hw/pci: add comment to explain checking for available function 0 in p…

…ci hotplug

This change is cosmetic. A comment is added explaining why we need to check for
the availability of function 0 when we hotplug a device.

CC: mst@redhat.com
CC: mjt@tls.msk.ru
Signed-off-by: Ani Sinha <anisinha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/pci/pci.c

@@ -1183,9 +1183,14 @@ static PCIDevice *do_pci_register_device(PCIDevice *pci_dev,
                    PCI_SLOT(devfn), PCI_FUNC(devfn), name,
                    bus->devices[devfn]->name, bus->devices[devfn]->qdev.id);
         return NULL;
-    } else if (dev->hotplugged &&
-               !pci_is_vf(pci_dev) &&
-               pci_get_function_0(pci_dev)) {
+    } /*
+       * Populating function 0 triggers a scan from the guest that
+       * exposes other non-zero functions. Hence we need to ensure that
+       * function 0 wasn't added yet.
+       */
+    else if (dev->hotplugged &&
+             !pci_is_vf(pci_dev) &&
+             pci_get_function_0(pci_dev)) {
         error_setg(errp, "PCI: slot %d function 0 already occupied by %s,"
                    " new func %s cannot be exposed to guest.",
                    PCI_SLOT(pci_get_function_0(pci_dev)->devfn),