Messages in this thread Patch in this message | | | Date | Thu, 4 Aug 2022 15:52:30 +0800 | Subject | Re: [PATCH v2 7/7] KVM: VMX: Simplify capability check when handling PERF_CAPABILITIES write | From | Like Xu <> |
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On 4/8/2022 3:26 am, Sean Christopherson wrote: > Explicitly check for the absence of host support for LBRs or PEBS when > userspace attempts to enable said features by writing PERF_CAPABILITIES. > Comparing host support against the incoming value is unnecessary and > weird since the checks are buried inside an if-statement that verifies > userspace wants to enable the feature.
If you mean this part in the KVM:
case MSR_IA32_PERF_CAPABILITIES: { ... if (data & ~msr_ent.data) return 1; ...
then this patch brings a flaw, for example:
a user space can successfully set 0x1 on a host that reports a value of 0x5, which should not happen since the semantics of 0x1 and 0x5 for LBR_FMT may be completely different from the guest LBR driver's perspective.
For such a model-specific feature, it needs to write to PERF_CAPABILITIES the exact value reported by the host/kvm.
A selftest is proposed in the hope of guarding this contract.
> > No functional change intended. > > Signed-off-by: Sean Christopherson <seanjc@google.com> > --- > arch/x86/kvm/vmx/vmx.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index d7f8331d6f7e..0ada0ee234b7 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -2323,15 +2323,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info) > if (data && !vcpu_to_pmu(vcpu)->version) > return 1; > if (data & PMU_CAP_LBR_FMT) { > - if ((data & PMU_CAP_LBR_FMT) != > - (vmx_get_perf_capabilities() & PMU_CAP_LBR_FMT)) > + if (!(vmx_get_perf_capabilities() & PMU_CAP_LBR_FMT)) > return 1; > if (!cpuid_model_is_consistent(vcpu)) > return 1; > } > if (data & PERF_CAP_PEBS_FORMAT) { > - if ((data & PERF_CAP_PEBS_MASK) != > - (vmx_get_perf_capabilities() & PERF_CAP_PEBS_MASK)) > + if (!(vmx_get_perf_capabilities() & PERF_CAP_PEBS_MASK)) > return 1; > if (!guest_cpuid_has(vcpu, X86_FEATURE_DS)) > return 1;
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