Re: [Xen-ia64-devel] SMP guest and itc

[Alex Williamson <alex.williamson@xxxxxx>]

On Mon, 2006-02-13 at 09:33 +0100, Tristan Gingold wrote:
> 
> > That said, I agree that Xen needs to pre-synchronize ITC
> > for host SMP to work properly.  I don't think guest SMP
> > for Linux will force ITC to be paravirtualized, but
> > I guess Tristan will let us know if he discovers otherwise.
> Currently this is my option: Xen pre-synchronize ITC and no more ITC 
> virtualization is performed.

   Thinking out loud... I wonder if it would be a better long term
approach to report the ITC as having drift to guest domains and
introduce a paravirtualized xen time interpolator.  I'm worried that if
the ITC is fully virtualized, things like gettimeofday() on an SMP guest
could cause lots of ring crossings.  Using a time interpolator, we might
be able to let any jitter protection or contention algorithms happen in
xen and get a little more efficiency.  At some point we'll need to run
on a system where the ITCs drift so we can't count on pre-sync'd ITCs as
the only long term solutions.  Thanks,

        Alex

-- 
Alex Williamson                             HP Linux & Open Source Lab


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