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.
