[Xen-devel] RE: [PATCH] strictly increasing hvm guest time

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

> > This simple one-line patch changes hvm guest time from
> > monotonically non-decreasing to monotonically strictly-
> > increasing.  As a result, two consecutive reads of the
> > (virtual) hpet will never return the same value, thus
> > avoiding the appearance that time has stopped (which may
> > occur if there is skew between physical processor TSCs).
> 
> It does seem a little hack-ish, if we don't know of any 
> issues arising from
> the current code, and we expect cross-cpu deltas to be pretty 
> small.

Using "xm debug-key t; xm dmesg | tail -1" you can get an idea
of the deltas.  Even on my single-socket dual-core recent-vintage
Intel box, I'm frequently seeing Diff's > 300ns.  While this
is still relatively small (and part of it may be SMP cache
synchronization time), this is supposed to be a "good TSC"
box.

I'm spinning a small patch capturing the maximum so that can
be output via debug-key t also.

> Also
> guests will often convert HPET reads to well-known units (e.g.,
> microseconds, milliseconds) before using them, in which case 
> even a delta of
> one may not result in differing converted time values.

Yes, but most newer Linux systems have a high-res timer API
that returns nanoseconds, though admittedly it is not widely
used yet.

Thanks,
Dan



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