perf pmus: Add placeholder core PMU

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>
Tested-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: James Clark <james.clark@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Link: https://lore.kernel.org/r/20230627182834.117565-1-irogers@google.comSigned-off-by: Namhyung Kim <namhyung@kernel.org>

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;

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 */

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;