[Xen-devel] RE: [PATCH] clocksource=tsc

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

> > Hmmm... One thing I was trying to do with the special
> > casing in get_s_time() was to avoid using a dynamically
> > changing scaling (t->tsc_scale).  If I'm not mistaken,
> > t->tsc_scale is recalculated every EPOCH and thus is
> > a potential source of stime jitter.  Seems unnecessary
> > just to make the code look cleaner.
> 
> My patch disables the per-epoch calibration.

OK, then I think it is the calls to calibrate_tsc_ap() that
result in different values of tsc_scale (potentially a different
one for every processor depending on the precision of the
calibration).  I still think it would be clearer and safer
to always use a fixed tsc_scale value.

> Actually in this mode of operation we hardly need a platform 
> timer *at all*.
> The idea is that we let the TSCs free-run, because we know 
> they will behave.
> Returning to 32-bit read_counter(), and having NULL read_counter when
> clocksource=tsc would be another possibility...

That's essentially what the original tscstable.patch did, though
I was perhaps much uglier in the miscellaneous parts.

Thanks,
Dan


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