Re: [PATCH v5 11/13] KVM: s390: implement mediated device open callback

Tony Krowiak
2018-06-11T12:52:05Z

On 06/11/2018 07:32 AM, Halil Pasic wrote:
>
>
> On 06/11/2018 11:23 AM, Pierre Morel wrote:
>> On 08/06/2018 23:59, Tony Krowiak wrote:
>>> On 06/07/2018 01:15 PM, Pierre Morel wrote:
>>>>
>>
>> ...snip...
>>
>>>>>>
>>>>>>>
>>>>>>> Why maintain a list of kvm_ap_matrix structures if we don't have
>>>>>>> to; it is stored
>>>>>>> with the mediated matrix device which is passed in to all of the
>>>>>>> vfio_ap driver
>>>>>>> callbacks.
>>>>>>
>>>>>> Because using the vm_list which is a static in kvm makes you
>>>>>> stick inside the kvm code.
>>>
>>> I understand your point here, but even if we did maintain a list of
>>> kvm_ap_matrix structures,
>>> we still need the kvm code to configure the guest's CRYCB and
>>> eventually ECA.28. There is
>>> also code in kvm-ap.c that is called from KVM.
>>
>> The only code from kvm-ap which is called from KVM is temporary code
>> waiting for Harald to offer the clean interface to AP instructions.
>>
>>> The idea behind kvm-ap.c is that all code
>>> related to configuration of AP structures in KVM is in this one spot.
>>
>> This I understand, but the code can be in one spot inside VFIO_AP
>> instead
>> of inside KVM.
>> Putting the code inside KVM induce dependencies between KVM and AP
>> while the kvm/vfio interface allows to avoid this dependency.
>>
>> The purpose of VFIO_AP is to handle the CRYCB, all get/clear/set
>> crycb masks
>> functions should be in VFIO AP.
>>
>> If we use wrappers in KVM, since the CRYCB is an a SIE extension,
>> it is legitimate, the KVM interface to the CRYCB should only
>> handle bitmaps and be unaware of the vfio_ap internal structures.
>>
>>
>> Another concern, the kvm_ap_validate_queue_sharing() should not be
>> inside KVM because it is a decision of current VFIO_AP driver
>> to not share the queues between guest of level 2.
>>
>> The Z architecture does not allow to share AP queues between
>> guests of level 1 but we could re-engineer the AP bus and the '
>> VFIO AP to offer queue sharing for guest level 2.
>>
>> This would be a new VFIO_AP driver (and an AP bus extension).
>> We should not have to change KVM for this.
>>
>
>
> Pierre's proposal makes a lot of sense to me. We would not need to take
> the kvm_lock (which we need to traverse the vm_list safely) for the
> validation, and we could have immediate validation (which is in my
> opinion
> better).
>
> Also your refcount (which is not a refcout) could go away. You simply
> traverse your list and check for duplicates when hooking up the mdev
> with KVM.
>
> And my opinion is if we don't have to add code to the kvm module we
> better not.
>
> @Janosch: Does core KVM share my opinion?

Okay, I'll make the change.

>
>
> Regards,
> Halil