RE: [Xen-devel] Time skew on HP DL785 (and possibly other boxes)

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> > The goal for Xen timekeeping is to ensure that if a guest
> > could somehow magically read any of its virtual clocks
> > (tsc, pit, hpet, pmtimer, ??) on all its virtual processors
> > simultaneously, the values read must always obey this
> > "virtual clock law":
> 
> We can do this for all except TSC for HVM guests because 

I understand that this is true IFF Xen system time itself
obeys the virtual clock law.  I am concerned that maybe
it cannot on machines such as this.  If not, NO HVM guest
clock will obey the law, correct?

> Everything else builds on Xen system time, and Xen system 
> time should just
> require each CPU's TSC to be individually stable.
> ...I think the benefit of your patch was in 
> sync'ing system
> time across all CPUs at the same time, which significantly 
> reduced maximum divergence.

The problem was, in our testing on this DL785, the maximum
divergence was not reduced enough!  This was tested with
xen-unstable (not sure what c/s).

> One concern I have however, is Intel's 
> X86_FEATURE_CONSTANT_TSC logic.

It's possible that this (or some other problem) has resulted
in the divergenece on the DL785.  So more testing is in
order.

Dan

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