linux-kernel - [RFC V2 8/8] arm64/perf: Enable branch stack sampling

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Message-Id: <20220412115455.293119-9-anshuman.khandual@arm.com>
Date:   Tue, 12 Apr 2022 17:24:55 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        James Clark <james.clark@....com>,
        Rob Herring <robh@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-perf-users@...r.kernel.org
Subject: [RFC V2 8/8] arm64/perf: Enable branch stack sampling

Now that all the required pieces are already in place, just enable the perf
branch stack sampling support on arm64 platform, by removing the gate which
blocks it in armpmu_event_init().

Cc: Mark Rutland <mark.rutland@....com>
Cc: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: linux-kernel@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
---
 drivers/perf/arm_pmu.c | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
index ab187a3bcdab..8b08811f662a 100644
--- a/drivers/perf/arm_pmu.c
+++ b/drivers/perf/arm_pmu.c
@@ -539,9 +539,35 @@ static int armpmu_event_init(struct perf_event *event)
 		!cpumask_test_cpu(event->cpu, &armpmu->supported_cpus))
 		return -ENOENT;
 
-	/* does not support taken branch sampling */
-	if (has_branch_stack(event))
-		return -EOPNOTSUPP;
+	if (has_branch_stack(event)) {
+		/*
+		 * BRBE support is absent. Select CONFIG_ARM_BRBE_PMU
+		 * in the config, before branch stack sampling events
+		 * can be requested.
+		 */
+		if (!IS_ENABLED(CONFIG_ARM_BRBE_PMU)) {
+			pr_warn_once("BRBE is disabled, select CONFIG_ARM_BRBE_PMU\n");
+			return -EOPNOTSUPP;
+		}
+
+		if (event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_KERNEL) {
+			if (!perfmon_capable()) {
+				pr_warn_once("does not have permission for kernel branch filter\n");
+				return -EPERM;
+			}
+		}
+
+		/*
+		 * Branch stack sampling event can not be supported in
+		 * case either the required driver itself is absent or
+		 * BRBE buffer, is not supported. Besides checking for
+		 * the callback prevents a crash in case it's absent.
+		 */
+		if (!armpmu->brbe_supported || !armpmu->brbe_supported(event)) {
+			pr_warn_once("BRBE is not supported\n");
+			return -EOPNOTSUPP;
+		}
+	}
 
 	if (armpmu->map_event(event) == -ENOENT)
 		return -ENOENT;
-- 
2.25.1