selftests/bpf: Skip the second half of get_branch_snapshot in vm

VMs running on upstream 5.12+ kernel support LBR. However,
bpf_get_branch_snapshot couldn't stop the LBR before too many entries
are flushed. Skip the hit/waste test for VMs before we find a proper fix
for LBR in VM.

Fixes: 025bd7c ("selftests/bpf: Add test for bpf_get_branch_snapshot")
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211007050231.728496-1-songliubraving@fb.com

tools/testing/selftests/bpf/prog_tests/get_branch_snapshot.c

@@ -6,6 +6,30 @@
 static int *pfd_array;
 static int cpu_cnt;
 
+static bool is_hypervisor(void)
+{
+	char *line = NULL;
+	bool ret = false;
+	size_t len;
+	FILE *fp;
+
+	fp = fopen("/proc/cpuinfo", "r");
+	if (!fp)
+		return false;
+
+	while (getline(&line, &len, fp) != -1) {
+		if (!strncmp(line, "flags", 5)) {
+			if (strstr(line, "hypervisor") != NULL)
+				ret = true;
+			break;
+		}
+	}
+
+	free(line);
+	fclose(fp);
+	return ret;
+}
+
 static int create_perf_events(void)
 {
 	struct perf_event_attr attr = {0};
@@ -83,6 +107,16 @@ void test_get_branch_snapshot(void)
 		goto cleanup;
 	}
 
+	if (is_hypervisor()) {
+		/* As of today, LBR in hypervisor cannot be stopped before
+		 * too many entries are flushed. Skip the hit/waste test
+		 * for now in hypervisor until we optimize the LBR in
+		 * hypervisor.
+		 */
+		test__skip();
+		goto cleanup;
+	}
+
 	ASSERT_GT(skel->bss->test1_hits, 6, "find_looptest_in_lbr");
 
 	/* Given we stop LBR in software, we will waste a few entries.