I find that Xen has a settings file named “permitted_clock_jitter” in /proc/sys/xen. I found this out when trying to discover why my DomU machine won’t shut down, but instead claims that it “Cannot access the Hardware Clock via any known method” and that “Time went backwards.” The latter claim, particularly, seems dubious, but the shutdown log is pretty adamant.

Note: if you got here by searching for that error message, I’d like to first say “ugh, sorry,” and then let you know that I solved this in the end by giving up on Xen and doing a normal install of Debian on the server I was setting up.