[PATCH RFC V2 2/4] perf/x86/intel/pt: Add support for pause / resume

[Adrian Hunter]

Prevent tracing to start if aux_paused.

Implement support for PERF_EF_PAUSE / PERF_EF_RESUME. When aux_paused, stop
tracing. When not aux_paused, only start tracing if it isn't currently
meant to be stopped.

Signed-off-by: Adrian Hunter <adrian.hunter at intel.com>
---
 arch/x86/events/intel/pt.c | 63 ++++++++++++++++++++++++++++++++++++--
 arch/x86/events/intel/pt.h |  4 +++
 2 files changed, 64 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index 42a55794004a..692b51849d1c 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -418,6 +418,9 @@ static void pt_config_start(struct perf_event *event)
 	struct pt *pt = this_cpu_ptr(&pt_ctx);
 	u64 ctl = event->hw.config;
 
+	if (READ_ONCE(event->aux_paused))
+		return;
+
 	ctl |= RTIT_CTL_TRACEEN;
 	if (READ_ONCE(pt->vmx_on))
 		perf_aux_output_flag(&pt->handle, PERF_AUX_FLAG_PARTIAL);
@@ -534,7 +537,20 @@ static void pt_config(struct perf_event *event)
 	reg |= (event->attr.config & PT_CONFIG_MASK);
 
 	event->hw.config = reg;
+
+	/*
+	 * Allow resume before starting so as not to overwrite a value set by a
+	 * PMI.
+	 */
+	WRITE_ONCE(pt->resume_allowed, 1);
+
 	pt_config_start(event);
+
+	/*
+	 * Allow pause after starting so its pt_config_stop() doesn't race with
+	 * pt_config_start().
+	 */
+	WRITE_ONCE(pt->pause_allowed, 1);
 }
 
 static void pt_config_stop(struct perf_event *event)
@@ -1507,6 +1523,7 @@ void intel_pt_interrupt(void)
 		buf = perf_aux_output_begin(&pt->handle, event);
 		if (!buf) {
 			event->hw.state = PERF_HES_STOPPED;
+			pt->resume_allowed = 0;
 			return;
 		}
 
@@ -1515,6 +1532,7 @@ void intel_pt_interrupt(void)
 		ret = pt_buffer_reset_markers(buf, &pt->handle);
 		if (ret) {
 			perf_aux_output_end(&pt->handle, 0);
+			pt->resume_allowed = 0;
 			return;
 		}
 
@@ -1569,6 +1587,26 @@ static void pt_event_start(struct perf_event *event, int mode)
 	struct pt *pt = this_cpu_ptr(&pt_ctx);
 	struct pt_buffer *buf;
 
+	if (mode & PERF_EF_RESUME) {
+		if (READ_ONCE(pt->resume_allowed)) {
+			u64 status;
+
+			/*
+			 * Only if the trace is not active and the error and
+			 * stopped bits are clear, is it safe to start, but a
+			 * PMI might have just cleared these, so resume_allowed
+			 * must be checked again also.
+			 */
+			rdmsrl(MSR_IA32_RTIT_STATUS, status);
+			if (!(status & (RTIT_STATUS_TRIGGEREN |
+					RTIT_STATUS_ERROR |
+					RTIT_STATUS_STOPPED)) &&
+			   READ_ONCE(pt->resume_allowed))
+				pt_config_start(event);
+		}
+		return;
+	}
+
 	buf = perf_aux_output_begin(&pt->handle, event);
 	if (!buf)
 		goto fail_stop;
@@ -1597,6 +1635,16 @@ static void pt_event_stop(struct perf_event *event, int mode)
 {
 	struct pt *pt = this_cpu_ptr(&pt_ctx);
 
+	if (mode & PERF_EF_PAUSE) {
+		if (READ_ONCE(pt->pause_allowed))
+			pt_config_stop(event);
+		return;
+	}
+
+	/* Protect against racing */
+	WRITE_ONCE(pt->pause_allowed, 0);
+	WRITE_ONCE(pt->resume_allowed, 0);
+
 	/*
 	 * Protect against the PMI racing with disabling wrmsr,
 	 * see comment in intel_pt_interrupt().
@@ -1655,8 +1703,12 @@ static long pt_event_snapshot_aux(struct perf_event *event,
 	/*
 	 * Here, handle_nmi tells us if the tracing is on
 	 */
-	if (READ_ONCE(pt->handle_nmi))
+	if (READ_ONCE(pt->handle_nmi)) {
+		/* Protect against racing */
+		WRITE_ONCE(pt->pause_allowed, 0);
+		WRITE_ONCE(pt->resume_allowed, 0);
 		pt_config_stop(event);
+	}
 
 	pt_read_offset(buf);
 	pt_update_head(pt);
@@ -1673,8 +1725,11 @@ static long pt_event_snapshot_aux(struct perf_event *event,
 	 * Compiler barrier not needed as we couldn't have been
 	 * preempted by anything that touches pt->handle_nmi.
 	 */
-	if (pt->handle_nmi)
+	if (pt->handle_nmi) {
+		WRITE_ONCE(pt->resume_allowed, 1);
 		pt_config_start(event);
+		WRITE_ONCE(pt->pause_allowed, 1);
+	}
 
 	return ret;
 }
@@ -1790,7 +1845,9 @@ static __init int pt_init(void)
 	if (!intel_pt_validate_hw_cap(PT_CAP_topa_multiple_entries))
 		pt_pmu.pmu.capabilities = PERF_PMU_CAP_AUX_NO_SG;
 
-	pt_pmu.pmu.capabilities	|= PERF_PMU_CAP_EXCLUSIVE | PERF_PMU_CAP_ITRACE;
+	pt_pmu.pmu.capabilities		|= PERF_PMU_CAP_EXCLUSIVE |
+					   PERF_PMU_CAP_ITRACE |
+					   PERF_PMU_CAP_AUX_PAUSE;
 	pt_pmu.pmu.attr_groups		 = pt_attr_groups;
 	pt_pmu.pmu.task_ctx_nr		 = perf_sw_context;
 	pt_pmu.pmu.event_init		 = pt_event_init;
diff --git a/arch/x86/events/intel/pt.h b/arch/x86/events/intel/pt.h
index 96906a62aacd..b9527205e028 100644
--- a/arch/x86/events/intel/pt.h
+++ b/arch/x86/events/intel/pt.h
@@ -117,6 +117,8 @@ struct pt_filters {
  * @filters:		last configured filters
  * @handle_nmi:		do handle PT PMI on this cpu, there's an active event
  * @vmx_on:		1 if VMX is ON on this cpu
+ * @pause_allowed:	PERF_EF_PAUSE is allowed to stop tracing
+ * @resume_allowed:	PERF_EF_RESUME is allowed to start tracing
  * @output_base:	cached RTIT_OUTPUT_BASE MSR value
  * @output_mask:	cached RTIT_OUTPUT_MASK MSR value
  */
@@ -125,6 +127,8 @@ struct pt {
 	struct pt_filters	filters;
 	int			handle_nmi;
 	int			vmx_on;
+	int			pause_allowed;
+	int			resume_allowed;
 	u64			output_base;
 	u64			output_mask;
 };
-- 
2.34.1