Re: [Qemu-devel] usb: a problem of using libusb for usb pass through

[Hans de Goede]

Hi,

On 07/22/2013 02:58 PM, Gerd Hoffmann wrote:
> On 07/16/13 10:45, Gonglei (Arei) wrote:
> > Hi, Gerd
> >         My Qemu version is 1.5.1, and use libusb for usb pass through.
> >         I pass through a host usb device to the guest by bus number and 
> > physical port, when I unplug the usb device from
> > the host, and plug in on the same physical port immediately, but the usb device 
> > don't show up in the guest.
> >         The qemu log show:
> > libusb_release_interface: -4 [NO_DEVICE]
> > libusbx: error [_get_usbfs_fd] libusbx couldn't open USB device 
> > /dev/bus/usb/002/004: No such file or directory
> > libusbx: error [_get_usbfs_fd] libusbx couldn't open USB device 
> > /dev/bus/usb/002/004: No such file or directory
> > libusbx: error [_get_usbfs_fd] libusbx couldn't open USB device 
> > /dev/bus/usb/002/004: No such file or directory
> >
> >         I find the reason is that the global libusb_context has recorded the 
> > usb devices' devnum, but when the host device was unplugged and plugged
> > from the host, the devnum will change, finally cause the problem.
> >         Any ideas ? Thanks!
>
> Sounds like libusbx doesn't flush the cache on unplug even though it
> should.  Hans?

This looks like qemu's host-libusb code is re-using the libusb_device handle, 
what it should do after seeing an
unplug (ie ENODEV error), it should close both the handle, and free any devices 
it may have, then redo
the libusb_get_device_list() and you should get a libusb_device with the new 
address. This is what the
usb code in the spice-client does, and it has no problems with sc

> [ qemu uses one global libusb_context.  libusb_device and
>    libusb_device_handle are allocated and released on open/close ]

That is fine, do you also free the device_list, and re-do the 
libusb_get_device_list() ?

Regards,

Hans