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

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Date:   Mon, 11 Jun 2018 11:23:27 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...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
Subject: Re: [PATCH v5 11/13] KVM: s390: implement mediated device open
 callback

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.

Regards,

Pierre

>
>>
>>
>>
>

-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany