Re: [PATCH 0/3] KVM: x86: guest interface for SEV live migration

[Sean Christopherson]

On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> On 20/04/21 22:16, Sean Christopherson wrote:
> > On Tue, Apr 20, 2021, Sean Christopherson wrote:
> > > On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> > > > In this particular case, if userspace sets the bit in CPUID2 but doesn't
> > > > handle KVM_EXIT_HYPERCALL, the guest will probably trigger some kind of
> > > > assertion failure as soon as it invokes the HC_PAGE_ENC_STATUS hypercall.
> > 
> > Oh!  Almost forgot my hail mary idea.  Instead of a new capability, can we
> > reject the hypercall if userspace has _not_ set KVM_CAP_ENFORCE_PV_FEATURE_CPUID?
> > 
> > 			if (vcpu->arch.pv_cpuid.enforce &&
> > 			    !guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS)
> > 				break;
> 
> Couldn't userspace enable that capability and _still_ copy the supported
> CPUID blindly to the guest CPUID, without supporting the hypercall?

Yes.  I was going to argue that we get to define the behavior, but that's not
true because it would break existing VMMs that blindly copy.  Capability it is...