linux-kernel - Re: [PATCH 2/2] KVM: SVM: Raise #UD if VMMCALL instruction is not intercepted

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Message-ID: <aV1UpwppcDbOim_K@google.com>
Date: Tue, 6 Jan 2026 10:29:59 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Kevin Cheng <chengkev@...gle.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	yosry.ahmed@...ux.dev
Subject: Re: [PATCH 2/2] KVM: SVM: Raise #UD if VMMCALL instruction is not intercepted

On Tue, Jan 06, 2026, Kevin Cheng wrote:
> The AMD APM states that if VMMCALL instruction is not intercepted, the
> instruction raises a #UD exception.
> 
> Create a vmmcall exit handler that generates a #UD if a VMMCALL exit
> from L2 is being handled by L0, which means that L1 did not intercept
> the VMMCALL instruction.
> 
> Co-developed-by: Sean Christopherson <seanjc@...gle.com>
> Co-developed-by: Yosry Ahmed <yosry.ahmed@...ux.dev>

Co-developed-by requires a SoB.  As Yosry noted off-list, he only provided the
comment, and I have feedback on that :-)  Unless Yosry objects, just drop his.
Co-developed-by.

Ditt for me, just give me

  Suggested-by: Sean Christopherson <seanjc@...gle.com>

I don't need a Co-developed-by for a tossing a code snippet your way. though I
appreciate the offer. :-)

> Signed-off-by: Kevin Cheng <chengkev@...gle.com>
> ---
>  arch/x86/kvm/svm/svm.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index fc1b8707bb00c..482495ad72d22 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3179,6 +3179,20 @@ static int bus_lock_exit(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> +static int vmmcall_interception(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * If VMMCALL from L2 is not intercepted by L1, the instruction raises a
> +	 * #UD exception
> +	 */

Mentioning L2 and L1 is confusing.  It reads like arbitrary KVM behavior.  And
IMO the most notable thing is what's missing: an intercept check.  _That_ is
worth commenting, e.g.

	/*
	 * VMMCALL #UDs if it's not intercepted, and KVM reaches this point if
	 * and only if the VMCALL intercept is not set in vmcb12.
	 */

> +	if (is_guest_mode(vcpu)) {
> +		kvm_queue_exception(vcpu, UD_VECTOR);
> +		return 1;
> +	}
> +
> +	return kvm_emulate_hypercall(vcpu);
> +}
> +
>  static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
>  	[SVM_EXIT_READ_CR0]			= cr_interception,
>  	[SVM_EXIT_READ_CR3]			= cr_interception,
> @@ -3229,7 +3243,7 @@ static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
>  	[SVM_EXIT_TASK_SWITCH]			= task_switch_interception,
>  	[SVM_EXIT_SHUTDOWN]			= shutdown_interception,
>  	[SVM_EXIT_VMRUN]			= vmrun_interception,
> -	[SVM_EXIT_VMMCALL]			= kvm_emulate_hypercall,
> +	[SVM_EXIT_VMMCALL]			= vmmcall_interception,
>  	[SVM_EXIT_VMLOAD]			= vmload_interception,
>  	[SVM_EXIT_VMSAVE]			= vmsave_interception,
>  	[SVM_EXIT_STGI]				= stgi_interception,
> -- 
> 2.52.0.351.gbe84eed79e-goog
>