[PATCH v1] perf pmus: Add placeholder core PMU

[Ian Rogers <irogers@google.com>]

If loading a core PMU fails, legacy hardware/cache events may segv due
to there being no PMU. Create a placeholder empty PMU for this
case. This was discussed in:
https://lore.kernel.org/lkml/20230614151625.2077-1-yangjihong1@huawei.com/

Reported-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/pmu.c  | 25 +++++++++++++++++++++++++
 tools/perf/util/pmu.h  |  1 +
 tools/perf/util/pmus.c |  7 ++++++-
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
index 8d5ecd4ff1a9..7f984a7f16ca 100644
--- a/tools/perf/util/pmu.c
+++ b/tools/perf/util/pmu.c
@@ -928,6 +928,31 @@ struct perf_pmu *perf_pmu__lookup(struct list_head *pmus, int dirfd, const char
 	return NULL;
 }
 
+/* Creates the PMU when sysfs scanning fails. */
+struct perf_pmu *perf_pmu__create_placeholder_core_pmu(struct list_head *core_pmus)
+{
+	struct perf_pmu *pmu = zalloc(sizeof(*pmu));
+
+	if (!pmu)
+		return NULL;
+
+	pmu->name = strdup("cpu");
+	if (!pmu->name) {
+		free(pmu);
+		return NULL;
+	}
+
+	pmu->is_core = true;
+	pmu->type = PERF_TYPE_RAW;
+	pmu->cpus = cpu_map__online();
+
+	INIT_LIST_HEAD(&pmu->format);
+	INIT_LIST_HEAD(&pmu->aliases);
+	INIT_LIST_HEAD(&pmu->caps);
+	list_add_tail(&pmu->list, core_pmus);
+	return pmu;
+}
+
 void perf_pmu__warn_invalid_formats(struct perf_pmu *pmu)
 {
 	struct perf_pmu_format *format;
diff --git a/tools/perf/util/pmu.h b/tools/perf/util/pmu.h
index 8807a624e918..203b92860e3c 100644
--- a/tools/perf/util/pmu.h
+++ b/tools/perf/util/pmu.h
@@ -286,6 +286,7 @@ int perf_pmu__event_source_devices_fd(void);
 int perf_pmu__pathname_fd(int dirfd, const char *pmu_name, const char *filename, int flags);
 
 struct perf_pmu *perf_pmu__lookup(struct list_head *pmus, int dirfd, const char *lookup_name);
+struct perf_pmu *perf_pmu__create_placeholder_core_pmu(struct list_head *core_pmus);
 void perf_pmu__delete(struct perf_pmu *pmu);
 
 #endif /* __PMU_H */
diff --git a/tools/perf/util/pmus.c b/tools/perf/util/pmus.c
index 0866dee3fc62..3cd9de42139e 100644
--- a/tools/perf/util/pmus.c
+++ b/tools/perf/util/pmus.c
@@ -153,7 +153,12 @@ static void pmu_read_sysfs(bool core_only)
 
 	closedir(dir);
 	if (core_only) {
-		read_sysfs_core_pmus = true;
+		if (!list_empty(&core_pmus))
+			read_sysfs_core_pmus = true;
+		else {
+			if (perf_pmu__create_placeholder_core_pmu(&core_pmus))
+				read_sysfs_core_pmus = true;
+		}
 	} else {
 		read_sysfs_core_pmus = true;
 		read_sysfs_all_pmus = true;
-- 
2.41.0.162.gfafddb0af9-goog