During the last days I had some fun with a Xen DomU: The virtual machine had a very bad network performance, although the well-known ethtool line was present in /etc/network/interfaces like in any other DomU on the same host. But only this single DomU had these problems.
After some time, I found out that although the line was present, tx checksumming was not disabled in this DomU. Then I found some other strange problems, when I tried to restart networking. I had to perform a lot of debugging of the startup process before I discovered that udev tried to bring up the interface, failed and then left the interface configuration in an inconsistent state. This problem only occurred in this DomU as in contrast to the other DomUs /usr has its own partition in this DomU, which is not yet mouted when udev tries to startup the interface.