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

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 20, 2021, Sean Christopherson wrote:
> On Tue, Apr 20, 2021, Ashish Kalra wrote:
> > On Tue, Apr 20, 2021 at 05:31:07PM +0000, Sean Christopherson wrote:
> > > On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> > > > +	case KVM_HC_PAGE_ENC_STATUS: {
> > > > +		u64 gpa = a0, npages = a1, enc = a2;
> > > > +
> > > > +		ret = -KVM_ENOSYS;
> > > > +		if (!vcpu->kvm->arch.hypercall_exit_enabled)
> > > 
> > > I don't follow, why does the hypercall need to be gated by a capability?  What
> > > would break if this were changed to?
> > > 
> > > 		if (!guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS))
> > > 
> > 
> > But, the above indicates host support for page_enc_status_hc, so we want
> > to ensure that host supports and has enabled support for the hypercall
> > exit, i.e., hypercall has been enabled.
> 
> I still don't see how parroting back KVM_GET_SUPPORTED_CPUID, i.e. "unintentionally"
> setting KVM_FEATURE_HC_PAGE_ENC_STATUS, would break anything.  Sure, the guest
> does unnecessary hypercalls, but they're eaten by KVM.  On the flip side, gating
> the hypercall on the capability, and especially only the capability, creates
> weird scenarios where the guest can observe KVM_FEATURE_HC_PAGE_ENC_STATUS=1
> but fail the hypercall.  Those would be fairly clearcut VMM bugs, but at the
> same time KVM is essentially going out of its way to manufacture the problem.

Doh, I was thinking of the MSR behavior, not the hypercall.  I'll respond to
Paolo's mail, I have one more hail mary idea.