Re: [PATCH v3 1/2] s390/setup: diag318: remove bit check and refactor struct

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

On 03.04.19 14:09, David Hildenbrand wrote:
> On 03.04.19 13:28, Christian Borntraeger wrote:
>>
>>
>> On 02.04.19 19:46, Collin Walling wrote:
>>> Execution of DIAGNOSE 0x318 is fenced by checking an SCLP bit
>>> for the availability of hardware support for the instruction.
>>>
>>> In order to support this instruction for a KVM/QEMU guest, we
>>> would need to provide modifications to the SCLP Read SCP Info
>>> data, which will in turn reduce the maximum number of CPUs that
>>> may be provided to the guest. This issue introduces compatability
>>> and legacy concerns.
>>>
>>> Let's circumvent this issue by removing the bit check and blindly
>>> executing the instruction. An exception table rule is in place to
>>> catch the case where hardware does not support this instruction.
>>
>>
>> No, please keep the check. We have to extend the read scp field anyway
>> for future extensions.
> 
> Wasn't there already an SCLP-way of telling the guest that the read-scp
> info response is bigger than 4k? Somehow rings a bell ...

Yes, that would be a future extension (the Linux guest does not support this).
Until then we probably have to go back to a smaller cpu number (e.g. 240 for 
machine type 4.1 and newer).