treewide: remove redundant IS_ERR() before error code check

[linux/fpc-iii.git] / Documentation / s390 / vfio-ap.rst

===============================
Adjunct Processor (AP) facility
===============================


Introduction
============
The Adjunct Processor (AP) facility is an IBM Z cryptographic facility comprised
of three AP instructions and from 1 up to 256 PCIe cryptographic adapter cards.
The AP devices provide cryptographic functions to all CPUs assigned to a
linux system running in an IBM Z system LPAR.

The AP adapter cards are exposed via the AP bus. The motivation for vfio-ap
is to make AP cards available to KVM guests using the VFIO mediated device
framework. This implementation relies considerably on the s390 virtualization
facilities which do most of the hard work of providing direct access to AP
devices.

AP Architectural Overview
=========================
To facilitate the comprehension of the design, let's start with some
definitions:

* AP adapter

  An AP adapter is an IBM Z adapter card that can perform cryptographic
  functions. There can be from 0 to 256 adapters assigned to an LPAR. Adapters
  assigned to the LPAR in which a linux host is running will be available to
  the linux host. Each adapter is identified by a number from 0 to 255; however,
  the maximum adapter number is determined by machine model and/or adapter type.
  When installed, an AP adapter is accessed by AP instructions executed by any
  CPU.

  The AP adapter cards are assigned to a given LPAR via the system's Activation
  Profile which can be edited via the HMC. When the linux host system is IPL'd
  in the LPAR, the AP bus detects the AP adapter cards assigned to the LPAR and
  creates a sysfs device for each assigned adapter. For example, if AP adapters
  4 and 10 (0x0a) are assigned to the LPAR, the AP bus will create the following
  sysfs device entries::

    /sys/devices/ap/card04
    /sys/devices/ap/card0a

  Symbolic links to these devices will also be created in the AP bus devices
  sub-directory::

    /sys/bus/ap/devices/[card04]
    /sys/bus/ap/devices/[card04]

* AP domain

  An adapter is partitioned into domains. An adapter can hold up to 256 domains
  depending upon the adapter type and hardware configuration. A domain is
  identified by a number from 0 to 255; however, the maximum domain number is
  determined by machine model and/or adapter type.. A domain can be thought of
  as a set of hardware registers and memory used for processing AP commands. A
  domain can be configured with a secure private key used for clear key
  encryption. A domain is classified in one of two ways depending upon how it
  may be accessed:

    * Usage domains are domains that are targeted by an AP instruction to
      process an AP command.

    * Control domains are domains that are changed by an AP command sent to a
      usage domain; for example, to set the secure private key for the control
      domain.

  The AP usage and control domains are assigned to a given LPAR via the system's
  Activation Profile which can be edited via the HMC. When a linux host system
  is IPL'd in the LPAR, the AP bus module detects the AP usage and control
  domains assigned to the LPAR. The domain number of each usage domain and
  adapter number of each AP adapter are combined to create AP queue devices
  (see AP Queue section below). The domain number of each control domain will be
  represented in a bitmask and stored in a sysfs file
  /sys/bus/ap/ap_control_domain_mask. The bits in the mask, from most to least
  significant bit, correspond to domains 0-255.

* AP Queue

  An AP queue is the means by which an AP command is sent to a usage domain
  inside a specific adapter. An AP queue is identified by a tuple
  comprised of an AP adapter ID (APID) and an AP queue index (APQI). The
  APQI corresponds to a given usage domain number within the adapter. This tuple
  forms an AP Queue Number (APQN) uniquely identifying an AP queue. AP
  instructions include a field containing the APQN to identify the AP queue to
  which the AP command is to be sent for processing.

