Re: [Xen-ia64-devel] SMP-guest status

[Tristan Gingold <Tristan.Gingold@xxxxxxxx>]

Le Jeudi 20 Avril 2006 18:51, Alex Williamson a écrit :
> On Thu, 2006-04-20 at 18:26 +0200, Tristan Gingold wrote:
> > Le Jeudi 20 Avril 2006 17:11, Alex Williamson a écrit :
> > > On Thu, 2006-04-20 at 15:22 +0200, Tristan Gingold wrote:
> > > > The change in time.c is just to work around a kernel bug.  Linux
> > > > kernel requires at least an interpolator. [Hence I think there is no
> > > > platform without ITC drift].
> > >
> > >    I'm not sure I'd call it a bug.
> >
> > Here are the facts:  if you boot linux 2.6.16 with ITC_DRIFT bit set,
> > kernel crashes.  That's a bug to me.
>
>   I agree that the kernel shouldn't crash *if* you have an alternate
> time source such as an HPET.  Do you?  The ITC time interpolator is only
> designed for ITCs that are nearly synchronized and do not drift (ie.
> they're driven from the same clock source).  The ITC_DRIFT flag is an
> indication that the ITC are not driven by the same clock source and
> therefore may drift.  If we were to use the ITC interpolator on a system
> that reports ITC_DRIFT we would get sporadic jumps in time depending on
> which CPU's ITC we're using as a reference.
>
> > > The kernel requires some kind of
> > > timesource for an interpolator.  AFAIK, SGI systems are the only ones
> > > that report ITC drift and they have a platform timesource to
> > > compensate.
> >
> >                  ^^^ no ITC drift
> > Thank you for the info.
>
>   No, to be clear, SGI systems are the only systems that I know of that
> set ITC_DRIFT in the sal_platform_features.  Presumably their hardware
> does drive the ITC of the various nodes in the system from different
> clock sources.  They therefore cannot make use of the ITC interpolator.
> To compensate for this, they have a platform specific time interpolator
> registered for SN systems.  All ia64 platforms must provide either
> non-drifting ITCs (as indicated by not setting the ITC_DRIFT flag) or
> provide an alternate time source to be registered as a new time
> interpolator.  I suppose one could argue that a single ITC could be used
> as a timesource in the drifting case, but the overhead in doing this
> would be impractical and the current ITC interpolator is not designed to
> do so.  Thanks,
Sorry, I misread the ITC_DRIFT specification.  Mea culpa.

Tristan.



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