severity 534978 normal thanks I've made some more progress in understanding this behaviour, and have now figured out a workaround. I find the documentation at http://wiki.debian.org/Xen very misleading in several respects.
The domU kernel is receiving time info from the hypervisor as it should. My earlier suspicion that the vcpu_info wasn't being updated turned out to be both unfounded (I had missed the significance of "vcpu_info placement") and irrelevant (even without the vcpu_info updates the time as computed by pvclock_read_wallclock() drifts several orders of magnitude more slowly than observed). Linux' generic time code (in kernel/time/timekeeping.c), however, doesn't use the value from pvclock_read_wallclock() directly; instead, it uses the clocksource (xen_clocksource in this case) to compute an increment to the xtime kernel variable. For some reason I haven't fully worked out, this results in the drift I observed. I still suspect there is a bug here (the accuracy of the time calculation ought to be better than this) so I'm downgrading the severity to "normal" rather than "wishlist". The good news is that the NTP support code can correct for this. My workaround is therefore to run NTP in the domU. It is neither possible nor necessary to set xen.independent_wallclock=1 (that parameter is only supported by the featureset=xen kernels, which include the SuSE patch), and it is neither necessary nor desirable to change the domU clocksource to jiffies (I tried that and found that the time accuracy got much worse). I had been led to expect (again, by the wiki page, but also by common sense) that the domU was meant to get its clock from Xen and didn't need to run NTP. This appears to only be the case when the domU kernel includes the SuSE patch, not for the pv_ops-based approach. I would very much appreciate being relieved of the need to run NTP in each and every domU: that seems wasteful. -- To UNSUBSCRIBE, email to debian-kernel-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
