linux-kernel - [PATCH v2] perf/arm: adjust hwevents mappings on boot

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Message-Id: <20220324181458.3216262-1-eranian@google.com>
Date:   Thu, 24 Mar 2022 11:14:58 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, will@...nel.org, mark.rutland@....com
Subject: [PATCH v2] perf/arm: adjust hwevents mappings on boot

The mapping of perf_events generic hardware events to actual PMU events on
ARM PMUv3 may not always be correct. This is in particular true for the
PERF_COUNT_HW_BRANCH_INSTRUCTIONS event. Although the mapping points to an
architected event, it may not always be available. This can be seen with a
simple:

$ perf stat -e branches sleep 0
 Performance counter stats for 'sleep 0':

   <not supported>      branches

       0.001401081 seconds time elapsed

Yet the hardware does have an event that could be used for branches.
This patch fixes the problem by dynamically validating the generic hardware
events against the supported architected events. If a mapping is wrong it
can be replaced it with another. This is done for the event above at boot time
and the kernel will log the remapping:

armv8_pmuv3: hwevent HW_BRANCH_INSTRUCTIONS remapped  from 0xc to 0x21

And with that:

$ perf stat -e branches sleep 0

 Performance counter stats for 'sleep 0':

           166,739      branches

       0.000832163 seconds time elapsed

Signed-off-by: Stephane Eranian <eranian@...gle.com>
---
 arch/arm64/kernel/perf_event.c | 41 +++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index cab678ed6618..d438f5a46bdc 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -39,7 +39,7 @@
  * be supported on any given implementation. Unsupported events will
  * be disabled at run-time based on the PMCEID registers.
  */
-static const unsigned armv8_pmuv3_perf_map[PERF_COUNT_HW_MAX] = {
+static unsigned armv8_pmuv3_perf_map[PERF_COUNT_HW_MAX] = {
 	PERF_MAP_ALL_UNSUPPORTED,
 	[PERF_COUNT_HW_CPU_CYCLES]		= ARMV8_PMUV3_PERFCTR_CPU_CYCLES,
 	[PERF_COUNT_HW_INSTRUCTIONS]		= ARMV8_PMUV3_PERFCTR_INST_RETIRED,
@@ -1222,6 +1222,42 @@ static void armv8_pmu_register_sysctl_table(void)
 		register_sysctl("kernel", armv8_pmu_sysctl_table);
 }
 
+static void armv8pmu_fixup_perf_map(struct arm_pmu *cpu_pmu)
+{
+	int i, code;
+	unsigned *map = armv8_pmuv3_perf_map;
+
+	for (i = 0; i < PERF_COUNT_HW_MAX; i++) {
+retry:
+		code = map[i];
+		if (code == HW_OP_UNSUPPORTED)
+			continue;
+
+		if (test_bit(map[i], cpu_pmu->pmceid_bitmap))
+			continue;
+		/*
+		 * mapping does not exist,
+		 * let's see if we can fix it
+		 */
+		switch (i) {
+		case PERF_COUNT_HW_BRANCH_INSTRUCTIONS:
+			if (code == ARMV8_PMUV3_PERFCTR_PC_WRITE_RETIRED) {
+				map[i] = ARMV8_PMUV3_PERFCTR_BR_RETIRED;
+				pr_info("armv8_pmuv3: hwevent "
+					"HW_BRANCH_INSTRUCTIONS remapped "
+					" from 0x%x to 0x%x\n", code, map[i]);
+				goto retry;
+			}
+			break;
+		default:
+			pr_info("armv8_pmuv3: hwevent %d not supported\n", i);
+			map[i] = HW_OP_UNSUPPORTED;
+			break;
+		}
+
+	}
+}
+
 static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
 			  int (*map_event)(struct perf_event *event),
 			  const struct attribute_group *events,
@@ -1249,6 +1285,9 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
 
 	cpu_pmu->name			= name;
 	cpu_pmu->map_event		= map_event;
+
+	armv8pmu_fixup_perf_map(cpu_pmu);
+
 	cpu_pmu->attr_groups[ARMPMU_ATTR_GROUP_EVENTS] = events ?
 			events : &armv8_pmuv3_events_attr_group;
 	cpu_pmu->attr_groups[ARMPMU_ATTR_GROUP_FORMATS] = format ?
-- 
2.35.1.1021.g381101b075-goog