  The AP bus will create a sysfs device for each APQN that can be derived from
  the cross product of the AP adapter and usage domain numbers detected when the
  AP bus module is loaded. For example, if adapters 4 and 10 (0x0a) and usage
  domains 6 and 71 (0x47) are assigned to the LPAR, the AP bus will create the
  following sysfs entries::

    /sys/devices/ap/card04/04.0006
    /sys/devices/ap/card04/04.0047
    /sys/devices/ap/card0a/0a.0006
    /sys/devices/ap/card0a/0a.0047

  The following symbolic links to these devices will be created in the AP bus
  devices subdirectory::

    /sys/bus/ap/devices/[04.0006]
    /sys/bus/ap/devices/[04.0047]
    /sys/bus/ap/devices/[0a.0006]
    /sys/bus/ap/devices/[0a.0047]

* AP Instructions:

  There are three AP instructions:

  * NQAP: to enqueue an AP command-request message to a queue
  * DQAP: to dequeue an AP command-reply message from a queue
  * PQAP: to administer the queues

  AP instructions identify the domain that is targeted to process the AP
  command; this must be one of the usage domains. An AP command may modify a
  domain that is not one of the usage domains, but the modified domain
  must be one of the control domains.

AP and SIE
==========
Let's now take a look at how AP instructions executed on a guest are interpreted
by the hardware.

A satellite control block called the Crypto Control Block (CRYCB) is attached to
our main hardware virtualization control block. The CRYCB contains three fields
to identify the adapters, usage domains and control domains assigned to the KVM
guest:

* The AP Mask (APM) field is a bit mask that identifies the AP adapters assigned
  to the KVM guest. Each bit in the mask, from left to right (i.e. from most
  significant to least significant bit in big endian order), corresponds to
  an APID from 0-255. If a bit is set, the corresponding adapter is valid for
  use by the KVM guest.

* The AP Queue Mask (AQM) field is a bit mask identifying the AP usage domains
  assigned to the KVM guest. Each bit in the mask, from left to right (i.e. from
  most significant to least significant bit in big endian order), corresponds to
  an AP queue index (APQI) from 0-255. If a bit is set, the corresponding queue
  is valid for use by the KVM guest.

* The AP Domain Mask field is a bit mask that identifies the AP control domains
  assigned to the KVM guest. The ADM bit mask controls which domains can be
  changed by an AP command-request message sent to a usage domain from the
  guest. Each bit in the mask, from left to right (i.e. from most significant to
  least significant bit in big endian order), corresponds to a domain from
  0-255. If a bit is set, the corresponding domain can be modified by an AP
  command-request message sent to a usage domain.

If you recall from the description of an AP Queue, AP instructions include
an APQN to identify the AP queue to which an AP command-request message is to be
sent (NQAP and PQAP instructions), or from which a command-reply message is to
be received (DQAP instruction). The validity of an APQN is defined by the matrix
calculated from the APM and AQM; it is the cross product of all assigned adapter
numbers (APM) with all assigned queue indexes (AQM). For example, if adapters 1
and 2 and usage domains 5 and 6 are assigned to a guest, the APQNs (1,5), (1,6),
(2,5) and (2,6) will be valid for the guest.

The APQNs can provide secure key functionality - i.e., a private key is stored
on the adapter card for each of its domains - so each APQN must be assigned to
at most one guest or to the linux host::

   Example 1: Valid configuration:
   ------------------------------
   Guest1: adapters 1,2  domains 5,6
   Guest2: adapter  1,2  domain 7

   This is valid because both guests have a unique set of APQNs:
      Guest1 has APQNs (1,5), (1,6), (2,5), (2,6);
      Guest2 has APQNs (1,7), (2,7)

   Example 2: Valid configuration:
   ------------------------------
   Guest1: adapters 1,2 domains 5,6
   Guest2: adapters 3,4 domains 5,6

   This is also valid because both guests have a unique set of APQNs:
      Guest1 has APQNs (1,5), (1,6), (2,5), (2,6);
      Guest2 has APQNs (3,5), (3,6), (4,5), (4,6)

   Example 3: Invalid configuration:
   --------------------------------
   Guest1: adapters 1,2  domains 5,6
   Guest2: adapter  1    domains 6,7

   This is an invalid configuration because both guests have access to
   APQN (1,6).

