perf/core, pt, bts: Get rid of itrace_started

I just noticed that hw.itrace_started and hw.config are aliased to the
same location. Now, the PT driver happens to use both, which works out
fine by sheer luck:

 - STORE(hw.itrace_start) is ordered before STORE(hw.config), in the
    program order, although there are no compiler barriers to ensure that,

 - to the perf_log_itrace_start() hw.itrace_start looks set at the same
   time as when it is intended to be set because both stores happen in the
   same path,

 - hw.config is never reset to zero in the PT driver.

Now, the use of hw.config by the PT driver makes more sense (it being a
HW PMU) than messing around with itrace_started, which is an awkward API
to begin with.

This patch replaces hw.itrace_started with an attach_state bit and an
API call for the PMU drivers to use to communicate the condition.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: vince@deater.net
Link: http://lkml.kernel.org/r/20170330153956.25994-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/events/intel/bts.c

@@ -268,7 +268,7 @@ static void bts_event_start(struct perf_event *event, int flags)
 	bts->ds_back.bts_absolute_maximum = cpuc->ds->bts_absolute_maximum;
 	bts->ds_back.bts_interrupt_threshold = cpuc->ds->bts_interrupt_threshold;
 
-	event->hw.itrace_started = 1;
+	perf_event_itrace_started(event);
 	event->hw.state = 0;
 
 	__bts_event_start(event);

arch/x86/events/intel/pt.c

@@ -471,8 +471,9 @@ static void pt_config(struct perf_event *event)
 	struct pt *pt = this_cpu_ptr(&pt_ctx);
 	u64 reg;
 
-	if (!event->hw.itrace_started) {
-		event->hw.itrace_started = 1;
+	/* First round: clear STATUS, in particular the PSB byte counter. */
+	if (!event->hw.config) {
+		perf_event_itrace_started(event);
 		wrmsrl(MSR_IA32_RTIT_STATUS, 0);
 	}
 

include/linux/perf_event.h

@@ -147,9 +147,6 @@ struct hw_perf_event {
 			struct list_head	cqm_groups_entry;
 			struct list_head	cqm_group_entry;
 		};
-		struct { /* itrace */
-			int			itrace_started;
-		};
 		struct { /* amd_power */
 			u64	pwr_acc;
 			u64	ptsc;
@@ -541,6 +538,7 @@ struct swevent_hlist {
 #define PERF_ATTACH_GROUP	0x02
 #define PERF_ATTACH_TASK	0x04
 #define PERF_ATTACH_TASK_DATA	0x08
+#define PERF_ATTACH_ITRACE	0x10
 
 struct perf_cgroup;
 struct ring_buffer;
@@ -864,6 +862,7 @@ extern int perf_aux_output_skip(struct perf_output_handle *handle,
 				unsigned long size);
 extern void *perf_get_aux(struct perf_output_handle *handle);
 extern void perf_aux_output_flag(struct perf_output_handle *handle, u64 flags);
+extern void perf_event_itrace_started(struct perf_event *event);
 
 extern int perf_pmu_register(struct pmu *pmu, const char *name, int type);
 extern void perf_pmu_unregister(struct pmu *pmu);

kernel/events/core.c

@@ -7301,6 +7301,11 @@ static void perf_log_throttle(struct perf_event *event, int enable)
 	perf_output_end(&handle);
 }
 
+void perf_event_itrace_started(struct perf_event *event)
+{
+	event->attach_state |= PERF_ATTACH_ITRACE;
+}
+
 static void perf_log_itrace_start(struct perf_event *event)
 {
 	struct perf_output_handle handle;
@@ -7316,7 +7321,7 @@ static void perf_log_itrace_start(struct perf_event *event)
 		event = event->parent;
 
 	if (!(event->pmu->capabilities & PERF_PMU_CAP_ITRACE) ||
-	    event->hw.itrace_started)
+	    event->attach_state & PERF_ATTACH_ITRACE)
 		return;
 
 	rec.header.type	= PERF_RECORD_ITRACE_START;