On Wed, Jan 5, 2022 at 2:22 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Jan 05, 2022, Yang Zhong wrote:
> > @@ -6399,6 +6424,26 @@ static void handle_interrupt_nmi_irqoff(struct kvm_vcpu *vcpu,
> > kvm_after_interrupt(vcpu);
> > }
> >
> > +static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu)
> > +{
> > + /*
> > + * Save xfd_err to guest_fpu before interrupt is enabled, so the
> > + * MSR value is not clobbered by the host activity before the guest
> > + * has chance to consume it.
> > + *
> > + * We should not blindly read xfd_err here, since this exception
>
> Nit, avoid "we", and explain what KVM does (or doesn't) do, not what KVM "should"
> do, e.g. just say
>
> * Do not blindly read ...
>
> > + * might be caused by L1 interception on a platform which doesn't
> > + * support xfd at all.
> > + *
> > + * Do it conditionally upon guest_fpu::xfd. xfd_err matters
> > + * only when xfd contains a non-zero value.
> > + *
> > + * Queuing exception is done in vmx_handle_exit. See comment there.
>
> Another nit, it's worth explaining why XFD_ERR needs to be read here regardless
> of is_guest_mode(). E.g.
>
> * Injecting the #NM back into the guest is handled in the standard path
> * as an #NM in L2 may be reflected into L1 as a VM-Exit. Read XFD_ERR
> * even if the #NM is from L2, as L1 may have exposed XFD to L2.
Do we have tests of L1 passing through XFD to L2?
