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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>
Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 26 May 2010 16:45:49 +0100
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On 26/05/2010 16:19, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Much of the TSC-based time infrastructure in Xen,
> especially as exposed to guests, is rather sensitive
> to sudden dramatic differences in TSC values between
> physical processors.  Hot-add of physical CPUs will
> introduce a huge difference.

True at the moment, but can we not just whack the TSC of the newly added CPU
on the head when it is brought online, to match the boot CPU? I think that
would suffice for systems with 'reliable tsc' which are the only ones we
don't emulate tsc by default?

 -- Keir

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