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

[Paolo Bonzini]

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?

(BTW, it's better to return a bitmask of hypercalls that will exit to 
userspace from KVM_CHECK_EXTENSION.  Userspace can still reject with 
-ENOSYS those that it doesn't know, but it's important that it knows in 
general how to handle KVM_EXIT_HYPERCALL).

Paolo