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

[Cornelia Huck]

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.