The Design
==========
The design introduces three new objects:

1. AP matrix device
2. VFIO AP device driver (vfio_ap.ko)
3. VFIO AP mediated matrix pass-through device

The VFIO AP device driver
-------------------------
The VFIO AP (vfio_ap) device driver serves the following purposes:

1. Provides the interfaces to secure APQNs for exclusive use of KVM guests.

2. Sets up the VFIO mediated device interfaces to manage a mediated matrix
   device and creates the sysfs interfaces for assigning adapters, usage
   domains, and control domains comprising the matrix for a KVM guest.

3. Configures the APM, AQM and ADM in the CRYCB referenced by a KVM guest's
   SIE state description to grant the guest access to a matrix of AP devices

Reserve APQNs for exclusive use of KVM guests
---------------------------------------------
The following block diagram illustrates the mechanism by which APQNs are
reserved::

                                +------------------+
                 7 remove       |                  |
           +--------------------> cex4queue driver |
           |                    |                  |
           |                    +------------------+
           |
           |
           |                    +------------------+          +----------------+
           |  5 register driver |                  | 3 create |                |
           |   +---------------->   Device core    +---------->  matrix device |
           |   |                |                  |          |                |
           |   |                +--------^---------+          +----------------+
           |   |                         |
           |   |                         +-------------------+
           |   | +-----------------------------------+       |
           |   | |      4 register AP driver         |       | 2 register device
           |   | |                                   |       |
  +--------+---+-v---+                      +--------+-------+-+
  |                  |                      |                  |
  |      ap_bus      +--------------------- >  vfio_ap driver  |
  |                  |       8 probe        |                  |
  +--------^---------+                      +--^--^------------+
  6 edit   |                                   |  |
    apmask |     +-----------------------------+  | 9 mdev create
    aqmask |     |           1 modprobe           |
  +--------+-----+---+           +----------------+-+         +----------------+
  |                  |           |                  |8 create |     mediated   |
  |      admin       |           | VFIO device core |--------->     matrix     |
  |                  +           |                  |         |     device     |
  +------+-+---------+           +--------^---------+         +--------^-------+
         | |                              |                            |
         | | 9 create vfio_ap-passthrough |                            |
         | +------------------------------+                            |
         +-------------------------------------------------------------+
                     10  assign adapter/domain/control domain

The process for reserving an AP queue for use by a KVM guest is:

1. The administrator loads the vfio_ap device driver
2. The vfio-ap driver during its initialization will register a single 'matrix'
   device with the device core. This will serve as the parent device for
   all mediated matrix devices used to configure an AP matrix for a guest.
3. The /sys/devices/vfio_ap/matrix device is created by the device core
4. The vfio_ap device driver will register with the AP bus for AP queue devices
   of type 10 and higher (CEX4 and newer). The driver will provide the vfio_ap
   driver's probe and remove callback interfaces. Devices older than CEX4 queues
   are not supported to simplify the implementation by not needlessly
   complicating the design by supporting older devices that will go out of
   service in the relatively near future, and for which there are few older
   systems around on which to test.
5. The AP bus registers the vfio_ap device driver with the device core
6. The administrator edits the AP adapter and queue masks to reserve AP queues
   for use by the vfio_ap device driver.
7. The AP bus removes the AP queues reserved for the vfio_ap driver from the
   default zcrypt cex4queue driver.
8. The AP bus probes the vfio_ap device driver to bind the queues reserved for
   it.
9. The administrator creates a passthrough type mediated matrix device to be
   used by a guest
10. The administrator assigns the adapters, usage domains and control domains
    to be exclusively used by a guest.

Set up the VFIO mediated device interfaces
------------------------------------------
The VFIO AP device driver utilizes the common interface of the VFIO mediated
device core driver to:

* Register an AP mediated bus driver to add a mediated matrix device to and
  remove it from a VFIO group.
* Create and destroy a mediated matrix device
* Add a mediated matrix device to and remove it from the AP mediated bus driver
* Add a mediated matrix device to and remove it from an IOMMU group

The following high-level block diagram shows the main components and interfaces
of the VFIO AP mediated matrix device driver::

