KVM: x86: clean/fix memory barriers in irqchip_in_kernel

The memory barriers are trying to protect against concurrent RCU-based
interrupt injection, but the IRQ routing table is not valid at the time
kvm->arch.vpic is written.  Fix this by writing kvm->arch.vpic last.
kvm_destroy_pic then need not set kvm->arch.vpic to NULL; modify it
to take a struct kvm_pic* and reuse it if the IOAPIC creation fails.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/i8259.c

@@ -651,15 +651,10 @@ struct kvm_pic *kvm_create_pic(struct kvm *kvm)
 	return NULL;
 }
 
-void kvm_destroy_pic(struct kvm *kvm)
+void kvm_destroy_pic(struct kvm_pic *vpic)
 {
-	struct kvm_pic *vpic = kvm->arch.vpic;
-
-	if (vpic) {
-		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &vpic->dev_master);
-		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &vpic->dev_slave);
-		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &vpic->dev_eclr);
-		kvm->arch.vpic = NULL;
-		kfree(vpic);
-	}
+	kvm_io_bus_unregister_dev(vpic->kvm, KVM_PIO_BUS, &vpic->dev_master);
+	kvm_io_bus_unregister_dev(vpic->kvm, KVM_PIO_BUS, &vpic->dev_slave);
+	kvm_io_bus_unregister_dev(vpic->kvm, KVM_PIO_BUS, &vpic->dev_eclr);
+	kfree(vpic);
 }

arch/x86/kvm/irq.h

@@ -74,7 +74,7 @@ struct kvm_pic {
 };
 
 struct kvm_pic *kvm_create_pic(struct kvm *kvm);
-void kvm_destroy_pic(struct kvm *kvm);
+void kvm_destroy_pic(struct kvm_pic *vpic);
 int kvm_pic_read_irq(struct kvm *kvm);
 void kvm_pic_update_irq(struct kvm_pic *s);
 
@@ -85,11 +85,11 @@ static inline struct kvm_pic *pic_irqchip(struct kvm *kvm)
 
 static inline int irqchip_in_kernel(struct kvm *kvm)
 {
-	int ret;
+	struct kvm_pic *vpic = pic_irqchip(kvm);
 
-	ret = (pic_irqchip(kvm) != NULL);
+	/* Read vpic before kvm->irq_routing.  */
 	smp_rmb();
-	return ret;
+	return vpic != NULL;
 }
 
 void kvm_pic_reset(struct kvm_kpic_state *s);

arch/x86/kvm/x86.c

@@ -3626,30 +3626,25 @@ long kvm_arch_vm_ioctl(struct file *filp,
 			r = kvm_ioapic_init(kvm);
 			if (r) {
 				mutex_lock(&kvm->slots_lock);
-				kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS,
-							  &vpic->dev_master);
-				kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS,
-							  &vpic->dev_slave);
-				kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS,
-							  &vpic->dev_eclr);
+				kvm_destroy_pic(vpic);
 				mutex_unlock(&kvm->slots_lock);
-				kfree(vpic);
 				goto create_irqchip_unlock;
 			}
 		} else
 			goto create_irqchip_unlock;
-		smp_wmb();
-		kvm->arch.vpic = vpic;
-		smp_wmb();
 		r = kvm_setup_default_irq_routing(kvm);
 		if (r) {
 			mutex_lock(&kvm->slots_lock);
 			mutex_lock(&kvm->irq_lock);
 			kvm_ioapic_destroy(kvm);
-			kvm_destroy_pic(kvm);
+			kvm_destroy_pic(vpic);
 			mutex_unlock(&kvm->irq_lock);
 			mutex_unlock(&kvm->slots_lock);
+			goto create_irqchip_unlock;
 		}
+		/* Write kvm->irq_routing before kvm->arch.vpic.  */
+		smp_wmb();
+		kvm->arch.vpic = vpic;
 	create_irqchip_unlock:
 		mutex_unlock(&kvm->lock);
 		break;