_counter_event_group(event))
+		if (is_pebs_counter_event_group(event)) {
 			static_call(x86_pmu_drain_pebs)(regs, &data);
+			/*
+			 * Draining the PEBS buffer can overwrite the shared
+			 * LBR stack, so mark lbr_need_update as true.
+			 */
+			lbr_need_update = true;
+		}
 
 		last_period = event->hw.last_period;
 
@@ -4012,8 +4023,14 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
 
 		perf_sample_data_init(&data, 0, last_period);
 
-		if (has_branch_stack(event))
+		if (has_branch_stack(event)) {
+			if (lbr_need_update) {
+				intel_pmu_lbr_read(&cpuc->lbr_stack,
+					cpuc->lbr_entries, cpuc->lbr_counters);
+				lbr_need_update = false;
+			}
 			intel_pmu_lbr_save_brstack(&data, cpuc, event);
+		}
 
 		x86_pmu_clear_perf_regs(regs);
 		x86_pmu_update_perf_regs(event, &data, regs, false);
-- 
2.34.1