[Xen-users] Solved, but rather a crutch (Was: Passing USB device to MS Windows guest)

[Willy Weisz <weisz@xxxxxxxxxxxxxxxxx>]

A second attempt without digital signature, which seems to impeach the
full publication on the xen-users list.
--

I've found the causes of my problems and have created a fix (but not a
real solution) for it.

The first step (which is a real solution):
'qemu-dm' uses the out-fashioned (pre-kernel 2.6) /proc file system to
access system information (more precisely the sub-directory
/proc/bus/usb)  and not the usbfs file system. The file /etc/fstab hass
an entry for the mount point /proc/bus/usb that hooks the usbfs up
there, but this is not done at boot time since it is prevented by the
option 'noauto'  in said entry.
The command 'mount /proc/bus/usb' must be entered manually or put in a
file in /etc/init.d/.

This first step allows qemu the access to the USB devices.

The second step should be to correct a conceptual bug in the handling of
'usbdevice' in the Python routines called by the commands 'xend' as well
as 'xm': 'usbdevice' is considered to be part of the platform definition
instead of being handled as a device entry. As part of the platform
definition it only accepts a single instantiation with a single value
(in my case only 'tablet' or 'host:Id1:Id2'), handled as a device, it
could be instantiated multiple times with different values.

Lacking a good documentation on the information flow through the Python
routines and the time to make a complete analysis of what parts of the
code to patch (at least the routines xend/image.py, xm/main.py,
xm/create.py and xm/xenapi_create.py), I decided to find a solution for
my problem only:
In order to be able to start a VM with multiple USB devices I extended
the 'usbdevice' option of the command 'xm create' to accept a character
string with a comma-separated list of USB devices (e.g.
usbdevice='tablet, host:05e3:0100' with white-spaces allowed before and
after any list entry) which translates to multiple 'usbdevice' options
for the 'qemu-dm' command. The necessary patch for the routine
xend/image.py is attached.

This patch isn't meant to be the solution, but rather a crutch solid
enough for my actual needs. I hope that XEN developers will work on a
real solution that will include the possibility to include multiple
'usbdevice' definitions in the configuration files for the VMs, moving
'usbdevice' from a platform entry to a device entry and defining a
syntax for the configuration file entries.

As for the unavailable entry 'usb_add' in the qemu-dm monitor it is the
result of a 'censorship' measure by the authors of the openSuSE rpm
package 'xen-tools'. As a consequence of the vulnerability CVE-2007-0998
reported by RedHat they deleted a few monitor commands thought to
endanger the confidentiality of some Dom0 informations - neither Mitre
nor NIST have yet fully assessed the 'danger' despite the fact that the
initial report dates from more than 2 years ago. The problem is not so
much the patch itself (it can easily be removed by commenting out the
corresponding patch in the xen.spec file of the source rpm or setting
the variable CONFIG_TRUSTED_CLIENT, and rebuilding the package), but
simply the fact that there is no mention of it anywhere in the doc files
accompanying the rpm package nor on the openSuSE web site.
The search for the reason for the 'missing monitor commands' has cost a
lot of the delay in finding a solution. Nevertheless the 'amputed' qemu
monitor is well suited for use with my patch.

I hope this lengthy mail helps some XEN users, and kicks off the
development of a real solution for the USB configuration of fully
virtualised VMs.

Regards
Willy

Willy Weisz wrote:
> The configuration:
>
> Dom0: openSuse 11.1 xen kernel for x86_64 and Xen 3.3.1
> DomU: MS Windows XP
>
> I want to pass a USB device with the identification 05e3:0100 to the DomU
>
> First attempt:
> I added a line
> usbdevice="host:05e3:0100"
> to the configuration file for the DomU already existing in /etc/vm. 
> The file also contains the lines
> usb=1
> usbdevice="tablet"
> .
>
> After rebooting the VM in DomU no USB device other than the "tablet" 
> (mouse equivalent) is available.
> A look at the log files of the Dom0 reveals no mention of 
> "host:05e3:0100".
>
> Second attempt:
> On the graphical console (vnc) of the VM running Windows I switched to 
> the qemu monitor (version 0.9.1) with Alt+Crl+2 and entered the 
> command "usb_add", and qemu answered: "unknown command: 'usb_add'; by 
> the way, "help" doesn't list "usb_add" either.
>
> So both approaches described in the Xen 3.3 User's Guide and in some 
> other places on the web don't work.
>
> Since the only reason for running Windows is using a software package 
> that isn't available for Linux and that uses the USB device as input 
> device, I'm stuck and need urgently help.
>
> Regards
> Willy

-- 
-----------------------------------------------------------
Willy Weisz

European Centre for Parallel Computing at Vienna (VCPC)
          Institute of Scientific Computing
               University of Vienna
                 Nordbergstrasse 15/C312
                 A-1090 Wien
Tel: (+43 1) 4277 - 39424          Fax: (+43 1) 4277 - 9394
                e-mail: weisz@xxxxxxxxxxxxxxxxx


--- xend/image.py.orig  2009-02-26 23:17:51.000000000 +0100
+++ xend/image.py       2009-03-02 00:11:16.000000000 +0100
@@ -742,6 +742,12 @@
                     if v: ret.append("-%s" % a)
                 except (ValueError, TypeError):
                     pass # if we can't convert it to a sane type, ignore it
+            # Handle usbdevice so that it passes multiple devices
+            if a == 'usbdevice':
+                d = v.split(',')
+                for u in d:
+                    ret.append("-%s" % a)
+                    ret.append("%s" % u.strip())
             else:
                 if v:
                     ret.append("-%s" % a)

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