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

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Date:	Thu, 15 Oct 2009 12:19:22 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Xen-devel <xen-devel@...ts.xensource.com>, kurt.hackel@...cle.com,
	arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.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 10/15/09 06:27, Dan Magenheimer wrote:
> I understand but you are now essentially emulating a
> reliable platform timer with a potentially unreliable
> (but still high resolution) per-CPU timer AND probably
> delivering that result to userland.
>
> Read_tsc should only be used if either CONSTANT_TSC
> or TSC_RELIABLE is true, so read_tsc is guaranteed
> to be monotonically-strictly-increasing by hardware
> (and enforced for CONSTANT_TSC by check_tsc_warp
> at boot).
>   

read_tsc clearly isn't expected to produce absolutely globally monotonic
results; if it were then the check wouldn't be necessary.

    J
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