Re: [Xen-devel] What is the target CPU "topology" of an SMP HVM machine?

[Matt Wilson <msw@xxxxxxxxxx>]

On Wed, Aug 14, 2013 at 10:52:54PM +0200, Dario Faggioli wrote:
> On mer, 2013-08-14 at 21:06 +0100, Andrew Cooper wrote:
> > On 14/08/13 20:23, Eric Shelton wrote:
> > > So, seeing as this information is being closely interrogated, what is
> > > the target virtual CPU topology?  How should this be reported via
> > > CPUID and MSR?  Darwin appears to be trying to determine or take into
> > > account things such as a number of packages, dies per package, cores
> > > per pie & package, and threads/logical CPUs per core & package; the
> > > degrees of sharing of caches by CPUs at various cache levels, and the
> > > presence of hyperthreading.
> > 
> > Xen by default advertises all VCPUs as separate sockets, to try and
> > dissuade "clever" schedulers from doing dumb things based on false
> > information.
> > 
> Are we absolutely sure about this? I'm asking because Elena run into a
> similar issue, i.e., seeing some vCPUs being advertised as
> threads/siblings (although that was a pv-guest)... Am I right Elena?

Yes, this is the current behavior when we set up HVM guests. My guest
NUMA patch for HVM guests adds the necessary features to adjust the
initial local APIC ID so that CPU topology enumeration works.

We should figure out how we want topology to be presented to PV
guests.

--msw

> I think she also has a patch that she may be able to share soon, which
> does right the masking of some of the CPUID stuff, as it looks like some
> false information was reaching out to the Linux Scheduler! :-O
> 
> I'm not sure this is the exact same issue, though.... Elena, could you
> tell something more about this?

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