On Thu, Sep 10, 2026, Ewan Hai-oc wrote:
> From: Frank Zhu <[email protected]>
>
> KVM allows userspace to query MSR_IA32_PERF_CAPABILITIES through
> KVM_GET_MSRS even when PDCM is not exposed in KVM_GET_SUPPORTED_CPUID.
> However, vCPU accesses to the MSR fail without PDCM.
Hrm, no, KVM is supposed to allow reads and drop writes of '0' for MSRs that are
advertised to userspace, but ultimately unsupported. I'm pretty sure this is a
KVM bug. I think MSR_IA32_ARCH_CAPABILITIES is also affected.
Completely untested, but I think this?
diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
index dd3bb04878ca..fa77cfdbda7e 100644
--- a/arch/x86/kvm/msrs.c
+++ b/arch/x86/kvm/msrs.c
@@ -512,9 +512,15 @@ static int kvm_get_feature_msr(struct kvm_vcpu *vcpu, u32
index, u64 *data,
switch (index) {
case MSR_IA32_ARCH_CAPABILITIES:
+ if (!kvm_cpu_cap_has(X86_FEATURE_ARCH_CAPABILITIES))
+ return KVM_MSR_RET_UNSUPPORTED;
+
*data = kvm_get_arch_capabilities();
break;
case MSR_IA32_PERF_CAPABILITIES:
+ if (!kvm_cpu_cap_has(X86_FEATURE_PDCM))
+ return KVM_MSR_RET_UNSUPPORTED;
+
*data = kvm_caps.supported_perf_cap;
break;
case MSR_PLATFORM_INFO:
