Re: [PATCH v2 2/2] s390/kvm: handle diagnose 318 instruction call

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On 13.12.18 11:21, Cornelia Huck wrote:
> On Thu, 13 Dec 2018 10:12:49 +0100
> David Hildenbrand <david@xxxxxxxxxx> wrote:
> 
>> On 12.12.18 14:08, Cornelia Huck wrote:
>>> On Wed, 12 Dec 2018 11:49:56 +0100
>>> David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>   
>>>> I am wondering if another way to handle this would maybe be even better.
>>>> It would allow also new QEMU on old KVM to get access to these values.
>>>> It will simply not be written to HW then (bad luck, there is no HW
>>>> support eventually)
>>>>
>>>>
>>>> It goes like this:
>>>>
>>>> 1. don't implement diag318 in the kernel, implement it in user space.
>>>> diag318 is already forwarded to QEMU as of now. (I don't consider
>>>> diag318 peformance relevant)
>>>>
>>>> 2. in user space, we can always support diag318, even without KVM/HW
>>>> support. (could glue to machines if really needed)
>>>>
>>>> 3. If we get a diag318 and the CPU feature is not enabled, handle it if
>>>> diag318 is not available (exception, just as we do now).
>>>>
>>>> 3. If we get a diag318 and the CPU feature is enabled, store both values
>>>> in QEMU (for migration and e.g. DUMPs) AND ...
>>>>
>>>> 4. ... if KVM supports KVM_S390_VM_MACHINE_CPC, also write the new
>>>> values (or even only the CPNC! ) to KVM.
>>>>
>>>> 5. During migration, if KVM supports KVM_S390_VM_MACHINE_CPC, also write
>>>> the migrated value to KVM.  
>>>
>>> What about that sclp thingie (I'm not quite sure what it signifies)?  
>>
>> You mean the indication of diag318? Well we might want to check in KVM
>> if the host has it (and therefore the CPNC in the SCB actually gets
>> used) and only then allow to set the CPNC. Or we could simply always
>> allow to write it to the SCB even though the value won't get used by HW.
> 
> Maybe always allow to write it, for simplicity's sake? Does it matter
> much whether it will get used by HW?
> 

Usually it does not matter. I remember that there were some fields that
should not be touched if the feature is not available (otherwise you
might get undefined behavior).

-- 

Thanks,

David / dhildenb



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