[PATCH 45/54] perf tools: Detect avalibility of write_backward

[Wang Nan]

Detect avalibility of write_backward and save the result into
record_opts. With write_backward the start pointer of a ring
buffer mapped read only can be found reliably.

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Zefan Li <lizefan@xxxxxxxxxx>
Cc: pi3orama@xxxxxxx
---
 tools/perf/perf.h        |  1 +
 tools/perf/util/record.c | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/tools/perf/perf.h b/tools/perf/perf.h
index 90129ac..00c25b1 100644
--- a/tools/perf/perf.h
+++ b/tools/perf/perf.h
@@ -71,6 +71,7 @@ struct record_opts {
 	bool	     sample_transaction;
 	unsigned     initial_delay;
 	bool         use_clockid;
+	bool	     has_write_backward;
 	clockid_t    clockid;
 	unsigned int proc_map_timeout;
 };
diff --git a/tools/perf/util/record.c b/tools/perf/util/record.c
index 0467367..d01f155 100644
--- a/tools/perf/util/record.c
+++ b/tools/perf/util/record.c
@@ -85,6 +85,11 @@ static void perf_probe_comm_exec(struct perf_evsel *evsel)
 	evsel->attr.comm_exec = 1;
 }
 
+static void perf_probe_write_backward(struct perf_evsel *evsel)
+{
+	evsel->attr.write_backward = 1;
+}
+
 static void perf_probe_context_switch(struct perf_evsel *evsel)
 {
 	evsel->attr.context_switch = 1;
@@ -105,6 +110,11 @@ bool perf_can_record_switch_events(void)
 	return perf_probe_api(perf_probe_context_switch);
 }
 
+static bool perf_can_write_backward(void)
+{
+	return perf_probe_api(perf_probe_write_backward);
+}
+
 bool perf_can_record_cpu_wide(void)
 {
 	struct perf_event_attr attr = {
@@ -235,6 +245,7 @@ static int record_opts__config_freq(struct record_opts *opts)
 
 int record_opts__config(struct record_opts *opts)
 {
+	opts->has_write_backward = perf_can_write_backward();
 	return record_opts__config_freq(opts);
 }
 
-- 
1.8.3.4