RE: [PATCH v1 05/23] KVM: VMX: Initialize FRED VM entry/exit controls in vmcs_config

Li, Xin3
2023-11-10T00:04:26Z

> > >+ if (cpu_feature_enabled(X86_FEATURE_FRED) &&
> > >+ !(_vmentry_control & VM_ENTRY_LOAD_IA32_FRED)) {
> > >+ pr_warn_once("FRED enabled but no VMX VM-Entry
> LOAD_IA32_FRED control: %x\n",
> > >+ _vmentry_control);
> >
> > Can we just hide FRED from guests like what KVM does for other
> > features which have similar dependencies? see vmx_set_cpu_caps().
>
> Both of these warnings should simply be dropped. The
> error_on_inconsistent_vmcs_config stuff is for inconsistencies within the allowed
> VMCS fields. Having a feature that is supported in bare metal but not virtualized
> is perfectly legal, if uncommon.

I deliberately keep it, at least for now, because these checks are helpful
during the development of nVMX FRED. It will be helpful for other VMMs
being developed/tested on KVM.

> What *is* needed is for KVM to refuse to virtualize FRED if the entry/exit controls
> aren't consistent. E.g. if at least one control is present, and at least one
> control is missing. I.e. KVM needs a version of vmcs_entry_exit_pairs that can
> deal with SECONDAY_VM_EXIT controls.

I agree there are better ways. But maybe after or before VMX FRED.

> I'll circle back to this when I give the
> series a proper review, which is going to be 3+ weeks.

The traffic in KVM mailing list is surprisingly high recently. So that is
totally expected.