Re: [PATCH v3 08/16] KVM: x86: Move bulk of emergency virtualizaton logic to virt subsystem

[Chao Gao]

On Fri, Feb 13, 2026 at 05:26:54PM -0800, Sean Christopherson wrote:
>Move the majority of the code related to disabling hardware virtualization
>in emergency from KVM into the virt subsystem so that virt can take full
>ownership of the state of SVM/VMX.  This will allow refcounting usage of
>SVM/VMX so that KVM and the TDX subsystem can enable VMX without stomping
>on each other.
>
>To route the emergency callback to the "right" vendor code, add to avoid

							     ^^^ and

>-void cpu_emergency_disable_virtualization(void)
>-{
>-	cpu_emergency_virt_cb *callback;
>-
>-	/*
>-	 * IRQs must be disabled as KVM enables virtualization in hardware via
>-	 * function call IPIs, i.e. IRQs need to be disabled to guarantee
>-	 * virtualization stays disabled.
>-	 */
>-	lockdep_assert_irqs_disabled();
>-
>-	rcu_read_lock();
>-	callback = rcu_dereference(cpu_emergency_virt_callback);
>-	if (callback)
>-		callback();
>-	rcu_read_unlock();

...

>+static void x86_virt_invoke_kvm_emergency_callback(void)
>+{
>+	cpu_emergency_virt_cb *kvm_callback;
>+
>+	kvm_callback = rcu_dereference(kvm_emergency_callback);
>+	if (kvm_callback)
>+		kvm_callback();

The RCU lock is dropped here. I assume this is intentional since the function
is only called with IRQs disabled, in which case the RCU lock isn't needed.

<snip>

>+int x86_virt_emergency_disable_virtualization_cpu(void)
>+{
>+	/* Ensure the !feature check can't get false positives. */
>+	BUILD_BUG_ON(!X86_FEATURE_SVM || !X86_FEATURE_VMX);
>+
>+	if (!virt_ops.feature)
>+		return -EOPNOTSUPP;
>+
>+	/*
>+	 * IRQs must be disabled as virtualization is enabled in hardware via
>+	 * function call IPIs, i.e. IRQs need to be disabled to guarantee
>+	 * virtualization stays disabled.
>+	 */

The comment is stale. Since this patch just moves the comment, it should be
fine to keep it as-is and fix it in a separate series.

>+	lockdep_assert_irqs_disabled();
>+
>+	/*
>+	 * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
>+	 * other CPUs may have virtualization enabled.
>+	 *
>+	 * TODO: Track whether or not virtualization might be enabled on other
>+	 *	 CPUs?  May not be worth avoiding the NMI shootdown...
>+	 */
>+	virt_ops.emergency_disable_virtualization_cpu();
>+	return 0;
>+}
>+
> void __init x86_virt_init(void)
> {
>-	x86_vmx_init();
>+	/*
>+	 * Attempt to initialize both SVM and VMX, and simply use whichever one
>+	 * is present.  Rsefuse to enable/use SVM or VMX if both are somehow

			^^^^^^^ Refuse

LGTM aside from the two typos above.

Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>