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Subject: Re: [PATCH v6 21/21] s390: doc: detailed specifications for AP
 virtualization
To:     Tony Krowiak <akrowiak@linux.vnet.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     freude@de.ibm.com, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, borntraeger@de.ibm.com,
        cohuck@redhat.com, kwankhede@nvidia.com,
        bjsdjshi@linux.vnet.ibm.com, pbonzini@redhat.com,
        alex.williamson@redhat.com, pmorel@linux.vnet.ibm.com,
        alifm@linux.vnet.ibm.com, mjrosato@linux.vnet.ibm.com,
        jjherne@linux.vnet.ibm.com, thuth@redhat.com,
        pasic@linux.vnet.ibm.com, berrange@redhat.com,
        fiuczy@linux.vnet.ibm.com, buendgen@de.ibm.com,
        Tony Krowiak <akrowiak@linux.ibm.com>
References: <1530306683-7270-1-git-send-email-akrowiak@linux.vnet.ibm.com>
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From:   Halil Pasic <pasic@linux.ibm.com>
Date:   Mon, 2 Jul 2018 18:28:42 +0200
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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?

> +* The admin binds the cc.qqqq queue to the vfio_ap device driver. This results
> +  in the ap_bus calling the device vfio_ap driver's probe interface to bind
> +  queue cc.qqqq to the driver. The vfio_ap device driver will store the APQN for
> +  the queue in the matrix device
> +
[..]