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

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Message-Id: <f1cc0d71-d2ff-6461-c450-07cc73a2973c@linux.ibm.com>
Date:   Fri, 8 Jun 2018 17:59:17 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     pmorel@...ux.ibm.com, Tony Krowiak <akrowiak@...ux.vnet.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 06/07/2018 01:15 PM, Pierre Morel wrote:
>
>>>
>>>
>>> We have internal structures like the ap_matrix and kvm_ap_matrix
>>> which look like the bus/devices we had previously but are differently
>>> or not at all integrated with the LDD.
>>
>> What is LDD? Are you talking about dependencies between the vfio_ap 
>> device
>> driver and KVM? If so, see my arguments below.
>>
>>>
>>>
>>> Also I think that with a little data structure refactoring you can 
>>> avoid most of
>>> the code in the arch/s390/kvm.
>>
>> How will structure refactoring help us avoid the code for updating 
>> the CRYCB
>> in the guest's SIE state description.
>>
>>>
>>>
>>> For example, storing the kvm pointer inside the kvm_ap_matrix and
>>> maintaining a list of the kvm_ap_matrix structures allows to easily 
>>> know
>>> if a guest already has an associated mediated device.
>>
>> How is that easier than storing the kvm pointer inside of the 
>> mediated matrix
>> device (i.e., struct ap_matrix_mdev) which also contains the struct 
>> kvm_ap_matrix?
>
> you can put it in ap_matrix_mdev but just the name "kvm_ap_matrix" 
> make the
> last one a better candidate for my opinion.

It's been in ap_matrix_mdev since v2, but I'll consider moving it to 
kvm_ap_matrix.

>
>
>> How does that allow us to avoid the code in arch/s390/kvm?
>
> This alone does not.
>
>> We still need the code
>> to update the CRYCB in the SIE block. I can obviously avoid placing 
>> that code in
>> kvm-ap.c and move it to the driver, but I already explained my 
>> reasoning for
>> keeping it in KVM. Let's face it, there is no way around the 
>> dependency between
>> the vfio_ap device driver and KVM unless guest matrix configuration 
>> is managed
>> solely by KVM through KVM interfaces.
>
> We get the pointer to KVM from the VFIO interface.
> That we both discuss on this is sterile.
> The only one who could say what is right is a S390 KVM maintainer.
> This would end the discussion.
> My point was just to say that we have an alternative.
>
>
>>
>> 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 idea behind kvm-ap.c 
is that all code
related to configuration of AP structures in KVM is in this one spot.

>
>
>