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

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Message-Id: <4f5f17f1-48ab-6f1e-6ef2-3741fc39e5f4@linux.ibm.com>
Date:   Mon, 11 Jun 2018 08:50:49 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>, pmorel@...ux.ibm.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com,
        Janosch Frank <frankja@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 11/13] KVM: s390: implement mediated device open
 callback

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