Re: [Qemu-devel] [qemu-s390x] [PATCH v1] s390: guest support for diagnose 318 and limit max VCPUs to 247

[David Hildenbrand <david@redhat.com>]

On 05.12.18 12:54, Cornelia Huck wrote:
> On Wed, 5 Dec 2018 09:27:44 +0100
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> 
>> On 05.12.2018 09:26, David Hildenbrand wrote:
>>> On 04.12.18 23:18, Collin Walling wrote:  
>>>> Add migration and reset support for diagnose 318. This is a new z14 GA2 
>>>> hardware feature, but we can provide guest support starting with the 
>>>> zEC12-full CPU model.
>>>>
>>>> Because new hardware introduces a new facility-availability byte in 
>>>> the Read SCP Info block, we lose one byte in the CPU entries list 
>>>> and must limit the maximum VCPUs to 247 (down from 248).  
>>>
>>> This could break setups that upgrade/migrate. At least forward migration
>>> can be broken. Do we care about that?  
>>
>> Can we maybe bind this feature and the cpu limit to the 4.0 machine?
> 
> I think that would make sense.

Won't be that easy, as we'll have different sizes of the struct size.
Something that might uglify the code quite a bit.

Also we'll get a dependence between the cpu model features (e.g. probed
via the NULL machine) and the compat machine. Have to think about this.

> 
>>>
>>> Can you split off
>>>
>>> 1. linux-header changes
>>> 2. CPU model changes? (introduction and definition of new feature, but
>>> not when it is used?)
>>>   
>>>>
>>>> Signed-off-by: Collin Walling <walling@linux.ibm.com>
>>>> ---


-- 

Thanks,

David / dhildenb