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    SubjectRe: [PATCH v2 3/4] KVM: nVMX: Don't muck with allowed sec exec controls on CPUID changes
    On Fri, Dec 23, 2022, Paolo Bonzini wrote:
    > 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.

    I assume you mean why KVM_SET_MSRS would be done before KVM_SET_CPUID2?

    This patch is mostly paranoia, AFAIK there is no userspace that is negatively
    affected by KVM's manipulations. The only case I can think of is if userspace
    wanted to emulate dynamic CPUID updates, e.g. set an MSR filter to intercept writes
    to MISC_ENABLES to update MONITOR/MWAIT support, but that behavior isn't allowed
    since commit feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN").

    There are scenarios where userspace might do KVM_SET_MSRS before KVM_SET_CPUID,
    e.g. QEMU's reuse of a vCPU for CPU hotplug, but in those cases I would expect
    userspace to follow up with another KVM_SET_MSRS.

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