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

[Gerd Hoffmann <kraxel@redhat.com>]

On 07/30/13 19:13, Hans de Goede wrote:
> 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

host-libusb.c doesn't do that, and I also can't reproduce the issue.
Running libusbx-1.0.14 at the moment.

I get slightly different errors:

libusb_release_interface: -4 [NO_DEVICE]
libusbx: error [_open_sysfs_attr] open
/sys/bus/usb/devices/10-1/bConfigurationValue failed ret=-1 errno=2

Which comes from host-libusb trying to release the (unplugged) device on
closing.  They are harmless.

>> [ 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() ?

Sure.  Each time it looks for new devices it calls libusb_get_device_list().

cheers,
  Gerd