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

From: Christian Borntraeger
Subject: Re: [Qemu-devel] [PATCH v4] s390: diagnose 318 info reset and migration support
Date: Tue, 14 May 2019 11:10:13 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

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 <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
> 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.

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