   +-------------+
   |             |
   | +---------+ | mdev_register_driver() +--------------+
   | |  Mdev   | +<-----------------------+              |
   | |  bus    | |                        | vfio_mdev.ko |
   | | driver  | +----------------------->+              |<-> VFIO user
   | +---------+ |    probe()/remove()    +--------------+    APIs
   |             |
   |  MDEV CORE  |
   |   MODULE    |
   |   mdev.ko   |
   | +---------+ | mdev_register_device() +--------------+
   | |Physical | +<-----------------------+              |
   | | device  | |                        |  vfio_ap.ko  |<-> matrix
   | |interface| +----------------------->+              |    device
   | +---------+ |       callback         +--------------+
   +-------------+

During initialization of the vfio_ap module, the matrix device is registered
with an 'mdev_parent_ops' structure that provides the sysfs attribute
structures, mdev functions and callback interfaces for managing the mediated
matrix device.

* sysfs attribute structures:

  supported_type_groups
    The VFIO mediated device framework supports creation of user-defined
    mediated device types. These mediated device types are specified
    via the 'supported_type_groups' structure when a device is registered
    with the mediated device framework. The registration process creates the
    sysfs structures for each mediated device type specified in the
    'mdev_supported_types' sub-directory of the device being registered. Along
    with the device type, the sysfs attributes of the mediated device type are
    provided.

    The VFIO AP device driver will register one mediated device type for
    passthrough devices:

      /sys/devices/vfio_ap/matrix/mdev_supported_types/vfio_ap-passthrough

    Only the read-only attributes required by the VFIO mdev framework will
    be provided::

        ... name
        ... device_api
        ... available_instances
        ... device_api

    Where:

        * name:
            specifies the name of the mediated device type
        * device_api:
            the mediated device type's API
        * available_instances:
            the number of mediated matrix passthrough devices
            that can be created
        * device_api:
            specifies the VFIO API
  mdev_attr_groups
    This attribute group identifies the user-defined sysfs attributes of the
    mediated device. When a device is registered with the VFIO mediated device
    framework, the sysfs attribute files identified in the 'mdev_attr_groups'
    structure will be created in the mediated matrix device's directory. The
    sysfs attributes for a mediated matrix device are:

    assign_adapter / unassign_adapter:
      Write-only attributes for assigning/unassigning an AP adapter to/from the
      mediated matrix device. To assign/unassign an adapter, the APID of the
      adapter is echoed to the respective attribute file.
    assign_domain / unassign_domain:
      Write-only attributes for assigning/unassigning an AP usage domain to/from
      the mediated matrix device. To assign/unassign a domain, the domain
      number of the the usage domain is echoed to the respective attribute
      file.
    matrix:
      A read-only file for displaying the APQNs derived from the cross product
      of the adapter and domain numbers assigned to the mediated matrix device.
    assign_control_domain / unassign_control_domain:
      Write-only attributes for assigning/unassigning an AP control domain
      to/from the mediated matrix device. To assign/unassign a control domain,
      the ID of the domain to be assigned/unassigned is echoed to the respective
      attribute file.
    control_domains:
      A read-only file for displaying the control domain numbers assigned to the
      mediated matrix device.

* functions:

  create:
    allocates the ap_matrix_mdev structure used by the vfio_ap driver to:

    * Store the reference to the KVM structure for the guest using the mdev
    * Store the AP matrix configuration for the adapters, domains, and control
      domains assigned via the corresponding sysfs attributes files

  remove:
    deallocates the mediated matrix device's ap_matrix_mdev structure. This will
    be allowed only if a running guest is not using the mdev.

* callback interfaces

  open:
    The vfio_ap driver uses this callback to register a
    VFIO_GROUP_NOTIFY_SET_KVM notifier callback function for the mdev matrix
    device. The open is invoked when QEMU connects the VFIO iommu group
    for the mdev matrix device to the MDEV bus. Access to the KVM structure used
    to configure the KVM guest is provided via this callback. The KVM structure,
    is used to configure the guest's access to the AP matrix defined via the
    mediated matrix device's sysfs attribute files.
  release:
    unregisters the VFIO_GROUP_NOTIFY_SET_KVM notifier callback function for the
    mdev matrix device and deconfigures the guest's AP matrix.

