device hotplug & file handles

[Gerd Hoffmann]

  Hi,

For usb device pass-through (aka -device usb-host) it would be very
useful to pass file handles from libvirt to qemu.  The workflow would
change from ...

  (1) libvirt enables access to /dev/usb/$bus/$dev
  (2) libvirt passes $bus + $dev (using hostbus + hostaddr properties)
      to qemu.
  (3) qemu opens /dev/usb/$bus/$dev

... to ...

  (1) libvirt opens /dev/usb/$bus/$dev
  (2) libvirt passes filehandle to qemu.

Question is how can we pass the file descriptor best?  My idea would be
to simply add an fd property to usb-host:

 * Coldplug would be "-device usb-host,fd=<nr>" (cmd line).
 * Hotplug would be "device_add usb-host,fd=<getfd-name>" (monitor).

Will that work from libvirt point of view?
Or does anyone have an better idea?

thanks,
  Gerd

PS: background: https://bugzilla.redhat.com/show_bug.cgi?id=1595525