[RFC PATCH] perf/arm64: Add BRBE support for bpf_get_branch_snapshot()

[Puranjay Mohan <puranjay@kernel.org>]

Enable the bpf_get_branch_snapshot() BPF helper on ARM64 by implementing
the perf_snapshot_branch_stack static call for ARM's Branch Record
Buffer Extension (BRBE).

The BPF helper bpf_get_branch_snapshot() allows BPF programs to capture
hardware branch records on-demand. This was previously only available on
x86 (Intel LBR, AMD BRS) but not on ARM64 despite BRBE being available
since ARMv9.

This implementation:

- Follows the x86 snapshot pattern (intel_pmu_snapshot_branch_stack)
- Performs atomic snapshot by pausing BRBE, reading records, and
  restoring previous state without disrupting ongoing perf events
- Reads branch records directly from BRBE registers without
  event-specific filtering to minimize branch pollution
- Handles all BRBE record types (complete, source-only, target-only)
- Complies with ARM ARM synchronization requirements (ISB barriers per
  rule PPBZP)
- Reuses existing BRBE infrastructure (select_brbe_bank,
  __read_brbe_regset, brbe_set_perf_entry_type, etc.)

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---

This patch is only compile tested as I don't have access to hardware with BRBE.

---
 drivers/perf/arm_brbe.c  | 95 ++++++++++++++++++++++++++++++++++++++++
 drivers/perf/arm_brbe.h  |  9 ++++
 drivers/perf/arm_pmuv3.c |  5 ++-
 3 files changed, 108 insertions(+), 1 deletion(-)

diff --git a/drivers/perf/arm_brbe.c b/drivers/perf/arm_brbe.c
index ba554e0c846c..cda7bf522c06 100644
--- a/drivers/perf/arm_brbe.c
+++ b/drivers/perf/arm_brbe.c
@@ -803,3 +803,98 @@ void brbe_read_filtered_entries(struct perf_branch_stack *branch_stack,
 done:
 	branch_stack->nr = nr_filtered;
 }
+
+/*
+ * ARM-specific callback invoked through perf_snapshot_branch_stack static
+ * call, defined in include/linux/perf_event.h. See its definition for API
+ * details. It's up to caller to provide enough space in *entries* to fit all
+ * branch records, otherwise returned result will be truncated to *cnt* entries.
+ *
+ * This is similar to brbe_read_filtered_entries but optimized for snapshot mode:
+ * - No filtering based on event attributes (captures everything)
+ * - Minimal branches to avoid polluting the branch buffer
+ * - Direct register reads without event-specific processing
+ */
+int arm_brbe_snapshot_branch_stack(struct perf_branch_entry *entries, unsigned int cnt)
+{
+	unsigned long flags;
+	int nr_hw, nr_banks, nr_copied = 0;
+	u64 brbidr, brbfcr, brbcr;
+
+	/*
+	 * The sequence of steps to freeze BRBE should be completely inlined
+	 * and contain no branches to minimize contamination of branch snapshot.
+	 */
+	local_irq_save(flags);
+
+	/* Save current BRBE configuration */
+	brbfcr = read_sysreg_s(SYS_BRBFCR_EL1);
+	brbcr = read_sysreg_s(SYS_BRBCR_EL1);
+
+	/* Pause BRBE to freeze the buffer */
+	write_sysreg_s(brbfcr | BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1);
+	isb();
+
+	/* Read BRBIDR to determine number of records */
+	brbidr = read_sysreg_s(SYS_BRBIDR0_EL1);
+	if (!valid_brbidr(brbidr))
+		goto out_restore;
+
+	nr_hw = FIELD_GET(BRBIDR0_EL1_NUMREC_MASK, brbidr);
+	nr_banks = DIV_ROUND_UP(nr_hw, BRBE_BANK_MAX_ENTRIES);
+
+	/* Read branch records from BRBE banks */
+	for (int bank = 0; bank < nr_banks; bank++) {
+		int nr_remaining = nr_hw - (bank * BRBE_BANK_MAX_ENTRIES);
+		int nr_this_bank = min(nr_remaining, BRBE_BANK_MAX_ENTRIES);
+
+		select_brbe_bank(bank);
+
+		for (int i = 0; i < nr_this_bank; i++) {
+			struct brbe_regset bregs;
+			struct perf_branch_entry *entry;
+
+			if (nr_copied >= cnt)
+				goto out_restore;
+
+			if (!__read_brbe_regset(&bregs, i))
+				goto out_restore;
+
+			entry = &entries[nr_copied];
+			perf_clear_branch_entry_bitfields(entry);
+
+			/* Simple conversion without filtering */
+			if (brbe_record_is_complete(bregs.brbinf)) {
+				entry->from = bregs.brbsrc;
+				entry->to = bregs.brbtgt;
+			} else if (brbe_record_is_source_only(bregs.brbinf)) {
+				entry->from = bregs.brbsrc;
+				entry->to = 0;
+			} else if (brbe_record_is_target_only(bregs.brbinf)) {
+				entry->from = 0;
+				entry->to = bregs.brbtgt;
+			}
+
+			brbe_set_perf_entry_type(entry, bregs.brbinf);
+			entry->cycles = brbinf_get_cycles(bregs.brbinf);
+
+			if (!brbe_record_is_target_only(bregs.brbinf)) {
+				entry->mispred = brbinf_get_mispredict(bregs.brbinf);
+				entry->predicted = !entry->mispred;
+			}
+
+			if (!brbe_record_is_source_only(bregs.brbinf))
+				entry->priv = brbinf_get_perf_priv(bregs.brbinf);
+
+			nr_copied++;
+		}
+	}
+
+out_restore:
+	/* Restore BRBE to its previous state */
+	write_sysreg_s(brbcr, SYS_BRBCR_EL1);
+	isb();
+	write_sysreg_s(brbfcr, SYS_BRBFCR_EL1);
+	local_irq_restore(flags);
+	return nr_copied;
+}
diff --git a/drivers/perf/arm_brbe.h b/drivers/perf/arm_brbe.h
index b7c7d8796c86..c2a1824437fb 100644
--- a/drivers/perf/arm_brbe.h
+++ b/drivers/perf/arm_brbe.h
@@ -10,6 +10,7 @@
 struct arm_pmu;
 struct perf_branch_stack;
 struct perf_event;
+struct perf_branch_entry;
 
