KVM: x86/pmu: Don't tell userspace to save MSRs for non-existent fixed PMCs

Limit the set of MSRs for fixed PMU counters based on the number of fixed
counters actually supported by the host so that userspace doesn't waste
time saving and restoring dummy values.
Signed-off-by: Like Xu <likexu@tencent.com>
[sean: split for !enable_pmu logic, drop min(), write changelog]
Link: https://lore.kernel.org/r/20230124234905.3774678-6-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/include/asm/kvm_host.h

@@ -514,6 +514,7 @@ struct kvm_pmc {
 #define MSR_ARCH_PERFMON_PERFCTR_MAX	(MSR_ARCH_PERFMON_PERFCTR0 + KVM_INTEL_PMC_MAX_GENERIC - 1)
 #define MSR_ARCH_PERFMON_EVENTSEL_MAX	(MSR_ARCH_PERFMON_EVENTSEL0 + KVM_INTEL_PMC_MAX_GENERIC - 1)
 #define KVM_PMC_MAX_FIXED	3
+#define MSR_ARCH_PERFMON_FIXED_CTR_MAX	(MSR_ARCH_PERFMON_FIXED_CTR0 + KVM_PMC_MAX_FIXED - 1)
 #define KVM_AMD_PMC_MAX_GENERIC	6
 struct kvm_pmu {
 	unsigned nr_arch_gp_counters;

arch/x86/kvm/x86.c

@@ -7055,6 +7055,11 @@ static void kvm_probe_msr_to_save(u32 msr_index)
 		    kvm_pmu_cap.num_counters_gp)
 			return;
 		break;
+	case MSR_ARCH_PERFMON_FIXED_CTR0 ... MSR_ARCH_PERFMON_FIXED_CTR_MAX:
+		if (msr_index - MSR_ARCH_PERFMON_FIXED_CTR0 >=
+		    kvm_pmu_cap.num_counters_fixed)
+			return;
+		break;
 	case MSR_IA32_XFD:
 	case MSR_IA32_XFD_ERR:
 		if (!kvm_cpu_cap_has(X86_FEATURE_XFD))