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

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Message-ID: <4AD6B1F0.6030904@goop.org>
Date:	Wed, 14 Oct 2009 22:24:00 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	kurt.hackel@...cle.com, arch/x86 maintainers <x86@...nel.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Avi Kivity <avi@...hat.com>, chris.mason@...cle.com
Subject: Re: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check

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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