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

[Collin Walling]

On 12/5/18 6:57 AM, David Hildenbrand wrote:
> On 05.12.18 12:54, Cornelia Huck wrote:
>> On Wed, 5 Dec 2018 09:27:44 +0100
>> Christian Borntraeger <address@hidden> 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.
> 

Are you referring to the struct CPUS390XState? Could we get away by adding
a one-byte "unused" field after the bitmap?

>>
>>>>
>>>> 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 <address@hidden>
>>>>> ---
> 
> 

I'll split the patches into:

1) linux-header changes
2) CPU model changes
3) diag318 usages / migration

-- 
Respectfully,
- Collin Walling