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Re: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs u

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits
From: Keir Fraser <keir.xen@xxxxxxxxx>
Date: Fri, 15 Apr 2011 18:28:09 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "winston.l.wang" <winston.l.wang@xxxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits
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On 15/04/2011 15:34, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>>> Agreed.  In fact, maybe it should be asserted in write_tsc?
>> We still write_tsc on CPU physical hot-add.
> Hmmm... IIRC the testing that Intel was doing for hot-add was
> not for processors that were actually electrically hot-plugged
> but only for processors that were powered-on at the same
> time as all other processors but left offline until needed
> (e.g. for capacity-on-demand).  For this situation, writing
> to tsc is still the wrong approach.  I don't think we finished
> the discussion about electrically hot-plugged processors
> because they didn't exist... don't know if they do yet either.
> IIRC I had proposed an unnamed boot parameter that said
> "this machine may add unsynchronized processors post-boot"
> and disallow hot-add processors if not specified (or if
> not specified AND a run-time check of a hot-add processor
> shows non-synchronization).

Well, I think the case I'm thinking of is electrical hot-plug. Not sure.
Either way I doubt anyone is actually using the feature.

 -- Keir

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