Re: [Qemu-devel] [PATCH v4] s390: diagnose 318 info reset and migration support

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 14.05.19 09:28, David Hildenbrand wrote:
>>>> But that can be tested using the runability information if I am not wrong.
>>>
>>> You mean the cpu level information, right?
> 
> Yes, query-cpu-definition includes for each model runability information
> via "unavailable-features" (valid under the started QEMU machine).
> 
>>>
>>>>
>>>>> and others that we have today.
>>>>>
>>>>> So yes, I think this would be acceptable.  
>>>>
>>>> I guess it is acceptable yes. I doubt anybody uses that many CPUs in
>>>> production either way. But you never know.
>>>
>>> I think that using that many cpus is a more uncommon setup, but I still
>>> think that having to wait for actual failure
>>
>> That can happen all the time today. You can easily say z14 in the xml when 
>> on a zEC12. Only at startup you get the error. The question is really:
> 
> "-smp 248 -cpu host" will no longer work, while e.g. "-smp 248 -cpu z12"
> will work. Actually, even "-smp 248" will no longer work on affected
> machines.
> 
> That is why wonder if it is better to disable the feature and print a
> warning. Similar to CMMA, where want want to tolerate when CMMA is not
> possible in the current environment (huge pages).
> 
> "Diag318 will not be enabled because it is not compatible with more than
> 240 CPUs".
> 
> However, I still think that implementing support for more than one SCLP
> response page is the best solution. Guests will need adaptions for > 240
> CPUs with Diag318, but who cares? Existing setups will continue to work.
> 
> Implementing that SCLP thingy will avoid any warnings and any errors. It
> just works from the QEMU perspective.
> 
> Is implementing this realistic?

Yes it is but it will take time. I will try to get this rolling. To make
progress on the diag318 thing, can we error on startup now and simply
remove that check when when have implemented a larger sccb? If we would
now do all kinds of "change the max number games" would be harder to "fix".



> 
>> do you want to error on definition of the xml or on startup.
> 
> I actually have no idea what the best practice on the libvirt side is.
> There seems to be a user for max-cpus and unavailable-features in QEMU.
> 
> And I think
>> startup is the better place here. This allows to create definitions that will
>> be useful in the future (pre-planning), e.g. if you know that you will update
>> your machine or the code soon.
> 
> 
>