[Qemu-devel] [libvirt] Modern CPU models cannot be used with libvirt

[Itamar Heim]

On 03/12/2012 09:50 PM, Anthony Liguori wrote:
> On 03/12/2012 02:12 PM, Itamar Heim wrote:
>> On 03/12/2012 09:01 PM, Anthony Liguori wrote:
>>>
>>> It's a trade off. From a RAS perspective, it's helpful to have
>>> information about the host available in the guest.
>>>
>>> If you're already exposing a compatible family, exposing the actual
>>> processor seems to be worth the extra effort.
>>
>> only if the entire cluster is (and will be?) identical cpu.
>
> At least in my experience, this isn't unusual.
>
>> or if you don't care about live migration i guess, which could be hte
>> case for
>> clouds, then again, not sure a cloud provider would want to expose the
>> physical
>> cpu to the tenant.
>
> Depends on the type of cloud you're building, I guess.
>
>>>> ovirt allows to set "cpu family" per cluster. assume tomorrow it could
>>>> do it an
>>>> even more granular way.
>>>> it could also do it automatically based on subset of flags on all
>>>> hosts - but
>>>> would it really make sense to expose a set of capabilities which
>>>> doesn't exist
>>>> in the real world (which iiuc, is pretty much aligned with the cpu
>>>> families?),
>>>> that users understand?
>>>
>>> No, I think the lesson we've learned in QEMU (the hard way) is that
>>> exposing a CPU that never existed will cause something to break. Often
>>> times, that something is glibc or GCC which tends to be rather epic in
>>> terms of failure.
>>
>> good to hear - I think this is the important part.
>> so from that perspective, cpu families sounds the right abstraction
>> for general
>> use case to me.
>> for ovirt, could improve on smaller/dynamic subsets of migration
>> domains rather
>> than current clusters
>> and sounds like you would want to see "expose host cpu for non migratable
>> guests, or for identical clusters".
>
> Would it be possible to have a "best available" option in oVirt-engine
> that would assume that all processors are of the same class and fail an
> attempt to add something that's an older class?
>
> I think that most people probably would start with "best available" and
> then after adding a node fails, revisit the decision and start lowering
> the minimum CPU family (I'm assuming that it's possible to modify the
> CPU family over time).

iirc, the original implementation for cpu family was start with an empty 
family, and use the best match from the first host added to the cluster.
not sure if that's still the behavior though.
worth mentioning the cpu families in ovirt have a 'sort' field to allow 
starting from best available.
and you can change the cpu family of a cluster today as well (with some 
validations hosts in the cluster match up)

>
>  From a QEMU perspective, I think that means having per-family CPU
> options and then Alex's '-cpu best'. But presumably it's also necessary
> to be able to figure out in virsh capabilities what '-cpu best' would be.

if sticking to cpu families, updating the config with name/prioirty of 
the families twice a year (or by user) seems good enough to me...

>
> Regards,
>
> Anthony Liguori
>
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