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

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Message-Id: <ea5c2c2f-27fe-b2bf-2aa8-fbe80e77ce81@linux.ibm.com>
Date:   Thu, 7 Jun 2018 17:20:40 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     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 07/06/2018 15:54, Tony Krowiak wrote:
> On 06/06/2018 01:40 PM, Pierre Morel wrote:
>> 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.
>
> The AP architecture makes virtualization of AP devices complex. We 
> tried the solution you
> described and found it to be sorely lacking which is why we ended up 
> where we are now.

I did not see any explanation on why between v1 and v2 as it was abandoned.


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.

Also I think that with a little data structure refactoring you can avoid 
most of
the code in the arch/s390/kvm.

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.

Pierre

>
>>
>>
>> Sorry for the noise.
>>
>> Regards,
>>
>> Pierre
>>
>>
>

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