VT-d spec specifies that all conventional PCI devices behind a PCIe-to
PCI/PCI-X bridge or conventional PCI bridge can only be collectively assigned
to the same guest. PCIe devices do not have this restriction.

If the device doesn't support MSI, and it shares IRQ with other devices,
then it cannot be assigned due to host irq sharing for assigned devices is
not supported. You will get warning message when you assign it. Notice this
also apply to the devices which only support MSI-X.


# lspci
......
07:00.0 USB controller: NEC Corporation uPD720200 USB 3.0 Host Controller
(rev 04)
......

$ lspci -n | grep 07
07:00.0 0c03: 1033:0194 (rev 04)

# modprobe pci_stub
# echo "1033 0194" > /sys/bus/pci/drivers/pci-stub/new_id
$ readlink /sys/bus/pci/devices/0000:07:00.0/driver
../../../../bus/pci/drivers/xhci_hcd

# echo "0000:07:00.0" > /sys/bus/pci/drivers/xhci_hcd/unbind
# echo "0000:07:00.0" > /sys/bus/pci/drivers/pci-stub/bind
# qemu-kvm -m 1024 -hda somefile.img -device pci-assign,host=0000:07:00.0