Re: [PATCH v2 3/4] KVM: nVMX: Don't muck with allowed sec exec controls on CPUID changes

[Sean Christopherson <seanjc@google.com>]

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.