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xen-devel
Re: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check
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To: |
Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> |
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Subject: |
Re: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check |
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From: |
Jeremy Fitzhardinge <jeremy@xxxxxxxx> |
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Date: |
Wed, 14 Oct 2009 22:24:00 -0700 |
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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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Wed, 14 Oct 2009 22:24:27 -0700 |
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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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