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

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Keir Fraser [mailto:keir.fraser@eu.citrix.com]
> Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC
> 
> On 26/05/2010 16:19, "Dan Magenheimer" <dan.magenheimer@oracle.com>
> 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.