s390x/vfio-ap: document hot plug/unplug of vfio-ap device

   'unassign_domain' file. This may be done multiple times to unassign more than

   one control domain.

Notes: Hot plug/unplug is not currently supported for mediated AP matrix

devices, so no changes to the AP matrix will be allowed while a guest using

Notes: No changes to the AP matrix will be allowed while a guest using

the mediated matrix device is running. Attempts to assign an adapter,

domain or control domain will be rejected and an error (EBUSY) returned.

                     for guest usage, no AP devices can be made accessible to a

                     guest started without APFT installed.

Hot plug a vfio-ap device into a running guest:

==============================================

Only one vfio-ap device can be attached to the virtual machine's ap-bus, so a

vfio-ap device can be hot plugged if and only if no vfio-ap device is attached

to the bus already, whether via the QEMU command line or a prior hot plug

action.

To hot plug a vfio-ap device, use the QEMU device_add command:

    (qemu) device_add vfio-ap,sysfsdev="$path-to-mdev"

    Where the '$path-to-mdev' value specifies the absolute path to a mediated

    device to which AP resources to be used by the guest have been assigned.

Note that on Linux guests, the AP devices will be created in the

/sys/bus/ap/devices directory when the AP bus subsequently performs its periodic

scan, so there may be a short delay before the AP devices are accessible on the

guest.

The command will fail if:

* A vfio-ap device has already been attached to the virtual machine's ap-bus.

* The CPU model features for controlling guest access to AP facilities are not

  enabled (see 'CPU model features' subsection in the previous section).

Hot unplug a vfio-ap device from a running guest:

================================================

A vfio-ap device can be unplugged from a running KVM guest if a vfio-ap device

has been attached to the virtual machine's ap-bus via the QEMU command line

or a prior hot plug action.

To hot unplug a vfio-ap device, use the QEMU device_del command:

    (qemu) device_del vfio-ap,sysfsdev="$path-to-mdev"

    Where $path-to-mdev is the same as the path specified when the vfio-ap

    device was attached to the virtual machine's ap-bus.

On a Linux guest, the AP devices will be removed from the /sys/bus/ap/devices

directory on the guest when the AP bus subsequently performs its periodic scan,

so there may be a short delay before the AP devices are no longer accessible by

the guest.

The command will fail if the $path-to-mdev specified on the device_del command

does not match the value specified when the vfio-ap device was attached to

the virtual machine's ap-bus.

Example: Configure AP Matrixes for Three Linux Guests:

=====================================================

Let's now provide an example to illustrate how KVM guests may be given

  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

* Dynamically assigning AP resources to or unassigning AP resources from a

  mediated matrix device - see 'Configuring an AP matrix for a linux guest'

  section above - while a running guest is using it is currently not supported.

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

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

  is using AP devices, the vfio-ap device configured for the guest must be

  unplugged before migrating the guest (see 'Hot unplug a vfio-ap device from a

  running guest' section above.