xen-pciback: Document when we FLR an PCI device.

When the toolstack wants us to drop or add an PCI device it
changes the XenBus state to Configuring - and as result of that
we find out which devices we should still be exporting out and
which ones not. For the ones we don't need anymore we need to
do an PCI reset so that it is ready for the next guest.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
diff --git a/drivers/xen/xen-pciback/xenbus.c b/drivers/xen/xen-pciback/xenbus.c
index a9ed867..4a7e6e0 100644
--- a/drivers/xen/xen-pciback/xenbus.c
+++ b/drivers/xen/xen-pciback/xenbus.c
@@ -93,6 +93,8 @@
 
 	xen_pcibk_disconnect(pdev);
 
+	/* N.B. This calls pcistub_put_pci_dev which does the FLR on all
+	 * of the PCIe devices. */
 	xen_pcibk_release_devices(pdev);
 
 	dev_set_drvdata(&pdev->xdev->dev, NULL);
@@ -286,6 +288,8 @@
 	dev_dbg(&dev->dev, "unregistering for %d\n", pdev->xdev->otherend_id);
 	xen_unregister_device_domain_owner(dev);
 
+	/* N.B. This ends up calling pcistub_put_pci_dev which ends up
+	 * doing the FLR. */
 	xen_pcibk_release_pci_dev(pdev, dev);
 
 out: