Re: [PATCH 2/2] KVM: nVMX: Defer SVI update to vmcs01 on EOI when L2 is active w/o VID

Sean Christopherson
2024-11-05T02:04:38Z

On Mon, Nov 04, 2024, Chao Gao wrote:
> >@@ -6873,6 +6873,23 @@ void vmx_hwapic_isr_update(struct kvm_vcpu *vcpu, int max_isr)
> > u16 status;
> > u8 old;
> >
> >+ /*
> >+ * If L2 is active, defer the SVI update until vmcs01 is loaded, as SVI
> >+ * is only relevant for if and only if Virtual Interrupt Delivery is
> >+ * enabled in vmcs12, and if VID is enabled then L2 EOIs affect L2's
> >+ * vAPIC, not L1's vAPIC. KVM must update vmcs01 on the next nested
> >+ * VM-Exit, otherwise L1 with run with a stale SVI.
> >+ */
> >+ if (is_guest_mode(vcpu)) {
> >+ /*
> >+ * KVM is supposed to forward intercepted L2 EOIs to L1 if VID
> >+ * is enabled in vmcs12; as above, the EOIs affect L2's vAPIC.
> >+ */
> >+ WARN_ON_ONCE(nested_cpu_has_vid(get_vmcs12(vcpu)));
>
> This function can be called in other scenarios, e.g., from
> kvm_apic_set_state(). Is it possible for the userspace VMM to set the APIC
> state while L2 is running and VID is enabled for L2? If so, this warning could
> be triggered by the userspace VMM.

Ugh, yeah, that's reachable. Bummer. Ooh! I think we can use the recently
added "wants_to_run" to filter out state stuffing. I.e. this

WARN_ON_ONCE(vcpu->wants_to_run &&
nested_cpu_has_vid(get_vmcs12(vcpu)));

plus an updated comment.