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Re: kern/45626

The following reply was made to PR kern/45626; it has been noted by GNATS.

From: Simon Nicolussi <>
Cc: Bernd Ernesti <>,
        Manuel Bouyer <>,
        Christos Zoulas <>
Subject: Re: kern/45626
Date: Wed, 25 Jun 2014 00:52:27 +0200

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 It has been a while since this report has last been touched, but maybe
 someone's still interested in my findings:
 I was seeing the same issue on an IBM ThinkCentre and didn't spot any
 obvious errors in the code where the TSC frequency is determined, but
 noticed that i8254_delay is sometimes taking some fraction of a second
 longer than it should take (as reflected in the TSC), despite waiting
 the correct number of i8254 ticks.
 SMM is the first thing that comes to mind as the reason for such weird
 behaviour. If a system receives SMIs right when executing i8254_delay,
 it might cause the two counters (i8254 and TSC) to diverge. One common
 application of SMM is USB keyboard support for real-mode operation [1]
 (e.g., for the BIOS). I'm fortunate enough to have a knob for that in
 my BIOS, so I disabled USB support there and, lo and behold, the bug
 no longer occurs.
 Simon Nicolussi, <>
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