xref: /openbmc/qemu/docs/system/s390x/vfio-ccw.rst (revision b86c6ba6)
1Subchannel passthrough via vfio-ccw
2===================================
3
4vfio-ccw (based upon the mediated vfio device infrastructure) allows to
5make certain I/O subchannels and their devices available to a guest. The
6host will not interact with those subchannels/devices any more.
7
8Note that while vfio-ccw should work with most non-QDIO devices, only ECKD
9DASDs have really been tested.
10
11Example configuration
12---------------------
13
14Step 1: configure the host device
15~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
16
17As every mdev is identified by a uuid, the first step is to obtain one::
18
19  [root@host ~]# uuidgen
20  7e270a25-e163-4922-af60-757fc8ed48c6
21
22Note: it is recommended to use the ``mdevctl`` tool for actually configuring
23the host device.
24
25To define the same device as configured below to be started
26automatically, use
27
28::
29
30   [root@host ~]# driverctl -b css set-override 0.0.0313 vfio_ccw
31   [root@host ~]# mdevctl define -u 7e270a25-e163-4922-af60-757fc8ed48c6 \
32                  -p 0.0.0313 -t vfio_ccw-io -a
33
34If using ``mdevctl`` is not possible or wanted, follow the manual procedure
35below.
36
37* Locate the subchannel for the device (in this example, ``0.0.2b09``)::
38
39    [root@host ~]# lscss | grep 0.0.2b09 | awk '{print $2}'
40    0.0.0313
41
42* Unbind the subchannel (in this example, ``0.0.0313``) from the standard
43  I/O subchannel driver and bind it to the vfio-ccw driver::
44
45    [root@host ~]# echo 0.0.0313 > /sys/bus/css/devices/0.0.0313/driver/unbind
46    [root@host ~]# echo 0.0.0313 > /sys/bus/css/drivers/vfio_ccw/bind
47
48* Create the mediated device (identified by the uuid)::
49
50    [root@host ~]# echo "7e270a25-e163-4922-af60-757fc8ed48c6" > \
51    /sys/bus/css/devices/0.0.0313/mdev_supported_types/vfio_ccw-io/create
52
53Step 2: configure QEMU
54~~~~~~~~~~~~~~~~~~~~~~
55
56* Reference the created mediated device and (optionally) pick a device id to
57  be presented in the guest (here, ``fe.0.1234``, which will end up visible
58  in the guest as ``0.0.1234``::
59
60    -device vfio-ccw,devno=fe.0.1234,sysfsdev=\
61    /sys/bus/mdev/devices/7e270a25-e163-4922-af60-757fc8ed48c6
62
63* Start the guest. The device (here, ``0.0.1234``) should now be usable::
64
65    [root@guest ~]# lscss -d 0.0.1234
66    Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPID
67    ----------------------------------------------------------------------
68    0.0.1234 0.0.0007  3390/0e 3990/e9      f0  f0  ff   1a2a3a0a 00000000
69    [root@guest ~]# chccwdev -e 0.0.1234
70    Setting device 0.0.1234 online
71    Done
72    [root@guest ~]# dmesg -t
73    (...)
74    dasd-eckd 0.0.1234: A channel path to the device has become operational
75    dasd-eckd 0.0.1234: New DASD 3390/0E (CU 3990/01) with 10017 cylinders, 15 heads, 224 sectors
76    dasd-eckd 0.0.1234: DASD with 4 KB/block, 7212240 KB total size, 48 KB/track, compatible disk layout
77    dasda:VOL1/  0X2B09: dasda1
78