KVM: SVM: Take and hold ir_list_lock across IRTE updates in IOMMU

Now that svm_ir_list_add() isn't overloaded with all manner of weird
things, fold it into avic_pi_update_irte(), and more importantly take
ir_list_lock across the irq_set_vcpu_affinity() calls to ensure the info
that's shoved into the IRTE is fresh.  While preemption (and IRQs) is
disabled on the task performing the IRTE update, thanks to irqfds.lock,
that task doesn't hold the vCPU's mutex, i.e. preemption being disabled
is irrelevant.

Link: https://lore.kernel.org/r/20250611224604.313496-40-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/avic.c

@@ -770,32 +770,6 @@ static void svm_ir_list_del(struct kvm_kernel_irqfd *irqfd)
 	spin_unlock_irqrestore(&to_svm(vcpu)->ir_list_lock, flags);
 }
 
-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;
-
-	irqfd->irq_bypass_data = pi->ir_data;
-
-	spin_lock_irqsave(&svm->ir_list_lock, flags);
-
-	/*
-	 * Update the target pCPU for IOMMU doorbells if the vCPU is running.
-	 * If the vCPU is NOT running, i.e. is blocking or scheduled out, KVM
-	 * will update the pCPU info when the vCPU awkened and/or scheduled in.
-	 * See also avic_vcpu_load().
-	 */
-	entry = svm->avic_physical_id_entry;
-	if (entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK)
-		amd_iommu_update_ga(entry & AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK,
-				    true, pi->ir_data);
-
-	list_add(&irqfd->vcpu_list, &svm->ir_list);
-	spin_unlock_irqrestore(&svm->ir_list_lock, flags);
-}
-
 int avic_pi_update_irte(struct kvm_kernel_irqfd *irqfd, struct kvm *kvm,
 			unsigned int host_irq, uint32_t guest_irq,
 			struct kvm_vcpu *vcpu, u32 vector)
@@ -824,8 +798,18 @@ int avic_pi_update_irte(struct kvm_kernel_irqfd *irqfd, struct kvm *kvm,
 			.vapic_addr = avic_get_backing_page_address(to_svm(vcpu)),
 			.vector = vector,
 		};
+		struct vcpu_svm *svm = to_svm(vcpu);
+		u64 entry;
 		int ret;
 
+		/*
+		 * Prevent the vCPU from being scheduled out or migrated until
+		 * the IRTE is updated and its metadata has been added to the
+		 * list of IRQs being posted to the vCPU, to ensure the IRTE
+		 * isn't programmed with stale pCPU/IsRunning information.
+		 */
+		guard(spinlock_irqsave)(&svm->ir_list_lock);
+
 		ret = irq_set_vcpu_affinity(host_irq, &pi_data);
 		if (ret)
 			return ret;
@@ -840,14 +824,19 @@ int avic_pi_update_irte(struct kvm_kernel_irqfd *irqfd, struct kvm *kvm,
 			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.
+		/*
+		 * Update the target pCPU for IOMMU doorbells if the vCPU is
+		 * running.  If the vCPU is NOT running, i.e. is blocking or
+		 * scheduled out, KVM will update the pCPU info when the vCPU
+		 * is awakened and/or scheduled in.  See also avic_vcpu_load().
 		 */
-		svm_ir_list_add(to_svm(vcpu), irqfd, &pi_data);
+		entry = svm->avic_physical_id_entry;
+		if (entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK)
+			amd_iommu_update_ga(entry & AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK,
+					    true, pi_data.ir_data);
+
+		irqfd->irq_bypass_data = pi_data.ir_data;
+		list_add(&irqfd->vcpu_list, &svm->ir_list);
 		return 0;
 	}
 	return irq_set_vcpu_affinity(host_irq, NULL);