KVM: x86/pmu: Update global enable_pmu when PMU is undetected

On some virt platforms (L1 guest w/o PMU), the value of module parameter
'enable_pmu' for nested L2 guests should be updated at initialisation.

Considering that there is no concept of "architecture pmu" in AMD or Hygon
and that the versions (prior to Zen 4) are all 0, but that the theoretical
available counters are at least AMD64_NUM_COUNTERS, the utility
check_hw_exists() is reused in the initialisation call path.

Opportunistically update Intel specific comments.

Fixes: 8eeac7e999e8 ("KVM: x86/pmu: Add kvm_pmu_cap to optimize perf_get_x86_pmu_capability")
Signed-off-by: Like Xu <likexu@tencent.com>
Message-Id: <20220518170118.66263-3-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/pmu.h

@@ -159,14 +159,19 @@ extern struct x86_pmu_capability kvm_pmu_cap;
 
 static inline void kvm_init_pmu_capability(void)
 {
+	bool is_intel = boot_cpu_data.x86_vendor == X86_VENDOR_INTEL;
+
 	perf_get_x86_pmu_capability(&kvm_pmu_cap);
 
-	/*
-	 * Only support guest architectural pmu on
-	 * a host with architectural pmu.
-	 */
-	if (!kvm_pmu_cap.version)
+	 /*
+	  * For Intel, only support guest architectural pmu
+	  * on a host with architectural pmu.
+	  */
+	if ((is_intel && !kvm_pmu_cap.version) || !kvm_pmu_cap.num_counters_gp) {
 		memset(&kvm_pmu_cap, 0, sizeof(kvm_pmu_cap));
+		enable_pmu = false;
+		return;
+	}
 
 	kvm_pmu_cap.version = min(kvm_pmu_cap.version, 2);
 	kvm_pmu_cap.num_counters_fixed = min(kvm_pmu_cap.num_counters_fixed,