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

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Date:   Wed, 6 Jun 2018 10:28:17 -0400
From:   Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
To:     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
Subject: Re: [PATCH v5 11/13] KVM: s390: implement mediated device open
 callback

On 06/05/2018 08:19 AM, Pierre Morel wrote:
> On 30/05/2018 16:33, Tony Krowiak wrote:
>> On 05/24/2018 05:08 AM, Pierre Morel wrote:
>>> On 23/05/2018 16:45, Tony Krowiak wrote:
>>>> On 05/16/2018 04:03 AM, Pierre Morel wrote:
>>>>> On 07/05/2018 17:11, Tony Krowiak wrote:
>>>>>> Implements the open callback on the mediated matrix device.
>>>>>> The function registers a group notifier to receive notification
>>>>>> of the VFIO_GROUP_NOTIFY_SET_KVM event. When notified,
>>>>>> the vfio_ap device driver will get access to the guest's
>>>>>> kvm structure. With access to this structure the driver will:
>>>>>>
>>>>>> 1. Ensure that only one mediated device is opened for the guest
>>>
>>> You should explain why.
>>>
>>>>>>
>>>>>> 2. Configure access to the AP devices for the guest.
>>>>>>
>>> ...snip...
>>>>>> +void kvm_ap_refcount_inc(struct kvm *kvm)
>>>>>> +{
>>>>>> +    atomic_inc(&kvm->arch.crypto.aprefs);
>>>>>> +}
>>>>>> +EXPORT_SYMBOL(kvm_ap_refcount_inc);
>>>>>> +
>>>>>> +void kvm_ap_refcount_dec(struct kvm *kvm)
>>>>>> +{
>>>>>> +    atomic_dec(&kvm->arch.crypto.aprefs);
>>>>>> +}
>>>>>> +EXPORT_SYMBOL(kvm_ap_refcount_dec);
>>>>>
>>>>> Why are these functions inside kvm-ap ?
>>>>> Will anyone use this outer of vfio-ap ?
>>>>
>>>> As I've stated before, I made the choice to contain all interfaces 
>>>> that
>>>> access KVM in kvm-ap because I don't think it is appropriate for 
>>>> the device
>>>> driver to have to have "knowledge" of the inner workings of KVM. 
>>>> Why does
>>>> it matter whether any entity outside of the vfio_ap device driver 
>>>> calls
>>>> these functions? I could ask a similar question if the interfaces were
>>>> contained in vfio-ap; what if another device driver needs access to 
>>>> these
>>>> interfaces?
>>>
>>> This is very driver specific and only used during initialization.
>>> It is not a common property of the cryptographic interface.
>>>
>>> I really think you should handle this inside the driver.
>>
>> We are going to have to agree to disagree on this one. Is it not 
>> possible
>> that future drivers - e.g., when full virtualization is implemented - 
>> will
>> require access to KVM?
>
> I do not think that an access to KVM is required for full virtualization.

You may be right, but at this point, there is no guarantee. I stand by my
design on this one.

>
>
>