hw/qdev: Can qdev_unrealize() ever fail?

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Hi,

QDev base class doesn't expect UNREALIZE to fail, and this
handler is only recommended for hot-plug devices:

/**
  * qdev_unrealize: Unrealize a device
  * @dev: device to unrealize
  *
  * Warning: most devices in QEMU do not expect to be unrealized. Only
  * devices which are hot-unpluggable should be unrealized (as part of
  * the unplugging process); all other devices are expected to last for
  * the life of the simulation and should not be unrealized and freed.
  */


   void qdev_unrealize(DeviceState *dev)
   {
       object_property_set_bool(OBJECT(dev), "realized",
                                false, &error_abort);
                                       ^^^^^^^^^^^^
   }

   static void device_unparent(Object *obj)
   {
       DeviceState *dev = DEVICE(obj);
       BusState *bus;

       if (dev->realized) {
           qdev_unrealize(dev);
       }
       while (dev->num_child_bus) {
           bus = QLIST_FIRST(&dev->child_bus);
           object_unparent(OBJECT(bus));
       }
       if (dev->parent_bus) {
           bus_remove_child(dev->parent_bus, dev);
           object_unref(OBJECT(dev->parent_bus));
           dev->parent_bus = NULL;
       }
   }

Now apparently some devices expect failures, see commit 7c0fa8dff8
("pcie: Add support for Single Root I/O Virtualization (SR/IOV)"):

   static void unregister_vfs(PCIDevice *dev)
   {
       uint16_t num_vfs = dev->exp.sriov_pf.num_vfs;
       uint16_t i;

       for (i = 0; i < num_vfs; i++) {
           Error *err = NULL;
           PCIDevice *vf = dev->exp.sriov_pf.vf[i];
           if (!object_property_set_bool(OBJECT(vf), "realized",
                                         false, &err)) {
                                                ^^^^
               error_reportf_err(err, "Failed to unplug: ");
           }
           object_unparent(OBJECT(vf));
           object_unref(OBJECT(vf));
       }
       ...
   }

(Note the failure path only emits a warning).

So instead of calling the QDev unrealize layer, this function is
calling the lower layer, QOM, bypassing the class handlers, leading
to further cleanups such commit 08f6328480 ("pcie: Release references
of virtual functions") or recent patch
https://lore.kernel.org/qemu-devel/20240210-reuse-v2-6-24ba2a502692@daynix.com/.

I couldn't find any explicit possible failure in:
  pci_qdev_unrealize()
  do_pci_unregister_device()
  pci_bus_unrealize()
so, what is the failure unregister_vfs() is trying to recover from?

I understand if a device is in a odd state, the kernel could reject
an unplug request. If so, how to deal with that cleanly?

Thanks,

Phil.