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

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Date:   Wed, 30 May 2018 10:33:13 -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 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?
>
> Pierre
>
>
> ...snip...
>
>