[libvirt] [PATCH/QEMU] s390x/kvm: use cpu_model_available for guarded storage on compat machines

[Boris Fiuczynski]

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.

Regarding the libvirt side of this:
When looking at https://libvirt.org/formatdomain.html#elementsCPU I 
found the following sentence:
Since the CPU definition is copied just before starting a domain, 
exactly the same XML can be used on different hosts while still 
providing the best guest CPU each host supports.

My interpretation of "the best guest CPU each host supports" is that 
besides limiting factors like hardware, kernel and qemu capabilities the 
requested machine type for the guest is a limiting factor as well.

Nevertheless if my interpretation is found to be incorrect than we 
should think about another new cpu mode that includes the machine type 
into the "best guest CPU" detection.
My assumption is that we must not require the users to know which cpu 
model they manually need to define to match a specific machine type
AND we want to guarantee that guests run without risking any side 
effects by tolerating any additional cpu features.

-- 
Mit freundlichen Grüßen/Kind regards
    Boris Fiuczynski

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