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Reply-To: pmorel@linux.ibm.com
Subject: Re: [PATCH v5 11/13] KVM: s390: implement mediated device open
 callback
From:   Pierre Morel <pmorel@linux.ibm.com>
To:     Tony Krowiak <akrowiak@linux.vnet.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
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Date:   Wed, 6 Jun 2018 19:40:30 +0200
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On 06/06/2018 18:08, Pierre Morel wrote:
> On 06/06/2018 16:28, Tony Krowiak wrote:
>> 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.
>
> I really regret that we abandoned the initial design with the matrix 
> bus and one
> single parent matrix device per guest.
> We would not have the problem of these KVM dependencies.
>
> It had the advantage of taking care of having only one device per guest
> (available_instance = 1), could take care of provisioning as you have
> sysfs entries available for a matrix without having a guest and a 
> mediated
> device.
>
> it also had advantage for virtualization to keep host side and guest 
> side matrix
> separate inside parent (host side) and mediated device (guest side).
>
> Shouldn't we treat this problem with a design using standard interfaces
> Instead of adding new dedicated interfaces?
>
> Regards,
>
> Pierre
>
>

Forget it.

I am not happy with the design but the design I was speaking of may not 
be the solution either.

Sorry for the noise.

Regards,

Pierre


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