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RE: [Xen-devel] [PATCH] Disable Xen PowerNow! support on Opteron 2nd gen and earlier processors

> > Surely it should be possible to inform Xen of frequency 
> changes (other
> > that T-states due to thermal throttling) and hence allow 
> Xen to ensure
> > monotonicity? Xen itself does not require that the CPUs run with the
> > same rate TSC. It does need to know what frequency they're 
> running at,
> > and when they change.
> Yes, and that is already done, hooked off the cpu notifier 
> callback chain.
> Nobbling the C- and P-state logic shouldn't be necessary.

And yet, people are still reporting "Time went backwards"
messages upwards of 30-40 times an hour on RevF Opterons
when PowerNow! is enabled on Xen.

I spent a month trying to debug this and I made no significant
progress.  If someone who understands Xen timekeeping wants to
help me dive into this, I'll be glad to take another stab at
it.  As it is, even with informing Xen of frequency changes,
Xen does not ensure monotinicity.

-Mark Langsdorf
Operating System Research Center

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