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.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kevin Cheng <chengkev@google.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 92a66b62cfabd..805267a5106ac 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3180,6 +3180,20 @@ static int bus_lock_exit(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
+static int vmmcall_interception(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * VMMCALL #UDs if it's not intercepted, and KVM reaches this point if
+	 * and only if the VMMCALL 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,
@@ -3230,7 +3244,7 @@ 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,
-- 
2.52.0.457.g6b5491de43-goog

On 1/12/2026 11:15 PM, 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.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Kevin Cheng <chengkev@google.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 92a66b62cfabd..805267a5106ac 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3180,6 +3180,20 @@ static int bus_lock_exit(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> +static int vmmcall_interception(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * VMMCALL #UDs if it's not intercepted, and KVM reaches this point if
> +	 * and only if the VMMCALL 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,
> @@ -3230,7 +3244,7 @@ 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,

Reviewed-by: Manali Shukla <manali.shukla@amd.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>

On Mon, Jan 12, 2026 at 05:45:35PM +0000, 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.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Kevin Cheng <chengkev@google.com>

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>

> ---
>  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 92a66b62cfabd..805267a5106ac 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3180,6 +3180,20 @@ static int bus_lock_exit(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> +static int vmmcall_interception(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * VMMCALL #UDs if it's not intercepted, and KVM reaches this point if
> +	 * and only if the VMMCALL 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,
> @@ -3230,7 +3244,7 @@ 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,
> -- 
> 2.52.0.457.g6b5491de43-goog
>