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Re: [Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest

On Tue, 2015-07-21 at 16:00 -0400, Boris Ostrovsky wrote:
> On 07/20/2015 10:43 AM, Boris Ostrovsky wrote:
> > On 07/20/2015 10:09 AM, Dario Faggioli wrote:

> > I'll need to see how LLC IDs are calculated, probably also from some 
> > CPUID bits.
> No, can't do this: LLC is calculated from CPUID leaf 4 (on Intel) which 
> use indexes in ECX register and xl syntax doesn't allow you to override 
> CPUIDs for such leaves.
Right. Which leaves us with the question of what should we do and/or
recommend users to do?

If there were a workaround that we could put in place, and document
somewhere, however tricky it was, I'd say to go for it, and call it
acceptable for now.

But, if there isn't, should we disable PV vnuma, or warn the user that
he may see issues? Can we identify, in Xen or in toolstack, whether an
host topology will be problematic, and disable/warn in those cases too?

I'm not sure, honestly. Disabling looks too aggressive, but it's an
issue I wouldn't like an user to be facing, without at least being
informed of the possibility... so, perhaps a (set of) warning(s)?

<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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