selftests/powerpc/pmu: Add selftest to check branch stack enablement will not...

selftests/powerpc/pmu: Add selftest to check branch stack enablement will not crash on any platforms

While enabling branch stack for an event, BHRB (Branch History
Rolling Buffer) filter is set using bhrb_filter_map() callback.
This callback is not defined for cases like generic_compat_pmu
or in case where there is no PMU registered. A fix was added
in kernel to address a crash issue observed while enabling branch
stack for environments which doesn't have this callback.
commit b460b512 ("powerpc/perf: Fix crashes with
generic_compat_pmu & BHRB").

Add perf sampling test to exercise this code path and make
sure enabling branch stack shouldn't crash in any platform.
Testcase uses software event cycles since software event is
available and can be used even in cases without PMU.
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220610134113.62991-11-atrajeev@linux.vnet.ibm.com

tools/testing/selftests/powerpc/pmu/sampling_tests/Makefile

@@ -6,7 +6,7 @@ TEST_GEN_PROGS := mmcr0_exceptionbits_test mmcr0_cc56run_test mmcr0_pmccext_test
 		   mmcr1_comb_test mmcr2_l2l3_test mmcr2_fcs_fch_test \
 		   mmcr3_src_test mmcra_thresh_marked_sample_test mmcra_thresh_cmp_test \
 		   mmcra_bhrb_ind_call_test mmcra_bhrb_any_test mmcra_bhrb_cond_test \
-		   mmcra_bhrb_disable_test
+		   mmcra_bhrb_disable_test bhrb_no_crash_wo_pmu_test
 
 top_srcdir = ../../../../../..
 include ../../../lib.mk

tools/testing/selftests/powerpc/pmu/sampling_tests/bhrb_no_crash_wo_pmu_test.c

+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2022, Athira Rajeev, IBM Corp.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "../event.h"
+#include "misc.h"
+#include "utils.h"
+
+/*
+ * A perf sampling test for making sure
+ * enabling branch stack doesn't crash in any
+ * environment, say:
+ *  - With generic compat PMU
+ *  - without any PMU registered
+ *  - With platform specific PMU
+ *  A fix for bhrb sampling crash was added in kernel
+ *  via commit: b460b512417a ("powerpc/perf: Fix crashes
+ *  with generic_compat_pmu & BHRB")
+ *
+ * This testcase exercises this code by doing branch
+ * stack enable for software event. s/w event is used
+ * since software event will work even in platform
+ * without PMU.
+ */
+static int bhrb_no_crash_wo_pmu_test(void)
+{
+	struct event event;
+
+	/*
+	 * Init the event for the sampling test.
+	 * This uses software event which works on
+	 * any platform.
+	 */
+	event_init_opts(&event, 0, PERF_TYPE_SOFTWARE, "cycles");
+
+	event.attr.sample_period = 1000;
+	event.attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
+	event.attr.disabled = 1;
+
+	/*
+	 * Return code of event_open is not
+	 * considered since test just expects no crash from
+	 * using PERF_SAMPLE_BRANCH_STACK. Also for environment
+	 * like generic compat PMU, branch stack is unsupported.
+	 */
+	event_open(&event);
+
+	event_close(&event);
+	return 0;
+}
+
+int main(void)
+{
+	return test_harness(bhrb_no_crash_wo_pmu_test, "bhrb_no_crash_wo_pmu_test");
+}