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[Xen-users] Re: [Xen-devel] Xen 4 TSC problems

To: Jan Beulich <JBeulich@xxxxxxxxxx>, Olivier Hanesse <olivier.hanesse@xxxxxxxxx>
Subject: [Xen-users] Re: [Xen-devel] Xen 4 TSC problems
From: Keir Fraser <keir@xxxxxxx>
Date: Thu, 24 Feb 2011 11:30:12 +0000
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Xen Users <xen-users@xxxxxxxxxxxxxxxxxxx>, Mark Adams <mark@xxxxxxxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] Xen 4 TSC problems
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On 24/02/2011 10:59, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:

>>>> On 24.02.11 at 10:59, Olivier Hanesse <olivier.hanesse@xxxxxxxxx> wrote:
>> (XEN) Platform timer appears to have unexpectedly wrapped 10 or more times.
>> Output of xm debug-key s :
>> (XEN) TSC has constant rate, deep Cstates possible, so not reliable,
>> warp=2684 (count=4)
> Did you try turning of use of C states ("cpuidle=0" on the Xen
> command line)?

Another thing to try is changing the platform timer that Xen uses. It's
using HPET on your machines, so try clocksource=pit on Xen command line, and
confirm that the 'Platform timer is xxx' message changes in xm dmesg.

However, this bug looks more like a CPU's TSC jumping forward (or maybe
backward) for some inexplicable reason. We added code post 3.2 to detect the
platform timer counter wrapping, and to account for that based on trusting
the CPU's 64-bit TSC. But if the TSC value is bogus then we can detect a
wrap when it didn't happen and the new code will do more harm than good. It
is not currently possible to disable the code via a boto parameter -- maybe
we could add that. However, if the problem is a jumpy TSC then it is better
to fix that as Xen relies so heavily on TSC for time handling.

 -- Keir

> Jan

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