[PATCH V3] arm64: perf: Retain PMCR.X of PMCR_EL0 during reset

[Srinivasarao Pathipati <quic_c_spathi@quicinc.com>]

Preserve the bit PMCR.X of PMCR_EL0 during PMU reset to export
PMU events. as it could be set before PMU initialization.

Exporting events could result in additional power consumption
so making it configurable.

Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@quicinc.com>
---
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 ++++
 arch/arm64/kernel/perf_event.c                  | 13 +++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index de3da15..cf3410e 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).
 
+	retain_pmcr_x
+			[KNL] Retain export bit of PMCR_EL0 that is getting reset
+			during pmu reset.
+
 	retain_initrd	[RAM] Keep initrd memory after extraction
 
 	rfkill.default_state=
diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index cb69ff1..1ac251f 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
 
+static int 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 +1026,14 @@ static int armv8pmu_filter_match(struct perf_event *event)
 	return evtype != ARMV8_PMUV3_PERFCTR_CHAIN;
 }
 
+static int __init retain_pmcr_x(char *str)
+{
+	export_pmu_events = 1;
+	return 0;
+}
+
+early_param("retain_pmcr_x", retain_pmcr_x);
+
 static void armv8pmu_reset(void *info)
 {
 	struct arm_pmu *cpu_pmu = (struct arm_pmu *)info;
@@ -1047,6 +1056,10 @@ static void armv8pmu_reset(void *info)
 	if (armv8pmu_has_long_event(cpu_pmu))
 		pmcr |= ARMV8_PMU_PMCR_LP;
 
+	/* Preserve PMCR_X to export PMU events */
+	if (export_pmu_events)
+		pmcr |= (armv8pmu_pmcr_read() & ARMV8_PMU_PMCR_X);
+
 	armv8pmu_pmcr_write(pmcr);
 }
 
-- 
2.7.4