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

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/04/21 01:20, Sean Christopherson wrote:
> If userspace blindly copies CPUID, but doesn't
> enable the capability, the guest will think the hypercall is supported.  The
> guest hopefully won't freak out too much on the resulting -KVM_ENOSYS, but it
> does make the CPUID flag rather useless.

Yes that's why the CPUID bit must *not* be in KVM_GET_SUPPORTED_CPUID.

> The
> guest hopefully won't freak out too much on the resulting -KVM_ENOSYS, but it
> does make the CPUID flag rather useless.
> 
> We can make it work with:
> 
> 		u64 gpa = a0, npages = a1, enc = a2;
> 
> 		if (!guest_pv_has(vcpu, KVM_FEATURE_HC_PAGE_ENC_STATUS))
> 			break;
> 
> 		if (!PAGE_ALIGNED(gpa) || !npages ||
> 		    gpa_to_gfn(gpa) + npages <= gpa_to_gfn(gpa)) {
> 			ret = -EINVAL;
> 			break;
> 		}
> 
> 		if (!vcpu->kvm->arch.hypercall_exit_enabled) {
> 			ret = 0;
> 			break;
> 		}
> 
> 		[...]

The interaction with KVM_CAP_ENFORCE_PV_FEATURE_CPUID scares me.  But 
I'll take it into account when posting v2.

>>>> (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).
>
> Speaking of bitmasks, what about also accepting a bitmask for enabling the
> capability?  (not sure if the above implies that).  E.g.

Yes, makes sense.

Paolo