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 19:31, Sean Christopherson 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))
> 
> The problem is that it's valid to take KVM_GET_SUPPORTED_CPUID and send it
> unmodified to KVM_SET_CPUID2.  For this reason, features that are
> conditional on other ioctls, or that require some kind of userspace support,
> must not be in KVM_GET_SUPPORTED_CPUID.  For example:
> 
> - TSC_DEADLINE because it is only implemented after KVM_CREATE_IRQCHIP (or
> after KVM_ENABLE_CAP of KVM_CAP_IRQCHIP_SPLIT)
> 
> - MONITOR only makes sense if userspace enables KVM_CAP_X86_DISABLE_EXITS
> 
> X2APIC is reported even though it shouldn't be.  Too late to fix that, I
> think.
> 
> 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.

Gah, I was thinking of the MSR behavior and forgot that the hypercall exiting
behavior intentionally doesn't require extra filtering.

It's also worth noting that guest_pv_has() is particularly useless since it
will unconditionally return true for older VMMs that dont' enable
KVM_CAP_ENFORCE_PV_FEATURE_CPUID.

Bummer.