linux-kernel - Re: [PATCH v3 1/4] KVM: SVM: Allow KVM_SET_NESTED

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Message-ID: <6ilulzhszphdjk3ta5jt7t222jicn3zj5e6em3fknzmudeqr3f@dogx6h7lsrax>
Date: Thu, 8 Jan 2026 20:38:11 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jim Mattson <jmattson@...gle.com>, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] KVM: SVM: Allow KVM_SET_NESTED_STATE to clear GIF
 when SVME==0

On Thu, Jan 08, 2026 at 10:58:40AM -0800, Sean Christopherson wrote:
> On Fri, Nov 21, 2025, Yosry Ahmed wrote:
> > From: Jim Mattson <jmattson@...gle.com>
> > 
> > GIF==0 together with EFER.SVME==0 is a valid architectural
> > state. Don't return -EINVAL for KVM_SET_NESTED_STATE when this
> > combination is specified.
> > 
> > Fixes: cc440cdad5b7 ("KVM: nSVM: implement KVM_GET_NESTED_STATE and KVM_SET_NESTED_STATE")
> > Signed-off-by: Jim Mattson <jmattson@...gle.com>
> > Reviewed-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> > Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> > ---
> >  arch/x86/kvm/svm/nested.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index c81005b24522..3e4bd8d69788 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -1784,8 +1784,8 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
> >  	 * EFER.SVME, but EFER.SVME still has to be 1 for VMRUN to succeed.
> >  	 */
> >  	if (!(vcpu->arch.efer & EFER_SVME)) {
> > -		/* GIF=1 and no guest mode are required if SVME=0.  */
> > -		if (kvm_state->flags != KVM_STATE_NESTED_GIF_SET)
> > +		/* GUEST_MODE must be clear when SVME==0 */
> > +		if (kvm_state->flags & KVM_STATE_NESTED_GUEST_MODE)
> 
> Hmm, this is technically wrong, as it will allow KVM_STATE_NESTED_RUN_PENDING.
> Now, arguably KVM already has a flaw there as KVM allows KVM_STATE_NESTED_RUN_PENDING
> without KVM_STATE_NESTED_GUEST_MODE for SVME=1, but I'd prefer not to make the
> hole bigger.
> 
> The nested if-statement is also unnecessary.
> 
> How about this instead?  (not yet tested)
> 
> 	/*
> 	 * If in guest mode, vcpu->arch.efer actually refers to the L2 guest's
> 	 * EFER.SVME, but EFER.SVME still has to be 1 for VMRUN to succeed.
> 	 * If SVME is disabled, the only valid states are "none" and GIF=1
> 	 * (clearing SVME does NOT set GIF, i.e. GIF=0 is allowed).
> 	 */
> 	if (!(vcpu->arch.efer & EFER_SVME) && kvm_state->flags &&
> 	    kvm_state->flags != KVM_STATE_NESTED_GIF_SET)
> 		return -EINVAL;

Looks good to me, with the tiny exception that at this point clearing
SVME does set GIF. Maybe re-order the patches?

Let me know if you want me to send a new version or if you'll fix it up
while applying.