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

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Message-ID: <aO7JjaymjPMBcjrz@google.com>
Date: Tue, 14 Oct 2025 15:07:09 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] KVM: SVM: Don't set GIF when clearing EFER.SVME

On Tue, Oct 14, 2025, Jim Mattson wrote:
> On Tue, Oct 14, 2025 at 2:18 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Tue, Oct 14, 2025, Jim Mattson wrote:
> > > On Mon, Oct 13, 2025 at 3:33 PM Sean Christopherson <seanjc@...gle.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.

Oh, for sure.  But unsurprisingly, people only care about cases that actually
matter in practice.  E.g. the other one that comes to mind is SHUTDOWN on AMD:

	/*
	 * VMCB is undefined after a SHUTDOWN intercept.  INIT the vCPU to put
	 * the VMCB in a known good state.  Unfortuately, KVM doesn't have
	 * KVM_MP_STATE_SHUTDOWN and can't add it without potentially breaking
	 * userspace.  At a platform view, INIT is acceptable behavior as
	 * there exist bare metal platforms that automatically INIT the CPU
	 * in response to shutdown.
	 *

> What about, say, misaligned accesses to xAPIC memory?

Drops all accesses (doesn't even set the destination on reads).