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Re: [Xen-devel] [PATCH] Disable Xen PowerNow! support on Opteron 2nd gen

To: "Langsdorf, Mark" <mark.langsdorf@xxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Disable Xen PowerNow! support on Opteron 2nd gen and earlier processors
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Sat, 02 Feb 2008 10:34:06 +0000
Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] [PATCH] Disable Xen PowerNow! support on Opteron 2nd gen and earlier processors
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What are the deltas when time goes backwards? Bear in mind we already
default to being silent for backwards deltas up to 10ms. You could just
fudge that a bit bigger if the powernow module is loaded. But if the deltas
you see are much bigger than 10ms then yes, there may be an issue to be

The issue here could well be that if you take a timer interrupt towards the
end of a frequency change but before Xen has been told that the frequency
has changed, the time values that Xen and dom0 calculate will be off by
potentially quite a bit. It's a small window of opportunity, but clearly you
hit it a few times each day. I'm not sure how easy it is to close off that
window cleanly with the MSR twiddling being controlled from dom0...

 -- Keir

On 1/2/08 16:39, "Langsdorf, Mark" <mark.langsdorf@xxxxxxx> wrote:

>> Changeset 16927 should fix the problem for you.
> I'm getting about 5 "Time went backwards" messages a day right
> now, which is a drastic improvement but still more than I'd
> like.  In three instances, the shadow time value does not
> increase.
> The weird thing is the cpu time values are always of the form:
> 0: xxxxxx7444952
> 1: yyyyyy7447746
> 2: yyyyyy7447746
> 3: yyyyyy7447746
> the y values are usually but not always the same as x or
> slightly higher.  When they are different, they differ
> by 1000000, 2000000, or 3000000.
> I suspect it is related to the fact that platform calibration
> is only performed if smp_processor_id() == 0, but I can't
> figure out why this is occurring.
> -Mark Langsdorf
> Operating System Research Center

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