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

From: David Hildenbrand
Subject: Re: [Qemu-devel] [PATCH v4] s390: diagnose 318 info reset and migration support
Date: Tue, 14 May 2019 10:59:20 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

On 14.05.19 10:49, Cornelia Huck wrote:
> On Tue, 14 May 2019 10:37:32 +0200
> Christian Borntraeger <address@hidden> 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


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.



David / dhildenb

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