x86: events: Do not return bogus capabilities if PMU is broken

If the PMU is broken due to firmware issues, check_hw_exists() will return
false but perf_get_x86_pmu_capability() will still return data from x86_pmu.
Likewise if some of the hotplug callbacks cannot be installed the contents
of x86_pmu will not be reverted.

Handle the failure in both cases by clearing x86_pmu if init_hw_perf_events()
or reverts to software events only.
Co-developed-by: Like Xu <likexu@tencent.com>
Signed-off-by: Like Xu <likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/events/core.c

@@ -2103,14 +2103,15 @@ static int __init init_hw_perf_events(void)
 	}
 	if (err != 0) {
 		pr_cont("no PMU driver, software events only.\n");
-		return 0;
+		err = 0;
+		goto out_bad_pmu;
 	}
 
 	pmu_check_apic();
 
 	/* sanity check that the hardware exists or is emulated */
 	if (!check_hw_exists(&pmu, x86_pmu.num_counters, x86_pmu.num_counters_fixed))
-		return 0;
+		goto out_bad_pmu;
 
 	pr_cont("%s PMU driver.\n", x86_pmu.name);
 
@@ -2219,6 +2220,8 @@ static int __init init_hw_perf_events(void)
 	cpuhp_remove_state(CPUHP_AP_PERF_X86_STARTING);
 out:
 	cpuhp_remove_state(CPUHP_PERF_X86_PREPARE);
+out_bad_pmu:
+	memset(&x86_pmu, 0, sizeof(x86_pmu));
 	return err;
 }
 early_initcall(init_hw_perf_events);
@@ -2990,6 +2993,11 @@ unsigned long perf_misc_flags(struct pt_regs *regs)
 
 void perf_get_x86_pmu_capability(struct x86_pmu_capability *cap)
 {
+	if (!x86_pmu_initialized()) {
+		memset(cap, 0, sizeof(*cap));
+		return;
+	}
+
 	cap->version		= x86_pmu.version;
 	/*
 	 * KVM doesn't support the hybrid PMU yet.