 #ifdef CONFIG_ARM64_BRBE
 void brbe_probe(struct arm_pmu *arm_pmu);
@@ -22,6 +23,8 @@ void brbe_disable(void);
 bool brbe_branch_attr_valid(struct perf_event *event);
 void brbe_read_filtered_entries(struct perf_branch_stack *branch_stack,
 				const struct perf_event *event);
+int arm_brbe_snapshot_branch_stack(struct perf_branch_entry *entries,
+				   unsigned int cnt);
 #else
 static inline void brbe_probe(struct arm_pmu *arm_pmu) { }
 static inline unsigned int brbe_num_branch_records(const struct arm_pmu *armpmu)
@@ -44,4 +47,10 @@ static void brbe_read_filtered_entries(struct perf_branch_stack *branch_stack,
 				       const struct perf_event *event)
 {
 }
+
+static inline int arm_brbe_snapshot_branch_stack(struct perf_branch_entry *entries,
+						 unsigned int cnt)
+{
+	return 0;
+}
 #endif
diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
index 8014ff766cff..1a9f129a0f94 100644
--- a/drivers/perf/arm_pmuv3.c
+++ b/drivers/perf/arm_pmuv3.c
@@ -1449,8 +1449,11 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
 	cpu_pmu->set_event_filter	= armv8pmu_set_event_filter;
 
 	cpu_pmu->pmu.event_idx		= armv8pmu_user_event_idx;
-	if (brbe_num_branch_records(cpu_pmu))
+	if (brbe_num_branch_records(cpu_pmu)) {
 		cpu_pmu->pmu.sched_task		= armv8pmu_sched_task;
+		static_call_update(perf_snapshot_branch_stack,
+				   arm_brbe_snapshot_branch_stack);
+	}
 
 	cpu_pmu->name			= name;
 	cpu_pmu->map_event		= map_event;

base-commit: c286e7e9d1f1f3d90ad11c37e896f582b02d19c4
-- 
2.47.3