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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] [RFC] Physical hot-add cpus and TSC
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 26 May 2010 09:44:15 -0700 (PDT)
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> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC
> 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?

Possibly... but the code for whacking the TSC of a CPU after
C3-state results in a TSC value that is poorly-aligned with other
running TSCs.  If there is a better way for "whacking" that
results in a nearly-perfectly-aligned TSC (that would pass
a "tsc warp test"), that is an option.

> I think that would suffice for systems with 'reliable tsc'
> which are the only ones we don't emulate tsc by default?

Yes, I'm particularly concerned with hot-add-physical-cpu
on any latest generation QPI/HT boxes where Invariant TSC
is set.

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