Configure the APM, AQM and ADM in the CRYCB
-------------------------------------------
Configuring the AP matrix for a KVM guest will be performed when the
VFIO_GROUP_NOTIFY_SET_KVM notifier callback is invoked. The notifier
function is called when QEMU connects to KVM. The guest's AP matrix is
configured via it's CRYCB by:

* Setting the bits in the APM corresponding to the APIDs assigned to the
  mediated matrix device via its 'assign_adapter' interface.
* Setting the bits in the AQM corresponding to the domains assigned to the
  mediated matrix device via its 'assign_domain' interface.
* Setting the bits in the ADM corresponding to the domain dIDs assigned to the
  mediated matrix device via its 'assign_control_domains' interface.

The CPU model features for AP
-----------------------------
The AP stack relies on the presence of the AP instructions as well as two
facilities: The AP Facilities Test (APFT) facility; and the AP Query
Configuration Information (QCI) facility. These features/facilities are made
available to a KVM guest via the following CPU model features:

1. ap: Indicates whether the AP instructions are installed on the guest. This
   feature will be enabled by KVM only if the AP instructions are installed
   on the host.

2. apft: Indicates the APFT facility is available on the guest. This facility
   can be made available to the guest only if it is available on the host (i.e.,
   facility bit 15 is set).

3. apqci: Indicates the AP QCI facility is available on the guest. This facility
   can be made available to the guest only if it is available on the host (i.e.,
   facility bit 12 is set).

Note: If the user chooses to specify a CPU model different than the 'host'
model to QEMU, the CPU model features and facilities need to be turned on
explicitly; for example::

     /usr/bin/qemu-system-s390x ... -cpu z13,ap=on,apqci=on,apft=on

A guest can be precluded from using AP features/facilities by turning them off
explicitly; for example::

     /usr/bin/qemu-system-s390x ... -cpu host,ap=off,apqci=off,apft=off

Note: If the APFT facility is turned off (apft=off) for the guest, the guest
will not see any AP devices. The zcrypt device drivers that register for type 10
and newer AP devices - i.e., the cex4card and cex4queue device drivers - need
the APFT facility to ascertain the facilities installed on a given AP device. If
the APFT facility is not installed on the guest, then the probe of device
drivers will fail since only type 10 and newer devices can be configured for
guest use.

Example
=======
Let's now provide an example to illustrate how KVM guests may be given
access to AP facilities. For this example, we will show how to configure
three guests such that executing the lszcrypt command on the guests would
look like this:

Guest1
------
=========== ===== ============
CARD.DOMAIN TYPE  MODE
=========== ===== ============
05          CEX5C CCA-Coproc
05.0004     CEX5C CCA-Coproc
05.00ab     CEX5C CCA-Coproc
06          CEX5A Accelerator
06.0004     CEX5A Accelerator
06.00ab     CEX5C CCA-Coproc
=========== ===== ============

Guest2
------
=========== ===== ============
CARD.DOMAIN TYPE  MODE
=========== ===== ============
05          CEX5A Accelerator
05.0047     CEX5A Accelerator
05.00ff     CEX5A Accelerator
=========== ===== ============

Guest2
------
=========== ===== ============
CARD.DOMAIN TYPE  MODE
=========== ===== ============
06          CEX5A Accelerator
06.0047     CEX5A Accelerator
06.00ff     CEX5A Accelerator
=========== ===== ============

These are the steps:

1. Install the vfio_ap module on the linux host. The dependency chain for the
   vfio_ap module is:
   * iommu
   * s390
   * zcrypt
   * vfio
   * vfio_mdev
   * vfio_mdev_device
   * KVM

   To build the vfio_ap module, the kernel build must be configured with the
   following Kconfig elements selected:
   * IOMMU_SUPPORT
   * S390
   * ZCRYPT
   * S390_AP_IOMMU
   * VFIO
   * VFIO_MDEV
   * VFIO_MDEV_DEVICE
   * KVM

   If using make menuconfig select the following to build the vfio_ap module::

     -> Device Drivers
        -> IOMMU Hardware Support
           select S390 AP IOMMU Support
        -> VFIO Non-Privileged userspace driver framework
           -> Mediated device driver frramework
              -> VFIO driver for Mediated devices
     -> I/O subsystem
        -> VFIO support for AP devices

