Identifying the physical machine running a Xen instance

Question

Given access to separate two Xen instances running Linux, is there some way to identify if the two instances are running on the same physical machine?

Answer 1

No. Anything you can try can be faked by the hypervisor. Even if you're not running in a hostile environment, the hypervisor blocks all of the interesting hardware calls you can make normally (lshw shows a wealth of component serial numbers on a real machine), and obviously the MAC addresses of all your domUs will be virtual and fake.

You're trying to answer the wrong question, though. Any possible scenario I can imagine this being useful in is better solved with either comprehensive system monitoring or contractual guarantees with visibility of compliance.