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

[Juergen Gross <jgross@xxxxxxxx>]

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.


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel