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 as0.0.1234
:-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