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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/22/2015 09:50 AM, Juergen Gross wrote:
> On 07/22/2015 03:36 PM, Dario Faggioli wrote:
> > 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)?
> > Thoughts?
> I think we have 2 possible solutions:
>
> 1. Try to handle this all in the hypervisor via CPUID mangling.
>
> 2. Add PV-topology support to the guest and indicate this capability via
>    elfnote; only enable PV-numa if this note is present.
>
> I'd prefer the second solution. If you are okay with this, I'd try to do
> some patches for the pvops kernel.
What if I configure a guest to follow HW topology? I.e. I pin VCPUs to 
appropriate cores/threads? With elfnote I am stuck with disabled topology.
Besides, this is not necessarily a NUMA-only issue, it's a scheduling 
one (inside the guest) as well.
-boris

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