KVM: SVM: Revert IRTE to legacy mode if IOMMU doesn't provide IR metadata

Revert the IRTE back to remapping mode if the AMD IOMMU driver mucks up
and doesn't provide the necessary metadata.  Returning an error up the
stack without actually handling the error is useless and confusing.

Link: https://lore.kernel.org/r/20250611224604.313496-39-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/avic.c

@@ -770,16 +770,13 @@ static void svm_ir_list_del(struct kvm_kernel_irqfd *irqfd)
 	spin_unlock_irqrestore(&to_svm(vcpu)->ir_list_lock, flags);
 }
 
-static int svm_ir_list_add(struct vcpu_svm *svm,
-			   struct kvm_kernel_irqfd *irqfd,
-			   struct amd_iommu_pi_data *pi)
+static void svm_ir_list_add(struct vcpu_svm *svm,
+			    struct kvm_kernel_irqfd *irqfd,
+			    struct amd_iommu_pi_data *pi)
 {
 	unsigned long flags;
 	u64 entry;
 
-	if (WARN_ON_ONCE(!pi->ir_data))
-		return -EINVAL;
-
 	irqfd->irq_bypass_data = pi->ir_data;
 
 	spin_lock_irqsave(&svm->ir_list_lock, flags);
@@ -797,7 +794,6 @@ static int svm_ir_list_add(struct vcpu_svm *svm,
 
 	list_add(&irqfd->vcpu_list, &svm->ir_list);
 	spin_unlock_irqrestore(&svm->ir_list_lock, flags);
-	return 0;
 }
 
 int avic_pi_update_irte(struct kvm_kernel_irqfd *irqfd, struct kvm *kvm,
@@ -834,14 +830,25 @@ int avic_pi_update_irte(struct kvm_kernel_irqfd *irqfd, struct kvm *kvm,
 		if (ret)
 			return ret;
 
+		/*
+		 * Revert to legacy mode if the IOMMU didn't provide metadata
+		 * for the IRTE, which KVM needs to keep the IRTE up-to-date,
+		 * e.g. if the vCPU is migrated or AVIC is disabled.
+		 */
+		if (WARN_ON_ONCE(!pi_data.ir_data)) {
+			irq_set_vcpu_affinity(host_irq, NULL);
+			return -EIO;
+		}
+
 		/**
 		 * Here, we successfully setting up vcpu affinity in
 		 * IOMMU guest mode. Now, we need to store the posted
 		 * interrupt information in a per-vcpu ir_list so that
 		 * we can reference to them directly when we update vcpu
 		 * scheduling information in IOMMU irte.
 		 */
-		return svm_ir_list_add(to_svm(vcpu), irqfd, &pi_data);
+		svm_ir_list_add(to_svm(vcpu), irqfd, &pi_data);
+		return 0;
 	}
 	return irq_set_vcpu_affinity(host_irq, NULL);
 }