RE: [PATCH v1 12/23] KVM: VMX: Handle FRED event data

Li, Xin3
2023-11-15T02:52:36Z

> >> >+ else if (is_nm_fault(intr_info) &&
> >> >+ vcpu->arch.guest_fpu.fpstate->xfd)
> >>
> >> does this necessarily mean the #NM is caused by XFD?
> >
> >Then the event data should be 0. Or I missed something obvious? I.e.,
> >it can be easily differentiated and we should just explicitly set it to
> >0.
>
> vcpu->arch.guest_fpu.fpstate->xfd just means the guest is enabling XFD.
> I don't think we can conclude that this #NM is caused by XFD only from this. i.e.,
> there may be some false positives.

Then we should get 0 in event data. Otherwise, it is a bug in how we deal
with IA32_XFD_ERR MSR, w/ or w/o FRED.

> >> >+ default:
> >> >+ WARN_ON(event_data != 0);
> >>
> >> I am not sure if this WARN_ON() can be triggeded by nested VMX. It is
> >> legitimate for L1 VMM to inject any event w/ an event_data.
> >>
> >> FRED spec says:
> >>
> >> Section 5.2.1 specifies the event data that FRED event delivery of
> >> certain events saves on the stack. When FRED event delivery is used
> >> for an event injected by VM entry, the event data saved is the value
> >> of the injected-event-data field in the VMCS. This value is used
> >> instead of what is specified in Section 5.2.1 and is done for __ALL__
> >> injected events using FRED event delivery
> >
> >5.2.1 Saving Information on the Regular Stack also says:
> >- For any other event, the event data are not currently defined and
> >will
> > be zero until they are.
> >
> >Or you mean something else?
>
> IIUC, L1 KVM can inject a nested exception whose vector isn't #DB, or #NM or #PF
> with a non-zero event_data to L2.

No, this is not allowed.

> If delivering the nested exception causes a VM-
> exit to L0 KVM, the assertion that event_data is always 0 for vectors other than
> #DB/#NM/#PF fails.