2. Secure the AP queues to be used by the three guests so that the host can not
   access them. To secure them, there are two sysfs files that specify
   bitmasks marking a subset of the APQN range as 'usable by the default AP
   queue device drivers' or 'not usable by the default device drivers' and thus
   available for use by the vfio_ap device driver'. The location of the sysfs
   files containing the masks are::

     /sys/bus/ap/apmask
     /sys/bus/ap/aqmask

   The 'apmask' is a 256-bit mask that identifies a set of AP adapter IDs
   (APID). Each bit in the mask, from left to right (i.e., from most significant
   to least significant bit in big endian order), corresponds to an APID from
   0-255. If a bit is set, the APID is marked as usable only by the default AP
   queue device drivers; otherwise, the APID is usable by the vfio_ap
   device driver.

   The 'aqmask' is a 256-bit mask that identifies a set of AP queue indexes
   (APQI). Each bit in the mask, from left to right (i.e., from most significant
   to least significant bit in big endian order), corresponds to an APQI from
   0-255. If a bit is set, the APQI is marked as usable only by the default AP
   queue device drivers; otherwise, the APQI is usable by the vfio_ap device
   driver.

   Take, for example, the following mask::

      0x7dffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff

    It indicates:

      1, 2, 3, 4, 5, and 7-255 belong to the default drivers' pool, and 0 and 6
      belong to the vfio_ap device driver's pool.

   The APQN of each AP queue device assigned to the linux host is checked by the
   AP bus against the set of APQNs derived from the cross product of APIDs
   and APQIs marked as usable only by the default AP queue device drivers. If a
   match is detected,  only the default AP queue device drivers will be probed;
   otherwise, the vfio_ap device driver will be probed.

   By default, the two masks are set to reserve all APQNs for use by the default
   AP queue device drivers. There are two ways the default masks can be changed:

   1. The sysfs mask files can be edited by echoing a string into the
      respective sysfs mask file in one of two formats:

      * An absolute hex string starting with 0x - like "0x12345678" - sets
        the mask. If the given string is shorter than the mask, it is padded
        with 0s on the right; for example, specifying a mask value of 0x41 is
        the same as specifying::

           0x4100000000000000000000000000000000000000000000000000000000000000

        Keep in mind that the mask reads from left to right (i.e., most
        significant to least significant bit in big endian order), so the mask
        above identifies device numbers 1 and 7 (01000001).

        If the string is longer than the mask, the operation is terminated with
        an error (EINVAL).

      * Individual bits in the mask can be switched on and off by specifying
        each bit number to be switched in a comma separated list. Each bit
        number string must be prepended with a ('+') or minus ('-') to indicate
        the corresponding bit is to be switched on ('+') or off ('-'). Some
        valid values are:

           - "+0"    switches bit 0 on
           - "-13"   switches bit 13 off
           - "+0x41" switches bit 65 on
           - "-0xff" switches bit 255 off

        The following example:

              +0,-6,+0x47,-0xf0

        Switches bits 0 and 71 (0x47) on

        Switches bits 6 and 240 (0xf0) off

        Note that the bits not specified in the list remain as they were before
        the operation.

   2. The masks can also be changed at boot time via parameters on the kernel
      command line like this:

         ap.apmask=0xffff ap.aqmask=0x40

         This would create the following masks::

            apmask:
            0xffff000000000000000000000000000000000000000000000000000000000000

            aqmask:
            0x4000000000000000000000000000000000000000000000000000000000000000

         Resulting in these two pools::

            default drivers pool:    adapter 0-15, domain 1
            alternate drivers pool:  adapter 16-255, domains 0, 2-255

