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Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 26 May 2010 19:01:07 +0100
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On 26/05/2010 18:39, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>> It ought to be possible to enhance the "whacking" code to set the TSC
>> based on the topologically nearest live CPU and then sanity-check
>> against all others, repeating a few times to protect against SMIs etc.
> Well obviously firmware can do it pre-boot, but I don't know
> what the impact of the mechanism is on running cpu's.  I'd assume
> that at least all guest activity would have to be stopped for
> some not-so-short period (~10-100msec?)

It depends how physical CPU hotplug is implemented doesn't it. I expect
there's sufficient firmware involved in such an operation that TSCs could
get synced up before host software gets a look in. I don't think we can
comment on whether or not there is an issue here without more information.
Also, one reason Intel pushed the CPU hotplug logic is for RAS, and
offlining CPUs that throw errors, which can clearly be supported with no
concerns over TSC sync.

 -- Keir

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