Re: [PATCH v2 2/2] KVM: SVM: Don't set GIF when clearing EFER.SVME

Jim Mattson
2025-10-14T21:31:14Z

On Tue, Oct 14, 2025 at 2:18 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Oct 14, 2025, Jim Mattson wrote:
> > On Mon, Oct 13, 2025 at 3:33 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > On Thu, Oct 09, 2025, Jim Mattson wrote:
> > > > Clearing EFER.SVME is not architected to set GIF.
> > >
> > > But it's also not architected to leave GIF set when the guest is running, which
> > > was the basic gist of the Fixes commit. I suspect that forcing GIF=1 was
> > > intentional, e.g. so that the guest doesn't end up with GIF=0 after stuffing the
> > > vCPU into SMM mode, which might actually be invalid.
> > >
> > > I think what we actually want is to to set GIF when force-leaving nested. The
> > > only path where it's not obvious that's "safe" is toggling SMM in
> > > kvm_vcpu_ioctl_x86_set_vcpu_events(). In every other path, setting GIF is either
> > > correct/desirable, or irrelevant because the caller immediately and unconditionally
> > > sets/clears GIF.
> > >
> > > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > > index a6443feab252..3392c7e22cae 100644
> > > --- a/arch/x86/kvm/svm/nested.c
> > > +++ b/arch/x86/kvm/svm/nested.c
> > > @@ -1367,6 +1367,8 @@ void svm_leave_nested(struct kvm_vcpu *vcpu)
> > > nested_svm_uninit_mmu_context(vcpu);
> > > vmcb_mark_all_dirty(svm->vmcb);
> > >
> > > + svm_set_gif(svm, true);
> > > +
> > > if (kvm_apicv_activated(vcpu->kvm))
> > > kvm_make_request(KVM_REQ_APICV_UPDATE, vcpu);
> > > }
> > >
> >
> > This seems dangerously close to KVM making up "hardware" behavior, but
> > I'm okay with that if you are.
>
> Regardless of what KVM does, we're defining hardware behavior, i.e. keeping GIF
> unchanged defines behavior just as much as setting GIF. The only way to truly
> avoid defining behavior would be to terminate the VM and completely prevent
> userspace from accessing its state.

This can't be the only instance of "undefined behavior" that KVM deals
with. What about, say, misaligned accesses to xAPIC memory?