Device Emulation

QEMU supports the emulation of a large number of devices from peripherals such network cards and USB devices to integrated systems on a chip (SoCs). Configuration of these is often a source of confusion so it helps to have an understanding of some of the terms used to describes devices within QEMU.

Common Terms

Device Front End

A device front end is how a device is presented to the guest. The type of device presented should match the hardware that the guest operating system is expecting to see. All devices can be specified with the --device command line option. Running QEMU with the command line options --device help will list all devices it is aware of. Using the command line --device foo,help will list the additional configuration options available for that device.

A front end is often paired with a back end, which describes how the host’s resources are used in the emulation.

Device Buses

Most devices will exist on a BUS of some sort. Depending on the machine model you choose (-M foo) a number of buses will have been automatically created. In most cases the BUS a device is attached to can be inferred, for example PCI devices are generally automatically allocated to the next free address of first PCI bus found. However in complicated configurations you can explicitly specify what bus (bus=ID) a device is attached to along with its address (addr=N).

Some devices, for example a PCI SCSI host controller, will add an additional buses to the system that other devices can be attached to. A hypothetical chain of devices might look like:

–device foo,bus=pci.0,addr=0,id=foo –device bar,bus=foo.0,addr=1,id=baz

which would be a bar device (with the ID of baz) which is attached to the first foo bus (foo.0) at address 1. The foo device which provides that bus is itself is attached to the first PCI bus (pci.0).

Device Back End

The back end describes how the data from the emulated device will be processed by QEMU. The configuration of the back end is usually specific to the class of device being emulated. For example serial devices will be backed by a --chardev which can redirect the data to a file or socket or some other system. Storage devices are handled by --blockdev which will specify how blocks are handled, for example being stored in a qcow2 file or accessing a raw host disk partition. Back ends can sometimes be stacked to implement features like snapshots.

While the choice of back end is generally transparent to the guest, there are cases where features will not be reported to the guest if the back end is unable to support it.

Device Pass Through

Device pass through is where the device is actually given access to the underlying hardware. This can be as simple as exposing a single USB device on the host system to the guest or dedicating a video card in a PCI slot to the exclusive use of the guest.