new file mode 100644
@@ -0,0 +1,77 @@
+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
+DASDs have really been tested.
+
+Example configuration
+---------------------
+
+Step 1: configure the host device
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As every mdev is identified by a uuid, the first step is to obtain one::
+
+ [root@host ~]# uuidgen
+ 7e270a25-e163-4922-af60-757fc8ed48c6
+
+Note: it is recommended to use the ``mdevctl`` tool for actually configuring
+the host device.
+
+To define the same device as configured below to be started
+automatically, use
+
+::
+
+ [root@host ~]# driverctl -b css set-override 0.0.0313 vfio_ccw
+ [root@host ~]# mdevctl define -u 7e270a25-e163-4922-af60-757fc8ed48c6 \
+ -p 0.0.0313 -t vfio-ccw_io -a
+
+If using ``mdevctl`` 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 the uuid)::
+
+ [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 CHPID
+ ----------------------------------------------------------------------
+ 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
+ Done
+ [root@guest ~]# dmesg -t
+ (...)
+ dasd-eckd 0.0.1234: A channel path to the device has become operational
+ dasd-eckd 0.0.1234: New DASD 3390/0E (CU 3990/01) with 10017 cylinders, 15 heads, 224 sectors
+ dasd-eckd 0.0.1234: DASD with 4 KB/block, 7212240 KB total size, 48 KB/track, compatible disk layout
+ dasda:VOL1/ 0X2B09: dasda1
@@ -25,6 +25,7 @@ or vfio-ap is also available.
s390x/vfio-ap
s390x/css
s390x/3270
+ s390x/vfio-ccw
Architectural features
======================