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

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}'
  • 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" > \

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=\
  • 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
    [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