KVM: x86/pmu: Advertise PERFCTR_CORE iff the min nr of counters is met

Enable and advertise PERFCTR_CORE if and only if the minimum number of
required counters are available, i.e. if perf says there are less than six
general purpose counters.

Opportunistically, use kvm_cpu_cap_check_and_set() instead of open coding
the check for host support.
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Like Xu <likexu@tencent.com>
[sean: massage shortlog and changelog]
Link: https://lore.kernel.org/r/20230603011058.1038821-9-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/svm/svm.c

@@ -5025,9 +5025,18 @@ static __init void svm_set_cpu_caps(void)
 	    boot_cpu_has(X86_FEATURE_AMD_SSBD))
 		kvm_cpu_cap_set(X86_FEATURE_VIRT_SSBD);
 
-	/* AMD PMU PERFCTR_CORE CPUID */
-	if (enable_pmu && boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
-		kvm_cpu_cap_set(X86_FEATURE_PERFCTR_CORE);
+	if (enable_pmu) {
+		/*
+		 * Enumerate support for PERFCTR_CORE if and only if KVM has
+		 * access to enough counters to virtualize "core" support,
+		 * otherwise limit vPMU support to the legacy number of counters.
+		 */
+		if (kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS_CORE)
+			kvm_pmu_cap.num_counters_gp = min(AMD64_NUM_COUNTERS,
+							  kvm_pmu_cap.num_counters_gp);
+		else
+			kvm_cpu_cap_check_and_set(X86_FEATURE_PERFCTR_CORE);
+	}
 
 	/* CPUID 0x8000001F (SME/SEV features) */
 	sev_set_cpu_caps();