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

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

On 14.05.19 11:20, David Hildenbrand wrote:
> On 14.05.19 11:10, Christian Borntraeger wrote:
>>
>>
>> On 14.05.19 10:59, David Hildenbrand wrote:
>>> On 14.05.19 10:49, Cornelia Huck wrote:
>>>> On Tue, 14 May 2019 10:37:32 +0200
>>>> Christian Borntraeger <borntraeger@de.ibm.com> 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".
>>>>
>>>> So, the idea right now is:
>>>>
>>>> - fail to start if you try to specify a diag318 device and more than
>>>>   240 cpus (do we need a knob to turn off the device?)
>>>> - in the future, support more than one SCLP response page
>>>>
>>>> I'm getting a bit lost in the discussion; but the above sounds
>>>> reasonable to me.
>>>>
>>>
>>> We can
>>>
>>> 1. Fail to start with #cpus > 240 when diag318=on
>>> 2. Remove the error once we support more than one SCLP response page
>>>
>>> Or
>>>
>>> 1. Allow to start with #cpus > 240 when diag318=on, but indicate only
>>>    240 CPUs via SCLP
>>> 2. Print a warning
>>> 3. Remove the restriction and the warning once we support more than one
>>>    SCLP response page
>>>
>>> While I prefer the second approach (similar to defining zPCI devices
>>> without zpci=on), I could also live with the first approach.
>>
>> I prefer approach 1.
>>
> 
> Isn't approach #2 what we discussed (limiting sclp, but of course to 247
> CPUs), but with an additional warning? I'm confused.

Different numbering interpretion. I was talking about 1 = "Allow to start with #cpus > 240 when diag318=on, but indicate only
240 CPUs via SCLP"