RE: [Xen-devel] x86 Physical CPUs at different frequencies - timer error messages

["Langsdorf, Mark" <mark.langsdorf@xxxxxxx>]

> > I'm experimenting with getting PowerNow! running on
> > SMP Opteron systems.
> > 
> > One problem I'm having is that Xen assumes that all
> > physical processors are incrementing the TSCs at
> > the same rate, and complains in the timer ISR that
> > time is going backward if this isn't the case:
> 
> Xen *is* able to cope with TSCs running at different frequencies. It
> calibrates each CPU every few seconds on the assumption that the
> frequencies are basically stable but different. 
> 
> To integrate with PowerNow, after you've changed the 
> frequency you need to run the code to generate a new
> time record and propagate it to the VCPU of the guest
> currently using the CPU. For the frequency field, you
> are best off scaling the frequency measured in the last
> calibration period rather than assuming the new target
> frequency is precise.

Could you give some rough hints on how to do that?  
Nothing in time-xen.c or the published hypercalls looks
more than vaguely related.  All the functions and
hypercalls deal with absolute offsets.

The only reference to a frequency field is in 
vcpu_time_info, but no functions take that as an 
argument.  vcpu_info doesn't have anything either.
What functions calls/hypercalls should I be dealing
with?

-Mark Langsdorf
Operating System Research Center
AMD



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