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

Jim Mattson
2025-10-14T20:52:12Z

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.