Unfortunately the Xen guest was not stable. It would crash after half an hour, sometimes after two hours. I tried increasing the amount of RAM allocated to the guest, tried both paravirtualized and fully virtualized guests and the result was the same. The guest would crash.

I tried again to use USB forwarding and pass only a single USB device to the guest. After a few more attempts I noticed that I could “partially” forward the USB device itself to a fully virtualized guest (HVM) using this syntax:

usb=1
usbdevice="host:xxxx:yyyy"

The values for xxxx and yyyy are the vendor ID and product ID for the USB device. They can be found out by using the lsusb command. For example on my laptop lsusb produces this output:

Bus 007 Device 001: ID 0000:0000
Bus 005 Device 003: ID 0b97:7772 O2 Micro, Inc.
Bus 005 Device 002: ID 0b97:7761 O2 Micro, Inc.
Bus 005 Device 001: ID 0000:0000
Bus 004 Device 009: ID 062a:0000 Creative Labs
Bus 004 Device 008: ID 413c:2002 Dell Computer Corp. SK-8125 Keyboard
Bus 004 Device 007: ID 413c:1002 Dell Computer Corp. Keyboard Hub
Bus 004 Device 001: ID 0000:0000
Bus 003 Device 001: ID 0000:0000
Bus 006 Device 001: ID 0000:0000
Bus 002 Device 001: ID 0000:0000
Bus 001 Device 001: ID 0000:0000

So if I wanted to forward the device Dell Computer Corp. SK-8125 Keyboard I’d put the following in the Xen guest configuration file:

usb=1
usbdevice="host:413c:2002"


Ok, so in theory this should have let me see the USB device in Xen guest. Well it did, kind of. One thing I noticed was that I was only able to see some output when running lsusb on the guest when the guest was started from command line using the xm create command. When starting the guest from Virtual Machine Manager GUI I’d see absolutely no USB devices in the guest. Second, even when starting the guest from command line I’d see this kind of output from lsusb:

Bus 001 Device 001: ID 0000:0000

So it looks like something was being forwarded to the guest but it wasn’t the device I wanted to export. It looks like USB pass-through works on Xen only with some USB devices. That matches with what I’ve read at xgu.ru.

The final result – at the moment I’m using the USB device from the physical host. At least until I have time to look at it again.

Related post(s)

• USB forwarding on Xen (1)
• Xen p2v conversion in seven simple steps (1)
• USB in Xen – how to make it work (0)
• Xen and CentOS 5.1 on PowerEdge 2950 (2)
• How to Xenify office servers (0)

To set it up, I followed instructions written by Jón Fairbairn at xensource.com. The pciback module in the CentOS 5.1 kernel doesn’t support the hide parameter so I left that out from my setup. It doesn’t seem to be necessary, anyway, for the driver for the USB controller in the host can be unbound from the PCI slot used by the controller.

To make sure the PCI slot is exported to Xen guests when the host system boots I created a xenpciexport script and placed it in the /etc/init.d/ directory. You can download the code from here xenpciexport.txt

The script xenpciexport is set up to start before xend and xendomains to make sure the PCI forwarding is working before any guest is running. It stops after xend and xendomains. That’s controlled by the line:

# chkconfig: 2345 97 02

you can see in the script header.

After copying the file to /etc/init.d you need to run

chkconfig --add xenpciexport

to create the appropriate links in the /etc/rc*.d/ directories.

To find out the slot number for the USB controller use lspci and lsusb commands. If the script is set up correctly you can plug in the USB device and run

service xenpciexport stop

After that the results of lsusb should include the USB device you want to export. Then run

service xenpciexport start

and now lsusb won’t show this device anymore – it’s hidden from the host and visible to Xen guests.

Before you start the Xen guest which is supposed to have access to the exported device add this line to its configuration file:

pci = [ '0000:00:1d.1' ]

The number in quotes must be the same as the SLOT variable in the xenpciexport script.

After that you need to patch your Xen installation. The version installed in CentOS 5.1 contains a bug which makes it impossible to use PCI pass-through. The fix can be found at CentOS Bug Tracker. Make sure you also fix the file pciif.py as specified in the note. In total there are three files which need to be edited: iopif.py, irqif.py and pciif.py.

Now you can finally start the Xen guest. When it finishes booting log into it and run lsusb there to confirm that the device is visible to the guest. lspci in the guest should also show you the USB controller. It is important to run the xenpciexport script before you start your guest, otherwise the USB device will not be visible to the guest OS.

The Xen guest has now full access to the USB device.

Here you can read more about PCI passthrough on Xen.

Update:
It turns out that this setup was causing the Xen guest to crash. You can read more in a new post.

Related post(s)

• USB forwarding on Xen – it just does not work (6)
• Xen p2v conversion in seven simple steps (1)
• Xen and CentOS 5.1 on PowerEdge 2950 (2)
• USB in Xen – how to make it work (0)
• Setting up UPS on CentOS 5.2 with SELinux, part 2 (1)