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

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Message-ID: <aV_-YLO4AVQc-ZmY@google.com>
Date: Thu, 8 Jan 2026 10:58:40 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
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 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;