Securing the APQNs for our example
----------------------------------
   To secure the AP queues 05.0004, 05.0047, 05.00ab, 05.00ff, 06.0004, 06.0047,
   06.00ab, and 06.00ff for use by the vfio_ap device driver, the corresponding
   APQNs can either be removed from the default masks::

      echo -5,-6 > /sys/bus/ap/apmask

      echo -4,-0x47,-0xab,-0xff > /sys/bus/ap/aqmask

   Or the masks can be set as follows::

      echo 0xf9ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff \
      > apmask

      echo 0xf7fffffffffffffffeffffffffffffffffffffffffeffffffffffffffffffffe \
      > aqmask

   This will result in AP queues 05.0004, 05.0047, 05.00ab, 05.00ff, 06.0004,
   06.0047, 06.00ab, and 06.00ff getting bound to the vfio_ap device driver. The
   sysfs directory for the vfio_ap device driver will now contain symbolic links
   to the AP queue devices bound to it::

     /sys/bus/ap
     ... [drivers]
     ...... [vfio_ap]
     ......... [05.0004]
     ......... [05.0047]
     ......... [05.00ab]
     ......... [05.00ff]
     ......... [06.0004]
     ......... [06.0047]
     ......... [06.00ab]
     ......... [06.00ff]

   Keep in mind that only type 10 and newer adapters (i.e., CEX4 and later)
   can be bound to the vfio_ap device driver. The reason for this is to
   simplify the implementation by not needlessly complicating the design by
   supporting older devices that will go out of service in the relatively near
   future and for which there are few older systems on which to test.

   The administrator, therefore, must take care to secure only AP queues that
   can be bound to the vfio_ap device driver. The device type for a given AP
   queue device can be read from the parent card's sysfs directory. For example,
   to see the hardware type of the queue 05.0004:

     cat /sys/bus/ap/devices/card05/hwtype

   The hwtype must be 10 or higher (CEX4 or newer) in order to be bound to the
   vfio_ap device driver.

3. Create the mediated devices needed to configure the AP matrixes for the
   three guests and to provide an interface to the vfio_ap driver for
   use by the guests::

     /sys/devices/vfio_ap/matrix/
     --- [mdev_supported_types]
     ------ [vfio_ap-passthrough] (passthrough mediated matrix device type)
     --------- create
     --------- [devices]

   To create the mediated devices for the three guests::

        uuidgen > create
        uuidgen > create
        uuidgen > create

        or

        echo $uuid1 > create
        echo $uuid2 > create
        echo $uuid3 > create

   This will create three mediated devices in the [devices] subdirectory named
   after the UUID written to the create attribute file. We call them $uuid1,
   $uuid2 and $uuid3 and this is the sysfs directory structure after creation::

     /sys/devices/vfio_ap/matrix/
     --- [mdev_supported_types]
     ------ [vfio_ap-passthrough]
     --------- [devices]
     ------------ [$uuid1]
     --------------- assign_adapter
     --------------- assign_control_domain
     --------------- assign_domain
     --------------- matrix
     --------------- unassign_adapter
     --------------- unassign_control_domain
     --------------- unassign_domain

     ------------ [$uuid2]
     --------------- assign_adapter
     --------------- assign_control_domain
     --------------- assign_domain
     --------------- matrix
     --------------- unassign_adapter
     ----------------unassign_control_domain
     ----------------unassign_domain

     ------------ [$uuid3]
     --------------- assign_adapter
     --------------- assign_control_domain
     --------------- assign_domain
     --------------- matrix
     --------------- unassign_adapter
     ----------------unassign_control_domain
     ----------------unassign_domain

4. The administrator now needs to configure the matrixes for the mediated
   devices $uuid1 (for Guest1), $uuid2 (for Guest2) and $uuid3 (for Guest3).

   This is how the matrix is configured for Guest1::

      echo 5 > assign_adapter
      echo 6 > assign_adapter
      echo 4 > assign_domain
      echo 0xab > assign_domain

   Control domains can similarly be assigned using the assign_control_domain
   sysfs file.

   If a mistake is made configuring an adapter, domain or control domain,
   you can use the unassign_xxx files to unassign the adapter, domain or
   control domain.

