[PATCH V5] arm64: perf: Make exporting of pmu events configurable

[Srinivasarao Pathipati <quic_c_spathi@quicinc.com>]

The PMU export bit (PMCR_EL0.X) is getting reset during pmu reset,
Make is configurable using sysctls to enable/disable at runtime.
It can also be enabled at early bootup with kernel arguments.

Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@quicinc.com>
---
Changes since V4:
	- Registering sysctls dynamically for only arm64 as suggested by Will
	- Not removed the code to configure with kernel parameters 
	  as the sysctl's kernel parameter(sysctl.kernel.export_pmu_events)
	  is not working at early bootup. pmu_reset() getting called before 
	  sysctl's kernel parameter is set.
Changes since V3:
	- export bit is now configurable with sysctl
	- enabling export bit on reset instead of retaining

Changes since V2:
	Done below changes as per Will's comments
	- enabling pmcr_x now configurable with kernel parameters and
	  by default it is disabled.
	
Changes since V1:
	- Preserving only PMCR_X bit as per Robin Murphy's comment.

---
 Documentation/admin-guide/kernel-parameters.txt |  5 +++++
 Documentation/admin-guide/sysctl/kernel.rst     |  9 +++++++++
 arch/arm64/kernel/perf_event.c                  | 24 ++++++++++++++++++++++++
 3 files changed, 38 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index de3da15..2bf1187 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5150,6 +5150,11 @@
 			Useful for devices that are detected asynchronously
 			(e.g. USB and MMC devices).
 
+	export_pmu_events
+			[KNL,ARM64] Sets the PMU export bit (PMCR_EL0.X), which enables
+			the exporting of events over an IMPLEMENTATION DEFINED PMU event
+			export bus to another device.
+
 	retain_initrd	[RAM] Keep initrd memory after extraction
 
 	rfkill.default_state=
diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index ddccd10..db42d4c 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -892,6 +892,15 @@ The default value is 0 (access disabled).
 
 See Documentation/arm64/perf.rst for more information.
 
+export_pmu_events (arm64 only)
+==============================
+Controls the PMU export bit (PMCR_EL0.X), which enables the exporting of
+events over an IMPLEMENTATION DEFINED PMU event export bus to another device.
+
+0: disables exporting of events (default).
+
+1: enables exporting of events.
+
 
 pid_max
 =======
diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index cb69ff1..d93e7c4 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -298,6 +298,7 @@ PMU_FORMAT_ATTR(long, "config1:0");
 PMU_FORMAT_ATTR(rdpmc, "config1:1");
 
 static int sysctl_perf_user_access __read_mostly;
+static int sysctl_export_pmu_events __read_mostly;
 
 static inline bool armv8pmu_event_is_64bit(struct perf_event *event)
 {
@@ -1025,6 +1026,17 @@ static int armv8pmu_filter_match(struct perf_event *event)
 	return evtype != ARMV8_PMUV3_PERFCTR_CHAIN;
 }
 
+static int __init export_pmu_events(char *str)
+{
+	/* Enable exporting of pmu events at early bootup with kernel
+	 * arguments.
+	 */
+	sysctl_export_pmu_events = 1;
+	return 0;
+}
+
+early_param("export_pmu_events", export_pmu_events);
+
 static void armv8pmu_reset(void *info)
 {
 	struct arm_pmu *cpu_pmu = (struct arm_pmu *)info;
@@ -1047,6 +1059,9 @@ static void armv8pmu_reset(void *info)
 	if (armv8pmu_has_long_event(cpu_pmu))
 		pmcr |= ARMV8_PMU_PMCR_LP;
 
+	if (sysctl_export_pmu_events)
+		pmcr |= ARMV8_PMU_PMCR_X;
+
 	armv8pmu_pmcr_write(pmcr);
 }
 
@@ -1221,6 +1236,15 @@ static struct ctl_table armv8_pmu_sysctl_table[] = {
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_ONE,
 	},
+	{
+		.procname       = "export_pmu_events",
+		.data           = &sysctl_export_pmu_events,
+		.maxlen         = sizeof(unsigned int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec_minmax,
+		.extra1         = SYSCTL_ZERO,
+		.extra2         = SYSCTL_ONE,
+	},
 	{ }
 };
 
-- 
2.7.4