[PATCH] perf stat: fix case where guest/host monitoring is not supported by kernel

[Stephane Eranian <eranian@google.com>]

By default, perf stat sets exclude_guest = 1. But when you run perf on a kernel
which does not support  host/guest filtering, then you get an error saying
the event in unsupported. This comes from the fact that when the
perf_event_attr struct passed by the user is larger than the one known to 
the kernel there is safety check which ensures that all unknown bits are
zero. But here, exclude_guest is 1 (part of the unknown bits) and thus the
perf_event_open() syscall return EINVAL.

To my surprise, running perf record on the same kernel did not exhibit
the problem. The reason is that perf record handles the problem by
catching the error and retrying with guest/host excludes set to zero.
For some reason, this was not done with perf stat. This patch fixes
this problem.

Signed-off-by: Stephane Eranian <eranian@google.com>
---
diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index dde9e17..6e1c1f9 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -283,6 +283,8 @@ static int create_perf_stat_counter(struct perf_evsel *evsel,
 {
 	struct perf_event_attr *attr = &evsel->attr;
 	struct xyarray *group_fd = NULL;
+	bool exclude_guest_missing = false;
+	int ret;
 
 	if (group && evsel != first)
 		group_fd = first->fd;
@@ -293,16 +295,39 @@ static int create_perf_stat_counter(struct perf_evsel *evsel,
 
 	attr->inherit = !no_inherit;
 
-	if (system_wide)
-		return perf_evsel__open_per_cpu(evsel, evsel_list->cpus,
+retry:
+	if (exclude_guest_missing)
+		evsel->attr.exclude_guest = evsel->attr.exclude_host = 0;
+
+	if (system_wide) {
+		ret = perf_evsel__open_per_cpu(evsel, evsel_list->cpus,
 						group, group_fd);
+		if (ret)
+			goto check_ret;
+		return 0;
+	}
+
 	if (!target_pid && !target_tid && (!group || evsel == first)) {
 		attr->disabled = 1;
 		attr->enable_on_exec = 1;
 	}
 
-	return perf_evsel__open_per_thread(evsel, evsel_list->threads,
-					   group, group_fd);
+	ret = perf_evsel__open_per_thread(evsel, evsel_list->threads,
+					  group, group_fd);
+	if (!ret)
+		return 0;
+	/* fall through */
+check_ret:
+	if (ret && errno == EINVAL) {
+		if (!exclude_guest_missing &&
+		    (evsel->attr.exclude_guest || evsel->attr.exclude_host)) {
+			pr_debug("Old kernel, cannot exclude "
+				 "guest or host samples.\n");
+			exclude_guest_missing = true;
+			goto retry;
+		}
+	}
+	return ret;
 }
 
 /*