Re: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 10/14/09 20:26, Dan Magenheimer wrote:
> As long as we are going through the trouble of making
> this monotonic, shouldn't it be monotonically increasing
> (rather than just monotonically non-decreasing)?  The
> rdtsc instruction and any suitably high-precision
> hardware timer will never return the same value
> on subsequent uses so this might be a reasonable
> precedent to obey.  E.g.
>
> +	return ret > xen_clocksource.cycle_last ?
> +		ret : ++xen_clocksource.cycle_last;
>   

No, cycle_last isn't updated on every read, only on timer ticks.  This
test doesn't seem to be intended to make sure that every
clocksource_read is globally monotonic, but just to avoid some boundary
conditions in the timer interrupt.  I just copied it directly from
read_tsc().

    J