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

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)?

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.


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