RE: Xen system skew MUCH worse than tsc skew (was RE: [Xen-devel] RE: [PATCH] record max stime skew (was RE: [PATCH] strictly increasing hvm guest time))

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

> > 1) by a boot option, or
> > 2) by the TSC_CONSTANT cpu flag, or
> > 3) when determined dynamically to be safe using code similar
> >    to arch/x86/tsc_sync.c in recent Linux kernels
> >
> > (1) is by far the easiest (perhaps not too late for 3.3?)
> > while (3) is clearly the best for users but adds lots of
> > code (bloat/untested)
> 
> (1) is perhaps fine.

OK, patch to follow.  I've used "clocksource=tsc"
 
> How does (2) work? The individual CPUs do not know whether they are
> synchronised across the mainboard. I think constant-tsc is necessary
> (individual CPUs must not vary their multiplier of the input 
> clock rate) but
> may not be sufficient.

Good point.

> I don't know how much code is involved in (3).

It's enough that I will take the "easy way" for now (boot option)
and look at submitting a dynamically-evaluate patch later.

Dan


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