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

Let's update the vfio-ap.txt document to include the hot plug/unplug
support.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Message-Id: <1550519397-25359-3-git-send-email-akrowiak@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

docs/vfio-ap.txt

@@ -440,8 +440,7 @@ unassign_control_domain
    '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
+Notes: No changes to the AP matrix will be allowed while a guest using
-devices, so 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.
 
@@ -562,6 +561,54 @@ facilities:
                      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
@@ -819,7 +866,11 @@ Limitations
   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
+* Dynamically assigning AP resources to or unassigning AP resources from a
-  hot(un)plug of AP devices for the guest) is currently not supported
+  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.