Device virtualization techniques

KVM on IBM® Z offers two major techniques to virtualize devices for its guests: virtio and VFIO.

Virtio devices

On IBM Z, the virtio framework virtualizes devices as virtio CCW devices. KVM guests access virtio CCW devices through a virtual z/Architecture® channel subsystem.

Virtio CCW devices can be paravirtualized host devices that resemble generic devices, as do paravirtualized devices on other architectures. Many of the characteristics of paravirtualized host devices are hidden from the guests.

The virtio CCW devices also include devices that are provided by QEMU and that are not based on physical host devices.

A special type of virtio CCW devices are virtual SCSI Host Bus Adapters (HBAs), which provide virtual SCSI devices to KVM guests. Virtual SCSI devices include SCSI pass-through devices, which are based on SCSI devices on the host.

VFIO pass-through devices

The VFIO framework can give guests direct access to specific host devices.

On the host, these VFIO pass-through devices are set up to be controlled by device-specific VFIO device drivers instead of their default device drivers. Depending on the device type, VFIO pass-through devices might require a VFIO mediated device that is based on host resources.

VFIO pass-through devices can block live migration of a virtual server, see VFIO pass-through devices.