Re: [PATCH v5 08/10] KVM: x86: nSVM: Save/restore gPAT with KVM_{GET,SET}_NESTED_STATE

[Sean Christopherson <seanjc@google.com>]

On Wed, Mar 04, 2026, Jim Mattson wrote:
> On Wed, Mar 4, 2026 at 9:11 AM Sean Christopherson <seanjc@google.com> wrote:
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index 991ee4c03363..099bf8ac10ee 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -1848,7 +1848,7 @@ static int svm_get_nested_state(struct kvm_vcpu *vcpu,
> >         if (is_guest_mode(vcpu)) {
> >                 kvm_state.hdr.svm.vmcb_pa = svm->nested.vmcb12_gpa;
> >                 if (nested_npt_enabled(svm)) {
> > -                       kvm_state.hdr.svm.flags |= KVM_STATE_SVM_VALID_GPAT;
> > +                       kvm_state->flags |= KVM_STATE_NESTED_GPAT_VALID;
> >                         kvm_state.hdr.svm.gpat = svm->vmcb->save.g_pat;
> >                 }
> >                 kvm_state.size += KVM_STATE_NESTED_SVM_VMCB_SIZE;
> > @@ -1914,7 +1914,8 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
> >
> >         if (kvm_state->flags & ~(KVM_STATE_NESTED_GUEST_MODE |
> >                                  KVM_STATE_NESTED_RUN_PENDING |
> > -                                KVM_STATE_NESTED_GIF_SET))
> > +                                KVM_STATE_NESTED_GIF_SET |
> > +                                KVM_STATE_NESTED_GPAT_VALID))
> >                 return -EINVAL;
> 
> Unless I'm missing something, this breaks forward compatibility
> completely. An older kernel will refuse to accept a nested state blob
> with GPAT_VALID set.

Argh, so we've painted ourselves into an impossible situation by restricting the
set of valid flags.  I.e. VMX's omission of checks on unknown flags is a feature,
not a bug.

Chatted with Jim offlist, and he pointed out that KVM's standard way to deal with
this is to make setting the flag opt-in, e.g. KVM_CAP_X86_TRIPLE_FAULT_EVENT and
KVM_CAP_EXCEPTION_PAYLOAD.

As much as I want to retroactively change KVM's documentation to state doing
KVM_SET_NESTED_STATE with data that didn't come from KVM_GET_NESTED_STATE is
unsupported, that feels too restrictive and could really bite us in the future.
And it doesn't help if there's already userspace that's putting garbage into the
header.

So yeah, I don't see a better option than adding yet another capability.

Can you send a new version based on `kvm-x86 next`?  (give me ~hour to drop these
and push).  This has snowballed beyond what I'm comfortable doing as fixup. :-(