[PATCH V4] arm64: perf: Set PMCR.X of PMCR_EL0 during pmu reset

[Srinivasarao Pathipati]

Enable exporting of events over PMU event export bus by setting
PMCR.X of PMCR_EL0 during pmu reset.

As it impacts power consumption make it configurable at bootup
with kernel arguments and at runtime with sysctl.

Signed-off-by: Srinivasarao Pathipati <quic_c_spathi at quicinc.com>
---
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 |  4 ++++
 Documentation/admin-guide/sysctl/kernel.rst     |  8 ++++++++
 arch/arm64/kernel/perf_event.c                  | 15 +++++++++++++++
 include/linux/perf_event.h                      |  1 +
 kernel/sysctl.c                                 | 12 ++++++++++++
 5 files changed, 40 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index de3da15..2139b81 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5150,6 +5150,10 @@
 			Useful for devices that are detected asynchronously
 			(e.g. USB and MMC devices).
 
+	export_pmu_events
+			[KNL] Sets export bit of PMCR_EL0 to enable the exporting of
+			events over PMU event export bus.
+
 	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..8fbc3a0 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -892,6 +892,14 @@ The default value is 0 (access disabled).
 
 See Documentation/arm64/perf.rst for more information.
 
+export_pmu_events
+=================
+Controls the export bit(4th bit) of PMCR_EL0 which enables the exporting of
+events over an IMPLEMENTATION DEFINED PMU event export bus to another device.
+
+0: disables exporting of events
+
+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..271a8c6 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -34,6 +34,7 @@
 #define ARMV8_THUNDER_PERFCTR_L1I_CACHE_PREF_ACCESS		0xEC
 #define ARMV8_THUNDER_PERFCTR_L1I_CACHE_PREF_MISS		0xED
 
+int sysctl_export_pmu_events __read_mostly;
 /*
  * ARMv8 Architectural defined events, not all of these may
  * be supported on any given implementation. Unsupported events will
@@ -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)
+{
+	/* Exporting of events can be enabled at runtime with sysctl or
+	 * statically at bootup with kernel parameters.
+	 */
+	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);
 }
 
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index da75956..7790328 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1311,6 +1311,7 @@ extern void put_callchain_entry(int rctx);
 
 extern int sysctl_perf_event_max_stack;
 extern int sysctl_perf_event_max_contexts_per_stack;
+extern int sysctl_export_pmu_events;
 
 static inline int perf_callchain_store_context(struct perf_callchain_entry_ctx *ctx, u64 ip)
 {
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e52b6e3..3b751a2e 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2008,6 +2008,18 @@ static struct ctl_table kern_table[] = {
 		.extra2		= SYSCTL_ONE_THOUSAND,
 	},
 #endif
+#ifdef CONFIG_HW_PERF_EVENTS
+	{
+		.procname       = "export_pmu_events",
+		.data           = &sysctl_export_pmu_events,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec_minmax,
+		.extra1         = SYSCTL_ZERO,
+		.extra2         = SYSCTL_ONE,
+
+	},
+#endif
 	{
 		.procname	= "panic_on_warn",
 		.data		= &panic_on_warn,
-- 
2.7.4