Features/Channel I/O Passthrough

Notice: this page is still under construction

The patch series introduce a basic channel I/O passthrough infrastructure based on vfio.

Focus on supporting dasd-eckd(cu_type/dev_type = 0x3990/0x3390) as the target device.
Support new qemu parameters in the style of:

   -machine s390-ccw-virtio(,s390-squash-mcss=on|off) \
   -device vfio-ccw,sysfsdev=$MDEV_PATH

We want to support real (i.e. not virtual) channel devices even for guests that do not support MCSS-E (where guests may see devices from any channel subsystem image at once). As all virtio-ccw devices are in css 0xfe (and show up in the default css 0 for guests not activating MCSS-E), we need an option to squash e.g. passed-through channel devices from their real css (0-3, or 0 for hosts not activating MCSS-E) into the default css, that is what the new machine option s390-squash-css is added.

Setup

This example setup is done with a Linux guest running in an s390x-ccw-virtio machine.

Prereqs

Kernel Configuration

 CONFIG_S390_CCW_IOMMU=m
 CONFIG_VFIO=m
 CONFIG_VFIO_MDEV=m
 CONFIG_VFIO_MDEV_DEVICE=m
 CONFIG_VFIO_CCW=m

Modules Required

 modprobe vfio.ko
 modprobe mdev.ko
 modprobe vfio_mdev.ko
 modprobe vfio_iommu_type1.ko
 modprobe vfio_ccw.ko

Prepare a Subchannel

Find a subchannel(0.0."%schid") of a DASD-ECKD device and bind it to vfio_ccw driver.

 #find the dasd you can use with lsdasd on your host. e.g.:
 devno="7e52"
 schid="16ca"
 #unbind the ccw device from the subchannel
 echo 0.0."$devno" > /sys/bus/ccw/devices/0.0."$devno"/driver/unbind
 #unbind the subchannel from io_subchannel driver
 echo 0.0."$schid" > /sys/bus/css/devices/0.0."$schid"/driver/unbind
 #bind the subchannel with vfio_ccw driver
 echo 0.0."$schid" > /sys/bus/css/drivers/vfio_ccw/bind

Create a Mediated Device

 #generate a uuid with uuidgen. e.g.:
 uuid="6dfd3ec5-e8b3-4e18-a6fe-57bc9eceb920"
 echo "$uuid" > \
 /sys/bus/css/devices/0.0."$schid"/mdev_supported_types/vfio_ccw-io/create

Starting QEMU

Pass-through this device to a vm:

 -M s390-ccw-virtio,s390-squash-css=on \
 -device vfio-ccw,sysfsdev=/sys/bus/mdev/devices/$uuid \
 ... ...