Re: [PATCH v6 21/21] s390: doc: detailed specifications for AP virtualization

[Tony Krowiak <akrowiak@linux.ibm.com>]

On 07/03/2018 07:52 AM, Cornelia Huck wrote:
> On Tue, 3 Jul 2018 11:22:10 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
>
>> On 07/03/2018 09:46 AM, Harald Freudenberger wrote:
>>> On 02.07.2018 18:28, Halil Pasic wrote:
>>>>
>>>> On 06/29/2018 11:11 PM, Tony Krowiak wrote:
>>>>> This patch provides documentation describing the AP architecture and
>>>>> design concepts behind the virtualization of AP devices. It also
>>>>> includes an example of how to configure AP devices for exclusive
>>>>> use of KVM guests.
>>>>>
>>>>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>>>>> ---
>>>> [..]
>>>>> +
>>>>> +Reserve APQNs for exclusive use of KVM guests
>>>>> +---------------------------------------------
>>>>> +The following block diagram illustrates the mechanism by which APQNs are
>>>>> +reserved:
>>>>> +
>>>>> +                              +------------------+
>>>>> +                 remove       |                  |   unbind
>>>>> +         +------------------->+ cex4queue driver +<-----------+
>>>>> +         |                    |                  |            |
>>>>> +         |                    +------------------+            |
>>>>> +         |                                                    |
>>>>> +         |                                                    |
>>>>> +         |                                                    |
>>>>> ++--------+---------+ register +------------------+      +-----+------+
>>>>> +|                  +<---------+                  | bind |            |
>>>>> +|      ap_bus      |          |  vfio_ap driver  +<-----+    admin   |
>>>>> +|                  +--------->+                  |      |            |
>>>>> ++------------------+  probe   +---+--------+-----+      +------------+
>>>>> +                                  |        |
>>>>> +                           create |        | store APQN
>>>>> +                                  |        |
>>>>> +                                  v        v
>>>>> +                              +---+--------+-----+
>>>>> +                              |                  |
>>>>> +                              |  matrix device   |
>>>>> +                              |                  |
>>>>> +                              +------------------+
>>>>> +
>>>>> +The process for reserving an AP queue for use by a KVM guest is:
>>>>> +
>>>>> +* The vfio-ap driver during its initialization will perform the following:
>>>>> +  * Create the 'vfio_ap' root device - /sys/devices/vfio_ap
>>>>> +  * Create the 'matrix' device in the 'vfio_ap' root
>>>>> +  * Register the matrix device with the device core
>>>>> +* Register with the ap_bus for AP queue devices of type 10 devices (CEX4 and
>>>>> +  newer) and to provide the vfio_ap driver's probe and remove callback
>>>>> +  interfaces. The reason why older devices are not supported is because there
>>>>> +  are no systems available on which to test.
>>>>> +* The admin unbinds queue cc.qqqq from the cex4queue device driver. This results
>>>>> +  in the ap_bus calling the the device driver's remove interface which
>>>>> +  unbinds the cc.qqqq queue device from the driver.
>>>> What if the queue cc.qqqq is already in use? AFAIU unbind is almost as radical as
>>>> pulling a cable. What is the proper procedure an admin should follow before doing
>>>> the unbind?
>>> What do you mean on this level with 'in use'? A unbind destroys the association
>>> between device and driver. There is no awareness of 'in use' or 'not in use' on this
>>> level. This is a hard unbind.
>>>>   
>>
>> Let me try to invoke the DASD analogy. If one for some reason wants to detach
>> a DASD the procedure to follow seems to be (see
>> https://www.ibm.com/support/knowledgecenter/en/linuxonibm/com.ibm.linux.z.lgdd/lgdd_t_dasd_online.html)
>> the following:
>> 1) Unmount.
>> 2) Offline possibly using safe_offline.
>> 3) Detach.
>>
>> Detaching a disk that is currently doing I/O asks for trouble, so the admin is encouraged
>> to make sure there is no pending I/O.
> I don't think we can use dasd (block devices) as a good analogy for
> every kind of device (for starters, consider network devices).
>
>> In case of AP you can interpret my 'in use' as the queue is not empty. In my understanding
>> unbind is supposed to be hard (I used the word radical). That's why I compared it to pulling
>> a cable. So that's why I ask is there stuff the admin is supposed to do before doing the
>> unbind.
> Are you asking for a kind of 'quiescing' operation? I would hope that
> the crypto drivers already can deal with that via flushing the queue,
> not allowing new requests, or whatever. This is not the block device
> case.

As I stated in Message ID: 
<89df60be-63d6-3ed9-4724-321e5b55d50d@linux.ibm.com>,
I believe the queue is flushed when the remove callback is invoked on the
driver.

>
> Anyway, this is an administrative issue. If you don't have a clear
> concept which devices are for host usage and which for guest usage, you
> already have problems.
>
> Speaking of administrative issues, is there libvirt support for vfio-ap
> under development? It would be helpful to validate the approach.

There is libvirt support under development although it is not very far
along at this point.

>