[PATCH v4 06/10] perf/x86: no counter allocation support

[Wei Wang]

In some cases, an event may be created without needing a counter
allocation. For example, an lbr event may be created by the host
only to help save/restore the lbr stack on the vCPU context switching.

This patch adds a "no_counter" attr boolean to let the callers
explicitly tell the perf core that no counter is needed.

Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
---
 arch/x86/events/core.c          | 12 ++++++++++++
 include/linux/perf_event.h      |  5 +++++
 include/uapi/linux/perf_event.h |  3 ++-
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 374a197..c09f03b 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -410,6 +410,9 @@ int x86_setup_perfctr(struct perf_event *event)
 	struct hw_perf_event *hwc = &event->hw;
 	u64 config;
 
+	if (is_no_counter_event(event))
+		return 0;
+
 	if (!is_sampling_event(event)) {
 		hwc->sample_period = x86_pmu.max_period;
 		hwc->last_period = hwc->sample_period;
@@ -1209,6 +1212,12 @@ static int x86_pmu_add(struct perf_event *event, int flags)
 	hwc = &event->hw;
 
 	n0 = cpuc->n_events;
+
+	if (is_no_counter_event(event)) {
+		n = n0;
+		goto done_collect;
+	}
+
 	ret = n = collect_events(cpuc, event, false);
 	if (ret < 0)
 		goto out;
@@ -1387,6 +1396,9 @@ static void x86_pmu_del(struct perf_event *event, int flags)
 	if (cpuc->txn_flags & PERF_PMU_TXN_ADD)
 		goto do_del;
 
+	if (is_no_counter_event(event))
+		goto do_del;
+
 	/*
 	 * Not a TXN, therefore cleanup properly.
 	 */
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 53c500f..e58a997 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1009,6 +1009,11 @@ static inline bool is_sampling_event(struct perf_event *event)
 	return event->attr.sample_period != 0;
 }
 
+static inline bool is_no_counter_event(struct perf_event *event)
+{
+	return event->attr.no_counter;
+}
+
 /*
  * Return 1 for a software event, 0 for a hardware event
  */
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 9de8780..ec97a70 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -372,7 +372,8 @@ struct perf_event_attr {
 				context_switch :  1, /* context switch data */
 				write_backward :  1, /* Write ring buffer from end to beginning */
 				namespaces     :  1, /* include namespaces data */
-				__reserved_1   : 35;
+				no_counter     :  1, /* no counter allocation */
+				__reserved_1   : 34;
 
 	union {
 		__u32		wakeup_events;	  /* wakeup every n events */
-- 
2.7.4