virtio: Don't use PAGE_SIZE in virtio_pci.c

The virtio PCI devices don't depend on the guest page size.  This matters
now PowerPC virtio is gaining ground (they like 64k pages).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_pci.c

@@ -245,7 +245,7 @@ static struct virtqueue *vp_find_vq(struct virtio_device *vdev, unsigned index,
 	}
 
 	/* activate the queue */
-	iowrite32(virt_to_phys(info->queue) >> PAGE_SHIFT,
+	iowrite32(virt_to_phys(info->queue) >> VIRTIO_PCI_QUEUE_ADDR_SHIFT,
 		  vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
 
 	/* create the vring */

include/linux/virtio_pci.h

@@ -53,4 +53,8 @@
 
 /* Virtio ABI version, this must match exactly */
 #define VIRTIO_PCI_ABI_VERSION		0
+
+/* How many bits to shift physical queue address written to QUEUE_PFN.
+ * 12 is historical, and due to x86 page size. */
+#define VIRTIO_PCI_QUEUE_ADDR_SHIFT	12
 #endif