On Wednesday, March 13, 2024 8:38 AM, Mingwei Zhang wrote: > Return correct value of IA32_PERF_CAPABILITIES when userspace tries to read > it after vCPU has already run. Previously, KVM will always return the guest > cached value on get_msr() even if guest CPUID lacks X86_FEATURE_PDCM. The > guest cached value on default is kvm_caps.supported_perf_cap. However, > when userspace sets the value during live migration, the call fails because of > the check on X86_FEATURE_PDCM. Could you point where in the set_msr path that could fail? (I don’t find there is a check of X86_FEATURE_PDCM in vmx_set_msr and kvm_set_msr_common) > > Initially, it sounds like a pure userspace issue. It is not. After vCPU has run, > KVM should faithfully return correct value to satisify legitimate requests from > userspace such as VM suspend/resume and live migrartion. In this case, KVM > should return 0 when guest cpuid lacks X86_FEATURE_PDCM. Some typos above (satisfy, migration, CPUID) Seems the description here isn’t aligned to your code below? The code below prevents userspace from reading the MSR value (not return 0 as the read value) in that case. >So fix the > problem by adding an additional check in vmx_set_msr(). > > Note that IA32_PERF_CAPABILITIES is emulated on AMD side, which is fine > because it set_msr() is guarded by kvm_caps.supported_perf_cap which is > always 0. > > Cc: Aaron Lewis <aaronlewis@xxxxxxxxxx> > Cc: Jim Mattson <jmattson@xxxxxxxxxx> > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx> > --- > arch/x86/kvm/vmx/vmx.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index > 40e3780d73ae..6d8667b56091 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -2049,6 +2049,17 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, > struct msr_data *msr_info) > msr_info->data = to_vmx(vcpu)->msr_ia32_sgxlepubkeyhash > [msr_info->index - MSR_IA32_SGXLEPUBKEYHASH0]; > break; > + case MSR_IA32_PERF_CAPABILITIES: > + /* > + * Host VMM should not get potentially invalid MSR value if > vCPU > + * has already run but guest cpuid lacks the support for the > + * MSR. > + */ > + if (msr_info->host_initiated && > + kvm_vcpu_has_run(vcpu) && > + !guest_cpuid_has(vcpu, X86_FEATURE_PDCM)) > + return 1; > + break; > case KVM_FIRST_EMULATED_VMX_MSR ... > KVM_LAST_EMULATED_VMX_MSR: > if (!guest_can_use(vcpu, X86_FEATURE_VMX)) > return 1; > > base-commit: fd89499a5151d197ba30f7b801f6d8f4646cf446 > -- > 2.44.0.291.gc1ea87d7ee-goog >
