I was asked to set up clock synchronization for a Xen VM, running on top of Xen 4.0, which in turn was running on top of a Debian Squeeze. As you may know, NTP on VMs just sucks, so I was looking for Xen 3.0's independent_wallclock setting, and have domU's clock follow dom0's.

With my surprise, /proc/sys/xen/independent_wallclock wasn't there anymore.

With some surprise (but not as much), my searches in Google returned just crap.

As it often happens, IRC came to the rescue. Or, well: no rescue, but at least I was provided with a reason:

(15:51:10) The topic for ##xen is: Xen 4.0 http://www.xen.org/downloads | XCP http://blog.xen.org/index.php/2010/11/24/the-xen-cloud-platform-xcp-1-0-beta-is-available-from-xen-org/ | Wiki: http://wiki.xen.org/ | Solaris: http://www.opensolaris.org/os/community/xen/ | Logs: http://zentific.com/irclogs | Management: check out #zentific
(15:54:48) bronto@freenode: hello! It's maybe an FAQ, but I couldn't find an answer anywhere. Xen 4.0 doesn't have /proc/sys/xen/independent_wallclock (at least, not in Debian Squeeze Linux). Is there any equivalent?
(15:55:14) bronto@freenode: I would like domUs to follow dom0's clock, and sync dom0 using NTP.
(15:55:42) pasik: bronto: it depends on the kernel
(15:55:47) pasik: bronto: pvops kernels don't have it
(15:55:57) pasik: bronto: old xenlinux kernels do have it
(15:56:33) bronto@freenode: pasik: thanks. hmmm… so, the solution is: use a different kernel?
(15:56:40) pasik: bronto: or ntp
(15:56:55) bronto@freenode: pasik: NTP on domU's? Uh, that just sucks…
(15:58:57) pasik: bronto: pvops (upstream) kernels don't have independent_wallclock because there was some problems with it, it wasn't thought to be accepted to upstream Linux
(15:59:03) pasik: bronto: I can't remember the details now
(15:59:27) bronto@freenode: pasik: ouch… 😦
(15:59:41) bronto@freenode: pasik: this is damn bad
(16:00:31) bronto@freenode: pasik: OK. I'll fall back to ntp, and hope it will work