Re: [Qemu-devel] [qemu-s390x] [PATCH v4] s390: diagnose 318 info reset and migration support

[Collin Walling <walling@linux.ibm.com>]

On 5/14/19 5:30 AM, Cornelia Huck wrote:
> On Tue, 14 May 2019 11:27:32 +0200
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 14.05.19 11:25, Christian Borntraeger wrote:
>>>
>>>
>>> On 14.05.19 11:23, Christian Borntraeger wrote:
>>>>
>>>>
>>>> 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:
> 
>>>>>>> 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
>>>>>>>b
>>>>>>> 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"
>>>
>>> So yes, variant 2 when I use your numbering. The only question is: do we need
>>> a warning? It probably does not hurt.
>>
>> After all, we are talking about 1 VCPU that the guest can only use by
>> indirect probing ... I leave that up to Collin :)
> 
> I'd prefer a warning... even if it is a corner case, I think it's
> better to be explicit instead of silent.
>

Thanks for the discussion. I'll implement diag318 as a CPU feature again
and adjust the max_cpu handling (to 247 now instead of 240).

Question: should diag318 be enabled by default for current CPU models?
What should we do in the case where diag318=on but KVM does not support
handling?

I am not familiar with how close QEMU and KVM versions stay in sync.