   To display the matrix configuration for Guest1::

         cat matrix

   This is how the matrix is configured for Guest2::

      echo 5 > assign_adapter
      echo 0x47 > assign_domain
      echo 0xff > assign_domain

   This is how the matrix is configured for Guest3::

      echo 6 > assign_adapter
      echo 0x47 > assign_domain
      echo 0xff > assign_domain

   In order to successfully assign an adapter:

   * The adapter number specified must represent a value from 0 up to the
     maximum adapter number configured for the system. If an adapter number
     higher than the maximum is specified, the operation will terminate with
     an error (ENODEV).

   * All APQNs that can be derived from the adapter ID and the IDs of
     the previously assigned domains must be bound to the vfio_ap device
     driver. If no domains have yet been assigned, then there must be at least
     one APQN with the specified APID bound to the vfio_ap driver. If no such
     APQNs are bound to the driver, the operation will terminate with an
     error (EADDRNOTAVAIL).

     No APQN that can be derived from the adapter ID and the IDs of the
     previously assigned domains can be assigned to another mediated matrix
     device. If an APQN is assigned to another mediated matrix device, the
     operation will terminate with an error (EADDRINUSE).

   In order to successfully assign a domain:

   * The domain number specified must represent a value from 0 up to the
     maximum domain number configured for the system. If a domain number
     higher than the maximum is specified, the operation will terminate with
     an error (ENODEV).

   * All APQNs that can be derived from the domain ID and the IDs of
     the previously assigned adapters must be bound to the vfio_ap device
     driver. If no domains have yet been assigned, then there must be at least
     one APQN with the specified APQI bound to the vfio_ap driver. If no such
     APQNs are bound to the driver, the operation will terminate with an
     error (EADDRNOTAVAIL).

     No APQN that can be derived from the domain ID and the IDs of the
     previously assigned adapters can be assigned to another mediated matrix
     device. If an APQN is assigned to another mediated matrix device, the
     operation will terminate with an error (EADDRINUSE).

   In order to successfully assign a control domain, the domain number
   specified must represent a value from 0 up to the maximum domain number
   configured for the system. If a control domain number higher than the maximum
   is specified, the operation will terminate with an error (ENODEV).

5. Start Guest1::

     /usr/bin/qemu-system-s390x ... -cpu host,ap=on,apqci=on,apft=on \
        -device vfio-ap,sysfsdev=/sys/devices/vfio_ap/matrix/$uuid1 ...

7. Start Guest2::

     /usr/bin/qemu-system-s390x ... -cpu host,ap=on,apqci=on,apft=on \
        -device vfio-ap,sysfsdev=/sys/devices/vfio_ap/matrix/$uuid2 ...

7. Start Guest3::

     /usr/bin/qemu-system-s390x ... -cpu host,ap=on,apqci=on,apft=on \
        -device vfio-ap,sysfsdev=/sys/devices/vfio_ap/matrix/$uuid3 ...

When the guest is shut down, the mediated matrix devices may be removed.

Using our example again, to remove the mediated matrix device $uuid1::

   /sys/devices/vfio_ap/matrix/
      --- [mdev_supported_types]
      ------ [vfio_ap-passthrough]
      --------- [devices]
      ------------ [$uuid1]
      --------------- remove

::

   echo 1 > remove

This will remove all of the mdev matrix device's sysfs structures including
the mdev device itself. To recreate and reconfigure the mdev matrix device,
all of the steps starting with step 3 will have to be performed again. Note
that the remove will fail if a guest using the mdev is still running.

It is not necessary to remove an mdev matrix device, but one may want to
remove it if no guest will use it during the remaining lifetime of the linux
host. If the mdev matrix device is removed, one may want to also reconfigure
the pool of adapters and queues reserved for use by the default drivers.

Limitations
===========
* The KVM/kernel interfaces do not provide a way to prevent restoring an APQN
  to the default drivers pool of a queue that is still assigned to a mediated
  device in use by a guest. It is incumbent upon the administrator to
  ensure there is no mediated device in use by a guest to which the APQN is
  assigned lest the host be given access to the private data of the AP queue
  device such as a private key configured specifically for the guest.

* Dynamically modifying the AP matrix for a running guest (which would amount to
  hot(un)plug of AP devices for the guest) is currently not supported

* Live guest migration is not supported for guests using AP devices.