[PATCH v3 bpf-next 1/3] perf: enable branch record for software events

[Song Liu]

The typical way to access branch record (e.g. Intel LBR) is via hardware
perf_event. For CPUs with FREEZE_LBRS_ON_PMI support, PMI could capture
reliable LBR. On the other hand, LBR could also be useful in non-PMI
scenario. For example, in kretprobe or bpf fexit program, LBR could
provide a lot of information on what happened with the function. Add API
to use branch record for software use.

Note that, when the software event triggers, it is necessary to stop the
branch record hardware asap. Therefore, static_call is used to remove some
branch instructions in this process.

Signed-off-by: Song Liu <songliubraving@xxxxxx>
---
 arch/x86/events/intel/core.c | 24 ++++++++++++++++++++++--
 include/linux/perf_event.h   | 24 ++++++++++++++++++++++++
 kernel/events/core.c         |  3 +++
 3 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index ac6fd2dabf6a2..d28d0e12c112c 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -2155,9 +2155,9 @@ static void __intel_pmu_disable_all(void)
 
 static void intel_pmu_disable_all(void)
 {
+	intel_pmu_lbr_disable_all();
 	__intel_pmu_disable_all();
 	intel_pmu_pebs_disable_all();
-	intel_pmu_lbr_disable_all();
 }
 
 static void __intel_pmu_enable_all(int added, bool pmi)
@@ -2186,6 +2186,20 @@ static void intel_pmu_enable_all(int added)
 	__intel_pmu_enable_all(added, false);
 }
 
+static int
+intel_pmu_snapshot_branch_stack(struct perf_branch_snapshot *br_snapshot)
+{
+	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+
+	intel_pmu_disable_all();
+	intel_pmu_lbr_read();
+	memcpy(br_snapshot->entries, cpuc->lbr_entries,
+	       sizeof(struct perf_branch_entry) * x86_pmu.lbr_nr);
+	br_snapshot->nr = x86_pmu.lbr_nr;
+	intel_pmu_enable_all(0);
+	return 0;
+}
+
 /*
  * Workaround for:
  *   Intel Errata AAK100 (model 26)
@@ -6283,9 +6297,15 @@ __init int intel_pmu_init(void)
 			x86_pmu.lbr_nr = 0;
 	}
 
-	if (x86_pmu.lbr_nr)
+	if (x86_pmu.lbr_nr) {
 		pr_cont("%d-deep LBR, ", x86_pmu.lbr_nr);
 
+		/* only support branch_stack snapshot for perfmon >= v2 */
+		if (x86_pmu.disable_all == intel_pmu_disable_all)
+			static_call_update(perf_snapshot_branch_stack,
+					   intel_pmu_snapshot_branch_stack);
+	}
+
 	intel_pmu_check_extra_regs(x86_pmu.extra_regs);
 
 	/* Support full width counters using alternative MSR range */
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index fe156a8170aa3..1f42e91668024 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -57,6 +57,7 @@ struct perf_guest_info_callbacks {
 #include <linux/cgroup.h>
 #include <linux/refcount.h>
 #include <linux/security.h>
+#include <linux/static_call.h>
 #include <asm/local.h>
 
 struct perf_callchain_entry {
@@ -1612,4 +1613,27 @@ extern void __weak arch_perf_update_userpage(struct perf_event *event,
 extern __weak u64 arch_perf_get_page_size(struct mm_struct *mm, unsigned long addr);
 #endif
 
+/*
+ * Snapshot branch stack on software events.
+ *
+ * Branch stack can be very useful in understanding software events. For
+ * example, when a long function, e.g. sys_perf_event_open, returns an
+ * errno, it is not obvious why the function failed. Branch stack could
+ * provide very helpful information in this type of scenarios.
+ *
+ * On software event, it is necessary to stop the hardware branch recorder
+ * fast. Otherwise, the hardware register/buffer will be flushed with
+ * entries af the triggering event. Therefore, static call is used to
+ * stop the hardware recorder.
+ */
+#define MAX_BRANCH_SNAPSHOT 32
+
+struct perf_branch_snapshot {
+	unsigned int nr;
+	struct perf_branch_entry entries[MAX_BRANCH_SNAPSHOT];
+};
+
+typedef int (perf_snapshot_branch_stack_t)(struct perf_branch_snapshot *);
+DECLARE_STATIC_CALL(perf_snapshot_branch_stack, perf_snapshot_branch_stack_t);
+
 #endif /* _LINUX_PERF_EVENT_H */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 011cc5069b7ba..22807864e913b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13437,3 +13437,6 @@ struct cgroup_subsys perf_event_cgrp_subsys = {
 	.threaded	= true,
 };
 #endif /* CONFIG_CGROUP_PERF */
+
+DEFINE_STATIC_CALL_RET0(perf_snapshot_branch_stack,
+			perf_snapshot_branch_stack_t);
-- 
2.30.2