virtio: Use PCI revision field to indicate virtio PCI ABI version

As Avi pointed out, as we continue to massage the virtio PCI ABI, we can make
things a little more friendly to users by utilizing the PCI revision field to
indicate which version of the ABI we're using.  This is a hard ABI version
and incrementing it will cause the guest driver to break.

This is the necessary changes to virtio_pci to support this.
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_pci.c

@@ -311,6 +311,12 @@ static int __devinit virtio_pci_probe(struct pci_dev *pci_dev,
 	if (pci_dev->device < 0x1000 || pci_dev->device > 0x103f)
 		return -ENODEV;
 
+	if (pci_dev->revision != VIRTIO_PCI_ABI_VERSION) {
+		printk(KERN_ERR "virtio_pci: expected ABI version %d, got %d\n",
+		       VIRTIO_PCI_ABI_VERSION, pci_dev->revision);
+		return -ENODEV;
+	}
+
 	/* allocate our structure and fill it out */
 	vp_dev = kzalloc(sizeof(struct virtio_pci_device), GFP_KERNEL);
 	if (vp_dev == NULL)

include/linux/virtio_pci.h

@@ -52,4 +52,6 @@
  * configuration space */
 #define VIRTIO_PCI_CONFIG		20
 
+/* Virtio ABI version, this must match exactly */
+#define VIRTIO_PCI_ABI_VERSION		0
 #endif