Re: [Qemu-devel] [PATCH/QEMU] s390x/kvm: use cpu_model_available for guarded storage on compat machines

[Marc Hartmayer <mhartmay@linux.vnet.ibm.com>]

On Wed, Oct 25, 2017 at 05:50 PM +0200, David Hildenbrand <david@redhat.com> wrote:
> On 25.10.2017 17:09, Boris Fiuczynski wrote:
>> On 10/25/2017 12:23 PM, David Hildenbrand wrote:
>>> On 25.10.2017 12:18, Christian Borntraeger wrote:
>>>> Ping, I plan to submit belows patch for 2.11. We can then still look into
>>>> a libvirt<->qemu interface for limiting host-model depending on machine versions
>>>> (or not).
>>>
>>> I think this would be sufficient for now.
>>>
>>> Having different host models, depending on the the machine type sounds
>>> wrong. But maybe we'll need it in the future.
>>>
>>
>> David, I disagree if your proposal is to generally tolerate new cpu
>> features in old machine types. This *might* work for gs but how do you
>> guaranty that guests do not behave differently/wrong when suddenly new
>> cpu features are made available to guest when (re-)starting them.
>> That is my feedback for the qemu side of this mater.
>
> Just re-reading this section, so what you mean is:
>
> a) guest is started, host model is copied and used. guest works.
> b) guest is stopped.
> c) QEMU/KVM/HW is updated.
> d) guest is started, new host model is copied. guest no longer works.
>
> d) could be because there are now some additional features with e.g.
> broken guest implementation or now missing features.
>
>
> What you propose (if I am not wrong) is a to bind features somehow to a
> QEMU machine. I think that should never be done. You could not catch now
> missing features.

What exactly do you mean by the last sentence?

>
> What would you think about a persistent host-model copy option? So
> instead of copying at every start, only copy and replace it once in the XML?
>
> Easy to specify by the user and no CPU feature changes will happend
> "blindly".
>
>
> --
>
> Thanks,
>
> David
>
--
Beste Grüße / Kind regards
   Marc Hartmayer

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