Messages in this thread | | | Date | Fri, 23 Dec 2022 18:30:20 +0100 | Subject | Re: [PATCH v2 3/4] KVM: nVMX: Don't muck with allowed sec exec controls on CPUID changes | From | Paolo Bonzini <> |
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On 12/13/22 07:23, Sean Christopherson wrote: > Don't modify the set of allowed secondary execution controls, i.e. the > virtual MSR_IA32_VMX_PROCBASED_CTLS2, in response to guest CPUID changes. > To avoid breaking old userspace that never sets the VMX MSRs, i.e. relies > on KVM to provide a consistent vCPU model, keep the existing behavior if > userspace has never written MSR_IA32_VMX_PROCBASED_CTLS2. > > KVM should not modify the VMX capabilities presented to L1 based on CPUID > as doing so may discard explicit settings provided by userspace. E.g. if > userspace does KVM_SET_MSRS => KVM_SET_CPUID and disables a feature in > the VMX MSRs but not CPUID (to prevent exposing the feature to L2), then > stuffing the VMX MSRs during KVM_SET_CPUID will expose the feature to L2 > against userspace's wishes.
The commit message doesn't explain *why* KVM_SET_CPUID would be done before KVM_SET_MSRS. The presence of certain MSRs or bits within is signaled by CPUID bits, and even though KVM is more lenient on host-initiated MSR writes it still verifies in general that the bits are valid.
For now I applied patch 1 and (with a reworded comment) patch 2. I'm not opposed to the rest, but I would like to better understand the reason for them. (If it has been reported to the mailing list, please add a "Link" trailer too).
Paolo
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