Re: [Qemu-devel] [libvirt] Question about the host-model CPU mode

[Christian Borntraeger]

On 10/20/2017 03:43 PM, David Hildenbrand wrote:
> On 20.10.2017 15:36, Christian Borntraeger wrote:
>>
>>
>> On 10/20/2017 03:16 PM, David Hildenbrand wrote:
>>>
>>>> Hi all,
>>>>
>>>> we recently encountered the problem that the 'host-model' [1] has to be
>>>> related to the machine type of a domain. We have following problem:
>>>>
>>>>    Let's assume we've a z13 system with a QEMU 2.9 and we define a
>>>>    domain using the default s390-virtio-ccw machine together with the
>>>>    host-model CPU mode [1]. The definition will have the machine
>>>>    expanded to s390-virtio-ccw-2.9 but retain the host-model CPU mode
>>>>    in the domain definition. In a next step we upgrade to QEMU 2.10
>>>>    (first version to recognize z14). Everything is still fine, even
>>>>    though the machine runs in 2.9 compatibility mode. Finally we
>>>>    upgrade to a z14. As a consequence it is not possible to start the
>>>>    domain anymore as the machine type doesn't support our CPU host
>>>>    model (which is expanded at start time of the domain).
>>>
>>> Actually, what is the cause of that problem? I assume it is the gs
>>> feature (gs_allowed)?
>>>
>>> We should really avoid such things (..._allowed) for CPU model features
>>> in the future and clue all new such stuff to cpumodel_allowed.
>>
>> Yes, starting a guest with
>>    <os>
>>     <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type>
>>   </os>
>>   <cpu mode='host-model'/>
>>
>> results in
>>
>> qemu-system-s390x: Some features requested in the CPU model are not 
>> available in the configuration: gs 
>>
>> Tying it to cpumodel_allowed would not help, migration-wise though.
>> libvirt would still transform
>>
>>    <os>
>>     <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type>
>>   </os>
>>   <cpu mode='host-model'/>
> 
> My point was, that the host model would have to be copied and _remain_
> there when s390-ccw-virtio was expanded to s390-ccw-virtio-2.9.
> 
> So really replacing <cpu mode='host-model'/> by the model z13-base....
> This would at least fix this issue. Just like s390-ccw-virtio get's
> replaced and remains thats way.
> 
> But this might for sure have other problems.

The problem goes much further.
A fresh guest with

    <os>
     <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type>
   </os>
   <cpu mode='host-model'/>

does not start. No migration from an older system is necessary.