xen/pciback: Add flag indicating device has been assigned by Xen

Device drivers that create and destroy SR-IOV virtual functions via
calls to pci_enable_sriov() and pci_disable_sriov can cause catastrophic
failures if they attempt to destroy VFs while they are assigned to
guest virtual machines.  By adding a flag for use by the Xen PCI back
to indicate that a device is assigned a device driver can check that
flag and avoid destroying VFs while they are assigned and avoid system
failures.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/xen/xen-pciback/xenbus.c

@@ -249,6 +249,7 @@ static int xen_pcibk_export_device(struct xen_pcibk_device *pdev,
 		goto out;
 
 	dev_dbg(&dev->dev, "registering for %d\n", pdev->xdev->otherend_id);
+	dev->dev_flags |= PCI_DEV_FLAGS_ASSIGNED;
 	if (xen_register_device_domain_owner(dev,
 					     pdev->xdev->otherend_id) != 0) {
 		dev_err(&dev->dev, "device has been assigned to another " \
@@ -288,6 +289,7 @@ static int xen_pcibk_remove_device(struct xen_pcibk_device *pdev,
 	}
 
 	dev_dbg(&dev->dev, "unregistering for %d\n", pdev->xdev->otherend_id);
+	dev->dev_flags &= ~PCI_DEV_FLAGS_ASSIGNED;
 	xen_unregister_device_domain_owner(dev);
 
 	xen_pcibk_release_pci_dev(pdev, dev);