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

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 14 Oct 2009 22:24:00 -0700
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, kurt.hackel@xxxxxxxxxx, arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Glauber de Oliveira Costa <gcosta@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, chris.mason@xxxxxxxxxx
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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

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