[PATCH 3/3] docs/s390x: document vfio-ccw

[Cornelia Huck <cohuck@redhat.com>]

Add a basic example for passing a dasd via vfio-ccw.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
---
 docs/system/s390x/vfio-ccw.rst | 58 ++++++++++++++++++++++++++++++++++
 docs/system/target-s390x.rst   |  1 +
 2 files changed, 59 insertions(+)
 create mode 100644 docs/system/s390x/vfio-ccw.rst

diff --git a/docs/system/s390x/vfio-ccw.rst b/docs/system/s390x/vfio-ccw.rst
new file mode 100644
index 000000000000..3b465578971f
--- /dev/null
+++ b/docs/system/s390x/vfio-ccw.rst
@@ -0,0 +1,58 @@
+Subchannel passthrough via vfio-ccw
+===================================
+
+vfio-ccw (based upon the mediated vfio device infrastructure) allows to
+make certain I/O subchannels and their devices available to a guest. The
+host will not interact with those subchannels/devices any more.
+
+Note that while vfio-ccw should work with most non-QDIO devices, only ECKD
+DASD have really been tested.
+
+Example configuration
+---------------------
+
+Step 1: configure the host device
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Note: it is recommended to use the ``mdevctl`` tool for this step. If this
+is not possible or wanted, follow the manual procedure below.
+
+* Locate the subchannel for the device (in this example, ``0.0.2b09``)::
+
+    [root@host ~]# lscss | grep 0.0.2b09 | awk '{print $2}'
+    0.0.0313
+
+* Unbind the subchannel (in this example, ``0.0.0313``) from the standard
+  I/O subchannel driver and bind it to the vfio-ccw driver::
+
+    [root@host ~]# echo 0.0.0313 > /sys/bus/css/devices/0.0.0313/driver/unbind
+    [root@host ~]# echo 0.0.0313 > /sys/bus/css/drivers/vfio_ccw/bind
+
+* Create the mediated device (identified by a uuid)::
+
+    [root@host ~]# uuidgen
+    7e270a25-e163-4922-af60-757fc8ed48c6
+    [root@host ~]# echo "7e270a25-e163-4922-af60-757fc8ed48c6" > /sys/bus/css/devices/0.0.0313/mdev_supported_types/vfio_ccw-io/create
+
+Step 2: configure QEMU
+~~~~~~~~~~~~~~~~~~~~~~
+
+* Reference the created mediated device and (optionally) pick a device id to
+  be presented in the guest (here, ``fe.0.1234``, which will end up visible
+  in the guest as ``0.0.1234``::
+
+    -device vfio-ccw,devno=fe.0.1234,sysfsdev=/sys/bus/mdev/devices/7e270a25-e163-4922-af60-757fc8ed48c6
+
+* Start the guest. The device (here, ``0.0.1234``) should now be usable::
+
+    [root@guest ~]# lscss -d 0.0.1234
+    Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs           
+    ----------------------------------------------------------------------
+    0.0.1234 0.0.0007  3390/0e 3990/e9      f0  f0  ff   1a2a3a0a 00000000
+    [root@guest ~]# chccwdev -e 0.0.1234
+    Setting device 0.0.1234 online
+    [  197.011652] dasd-eckd 0.0.1234: A channel path to the device has become operational
+    [  197.014468] dasd-eckd 0.0.1234: New DASD 3390/0E (CU 3990/01) with 10017 cylinders, 15 heads, 224 sectors
+    [  197.045606] dasd-eckd 0.0.1234: DASD with 4 KB/block, 7212240 KB total size, 48 KB/track, compatible disk layout
+    [  197.049034]  dasda:VOL1/  0X2B09: dasda1
+    Done
diff --git a/docs/system/target-s390x.rst b/docs/system/target-s390x.rst
index 2592a05303ef..644e404ef9fd 100644
--- a/docs/system/target-s390x.rst
+++ b/docs/system/target-s390x.rst
@@ -25,6 +25,7 @@ or vfio-ap is also available.
    s390x/vfio-ap
    s390x/css
    s390x/3270
+   s390x/vfio-ccw
 
 Architectural features
 ======================
-- 
2.21.3