> UML does something along these lines - the guests (domX: x > 1) get
> their timing from the host (dom0). But that works because the guest
> kernel there is running as a user process in the host environment. Xen
> would need to invoke interdomain messaging to pass queries, which
> offhand sounds more costly.
If you run NTPD in DOM0 then all other domains will be sync'ed to
DOM0's accurate timebase. This is how we run our own machines.
> I'm unsure if I replied earlier to the list, so let me point out that
> IME drifts up to 200 ppm can be found outside of Xen with contemporary
> PC hardware. That's about the same Adam's reported drift, or 4 minutes
> per week. This would all be more interesting if drift under Xen could
> be compared to the same piece of hardware's drift without Xen, holding
> all else as much the same as practical.
The two main places we get drift from when not using NTP are
inaccuracy of the PIT oscillator (can easily be 100 or more ppm), and
loss of significance when we program the PIT (we can't tell it to tick
precisely at 100Hz --- the closest we can get is out be a few 10s ppm
at least IIRC).
Not much we can do about the former -- we could deal with the latter
by stretching/shrinking jiffies or by inserting leap jiffies. 
 -- Keir
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