FROMLIST: arm64: perf: Make exporting of pmu events configurable

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.

Bug: 230559577
Bug: 427401803
Change-Id: I35dcfeed23e64ec9493f9a15dbb43e9966108664
Link: https://lore.kernel.org/lkml/1653306574-20946-1-git-send-email-quic_c_spathi@quicinc.com/
Signed-off-by: Srinivasarao Pathipati <quic_spathi@quicinc.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -1541,6 +1541,10 @@
 			Format: { "fix" }
 			Permit 'security.evm' to be updated regardless of
 			current integrity status.
+	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.
 
 	early_page_ext [KNL,EARLY] Enforces page_ext initialization to earlier
 			stages so cover more early boot allocations.

Documentation/admin-guide/sysctl/kernel.rst

@@ -286,6 +286,17 @@ domain names are in general different. For a detailed discussion
 see the ``hostname(1)`` man page.
 
 
+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.
+
+
 firmware_config
 ===============
 

drivers/perf/arm_pmuv3.c

@@ -325,6 +325,7 @@ GEN_PMU_FORMAT_ATTR(threshold_compare);
 GEN_PMU_FORMAT_ATTR(threshold);
 
 static int sysctl_perf_user_access __read_mostly;
+static int sysctl_export_pmu_events __read_mostly;
 
 static bool armv8pmu_event_is_64bit(struct perf_event *event)
 {
@@ -1053,6 +1054,17 @@ static int armv8pmu_set_event_filter(struct hw_perf_event *event,
 	return 0;
 }
 
+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;
@@ -1077,6 +1089,9 @@ 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);
 }
 
@@ -1280,6 +1295,15 @@ 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,
+	},
 };
 
 static void armv8_pmu_register_sysctl_table(void)