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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/07/15 05:43, Juergen Gross wrote:
> On 07/22/2015 04:44 PM, Boris Ostrovsky wrote:
>> On 07/22/2015 10:09 AM, Juergen Gross wrote:
>>> On 07/22/2015 03:58 PM, Boris Ostrovsky wrote:
>>>> 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.
>>
>> Why do you think that kernel patches are preferable to CPUID management?
>> This would be all in tools, I'd think. (Well, one problem that I can
>> think of is that AMD sometimes pokes at MSRs and/or Northbridge's PCI
>> registers to figure out nodeID --- that we may need to have to address
>> in the hypervisor)
>
> Doing it via CPUID is more HW specific. Trying to fake a topology for
> the guest from outside might lead to weird decisions in the guest e.g.
> regarding licenses based on socket counts.
>
> If you are doing it in the guest itself you are able to address the
> different problems (scheduling, licensing) in different ways.
>
>> And those patches won't help HVM guests, will they? How would they be
>> useful by user processes?
>
> HVM can use pv interfaces as well. It's called pv-NUMA :-)
>
> Hmm, I didn't think of user processes. Are you aware of cases where they
> are to be considered? The only case where user processes are involved I
> could think of is licensing again. Depending on the licensing model
> playing with CPUID is either good or bad. I can even imagine the CPUID
> configuration capabilities in xl are in use today for exactly this
> purpose. Using them for pv-NUMA as well will make this feature unusable
> for those users.

Userspace can also use things like hwloc which use cpuid to calculate
efficient allocation of resources, rather than for licensing purposes.

~Andrew

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