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, 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...

Hrm, that won't quite work though.  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.

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;
		}

		vcpu->run->exit_reason        = KVM_EXIT_HYPERCALL;
		vcpu->run->hypercall.nr       = KVM_HC_PAGE_ENC_STATUS;
		vcpu->run->hypercall.args[0]  = gpa;
		vcpu->run->hypercall.args[1]  = npages;
		vcpu->run->hypercall.args[2]  = enc;
		vcpu->run->hypercall.longmode = op_64_bit;
		vcpu->arch.complete_userspace_io = complete_hypercall_exit;

That's dancing pretty close to hypercall filtering, which I was hoping to avoid.
I guess it's not reaaaally filtering since the exit check happens after the
validity checks.

> > > (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.

		if (!(vcpu->kvm->arch.hypercall_exit_enabled & BIT_ULL(nr))) {
			ret = 0;
			break;
		}