virtio-pci: harden INTX interrupts

This patch tries to make sure the virtio interrupt handler for INTX
won't be called after a reset and before virtio_device_ready(). We
can't use IRQF_NO_AUTOEN since we're using shared interrupt
(IRQF_SHARED). So this patch tracks the INTX enabling status in a new
intx_soft_enabled variable and toggle it during in
vp_disable/enable_vectors(). The INTX interrupt handler will check
intx_soft_enabled before processing the actual interrupt.

Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20211019070152.8236-6-jasowang@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_pci_common.c

@@ -30,8 +30,16 @@ void vp_disable_cbs(struct virtio_device *vdev)
 	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
 	int i;
 
-	if (vp_dev->intx_enabled)
+	if (vp_dev->intx_enabled) {
+		/*
+		 * The below synchronize() guarantees that any
+		 * interrupt for this line arriving after
+		 * synchronize_irq() has completed is guaranteed to see
+		 * intx_soft_enabled == false.
+		 */
+		WRITE_ONCE(vp_dev->intx_soft_enabled, false);
 		synchronize_irq(vp_dev->pci_dev->irq);
+	}
 
 	for (i = 0; i < vp_dev->msix_vectors; ++i)
 		disable_irq(pci_irq_vector(vp_dev->pci_dev, i));
@@ -43,8 +51,16 @@ void vp_enable_cbs(struct virtio_device *vdev)
 	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
 	int i;
 
-	if (vp_dev->intx_enabled)
+	if (vp_dev->intx_enabled) {
+		disable_irq(vp_dev->pci_dev->irq);
+		/*
+		 * The above disable_irq() provides TSO ordering and
+		 * as such promotes the below store to store-release.
+		 */
+		WRITE_ONCE(vp_dev->intx_soft_enabled, true);
+		enable_irq(vp_dev->pci_dev->irq);
 		return;
+	}
 
 	for (i = 0; i < vp_dev->msix_vectors; ++i)
 		enable_irq(pci_irq_vector(vp_dev->pci_dev, i));
@@ -97,6 +113,9 @@ static irqreturn_t vp_interrupt(int irq, void *opaque)
 	struct virtio_pci_device *vp_dev = opaque;
 	u8 isr;
 
+	if (!READ_ONCE(vp_dev->intx_soft_enabled))
+		return IRQ_NONE;
+
 	/* reading the ISR has the effect of also clearing it so it's very
 	 * important to save off the value. */
 	isr = ioread8(vp_dev->isr);

drivers/virtio/virtio_pci_common.h

@@ -63,6 +63,7 @@ struct virtio_pci_device {
 	/* MSI-X support */
 	int msix_enabled;
 	int intx_enabled;
+	bool intx_soft_enabled;
 	cpumask_var_t *msix_affinity_masks;
 	/* Name strings for interrupts. This size should be enough,
 	 * and I'm too lazy to allocate each name separately. */