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

[David Hildenbrand <david@redhat.com>]

On 14.05.19 10:56, Christian Borntraeger wrote:
> 
> 
> On 14.05.19 10:50, David Hildenbrand wrote:
>> On 14.05.19 10:37, Christian Borntraeger wrote:
>>>
>>>
>>> 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".
>>
>>
>> Another idea for temporary handling: Simply only indicate 240 CPUs to
>> the guest if the response does not fit into a page. Once we have that
>> SCLP thingy, this will be fixed. Guest migration back and forth should
>> work, as the VCPUs are fully functional (and initially always stopped),
>> the guest will simply not be able to detect them via SCLP when booting
>> up, and therefore not use them.
> 
> Yes, that looks like a good temporary solution. In fact if the guest relies
> on simply probing it could even make use of the additional CPUs. Its just
> the sclp response that is limited to 240 (or make it 247?)

I think the limiting factor was more than a single CPU, but I don't
recall. We can do the math again and come up with the right number.

-- 

Thanks,

David / dhildenb