14.3 Using Devices in QEMU

QEMU virtual machines emulate all devices needed to run a VM Guest. QEMU supports, for example, several types of network cards, block devices (hard and removable drives), USB devices, character devices (serial and parallel ports), or multimedia devices (graphic and sound cards). For satisfactory operation and performance of the virtual machine, some or all of these devices must be configured correctly. This section introduces options to configure various types of supported devices.

HINT: If your device, such as -drive, needs a special driver and driver properties to be set, specify them with the -device option, and identify with drive= suboption. For example

qemu [...] -drive if=none,id=drive0,format=raw \
-device virtio-blk-pci,drive=drive0,scsi=off ...

To get help on available drivers and their properties, use -device ? and -device driver,?.

14.3.1 Block Devices

Block devices are vital for virtual machines. In general, these are fixed or removable storage media usually referred to as 'drives'. One of the connected hard drives typically holds the guest operating system to be virtualized.

Virtual machine drives are defined with -drive. This option uses many suboptions, some of which are described in this section. For their complete list, see the manual page (man 1 qemu-kvm).

Sub-options for the -drive Option

Specifies the path to the disk image which will be used with this drive. If not specified, an empty (removable) drive is assumed.


Specifies the type of interface to which the drive is connected. Currently only floppy, ide, or virtio are supported by SUSE. virtio defines a paravirtualized disk driver. Default is ide.


Specifies the index number of a connector on the disk interface (see the if option) where the drive is connected. If not specified, the index is automatically incremented.


Specifies the type of the media. Can be disk for hard disks, or cdrom for removable CD-ROM drives.


Specifies the format of the connected disk image. If not specified, the format is autodetected. Currently, SUSE supports qcow2, qed and raw formats for read and write access. The vmdk, vpc and vhd/vhdx formats are only supported in read-only mode.


Specifies the caching method for the drive. Possible values are unsafe, writethrough, writeback, directsync, or none. For the qcow2 image format, choose writeback if you care about performance. none disables the host page cache and, therefore, is the safest option. Default for image files is writeback.

HINT: To simplify defining of block devices, QEMU understands several shortcuts which you may find handy when entering the qemu-kvm command line.

You can use

qemu-kvm -cdrom /images/cdrom.iso

instead of

qemu-kvm -drive file=/images/cdrom.iso,index=2,media=cdrom


qemu-kvm -hda /images/imagei1.raw -hdb /images/image2.raw -hdc \
/images/image3.raw -hdd /images/image4.raw

instead of

qemu-kvm -drive file=/images/image1.raw,index=0,media=disk \
-drive file=/images/image2.raw,index=1,media=disk \
-drive file=/images/image3.raw,index=2,media=disk \
-drive file=/images/image4.raw,index=3,media=disk

HINT: Using Host Drives Instead of Images

Normally you will use disk images (see Section 13.2, Managing Disk Images with qemu-img) as disk drives of the virtual machine. However, you can also use existing VM Host Server disks, connect them as drives, and access them from VM Guest. Use the host disk device directly instead of disk image filenames.

To access the host CD-ROM drive, use

qemu-kvm [...] -drive file=/dev/cdrom,media=cdrom

To access the host hard disk, use

qemu-kvm [...] -drive file=/dev/hdb,media=disk

When accessing the host hard drive from VM Guest, always make sure the access is read-only. You can do so by modifying the host device permissions.


The virtio-blk-data-plane is a new performance feature for KVM. It enables a high-performance code path for I/O requests coming from VM Guests. More specifically, this feature introduces dedicated threads (one per virtual block device) to process I/O requests going through the virtio-blk driver. It makes use of Linux AIO (asynchronous I/O interface) support in the VM Host Server Kernel directly—without the need to go through the QEMU block layer. Therefore it can sustain very high I/O rates on storage setups.

The virtio-blk-data-plane feature can be enabled or disabled by the x-data-plane=on|off option on the qemu command line when starting the VM Guest:

qemu [...] -drive if=none,id=drive0,cache=none,aio=native,\
format=raw,file=filename -device virtio-blk-pci,drive=drive0,scsi=off,\
config-wce=off,x-data-plane=on [...]

As of now, the virtio-blk-data-plane has the following limitations:

  • Only raw image format is supported.

  • No support for live migration.

  • Block jobs and hot unplug operations fail with -EBUSY.

  • I/O throttling limits are ignored.

  • Only Linux VM Host Servers are supported because of the Linux AIO usage, but non-Linux VM Guests are supported.

IMPORTANT: The virtio-blk-data-plane feature is not yet supported in SUSE Linux Enterprise Server. It is released as a technical preview only.

14.3.2 Graphic Devices and Display Options

This section describes QEMU options affecting the type of the emulated video card and the way VM Guest graphical output is displayed.

Defining Video Cards

QEMU uses -vga to define a video card used to display VM Guest graphical output. The -vga option understands the following values:


Disables video cards on VM Guest (no video card is emulated). You can still access the running VM Guest via the QEMU monitor and the serial console.


Emulates a standard VESA 2.0 VBE video card. Use it if you intend to use high display resolution on VM Guest.


Emulates Cirrus Logic GD5446 video card. Good choice if you insist on high compatibility of the emulated video hardware. Most operating systems (even Windows 95) recognize this type of card.

HINT: For best video performance with the cirrus type, use 16-bit color depth both on VM Guest and VM Host Server.

Display Options

The following options affect the way VM Guest graphical output is displayed.


Disables QEMU's graphical output. The emulated serial port is redirected to the console.

After starting the virtual machine with -nographic, press Ctrl+A H in the virtual console to view the list of other useful shortcuts, for example, to toggle between the console and the QEMU monitor.

tux@venus:~> qemu-kvm -hda /images/sles11sp1_base.raw -nographic

C-a h    print this help
C-a x    exit emulator
C-a s    save disk data back to file (if -snapshot)
C-a t    toggle console timestamps
C-a b    send break (magic sysrq)
C-a c    switch between console and monitor
C-a C-a  sends C-a
(pressed C-a c)

QEMU 0.12.5 monitor - type 'help' for more information

Disables decorations for the QEMU window. Convenient for dedicated desktop workspace.


Starts QEMU graphical output in full screen mode.


Disables the 'close' button of QEMU window and prevents it from being closed by force.

-alt-grab, -ctrl-grab

By default QEMU window releases the 'captured' mouse after Ctrl+Alt is pressed. You can change the key combination to either Ctrl+Alt+Shift (-alt-grab), or Right Ctrl (-ctrl-grab).

14.3.3 USB Devices

There are basically two ways to create USB devices usable by the VM Guest in KVM: you can either emulate new USB devices inside a VM Guest, or assign an existing host USB device to a VM Guest. To use USB devices in QEMU you first need to enable the generic USB driver with the -usb option. Then you can specify individual devices with the -usbdevice option.

Emulating USB Devices in VM Guest

SUSE currently supports the following types of USB devices: disk, host, serial, braille, netmouse and tablet.

Types of USB devices for the -usbdevice option

Emulates a mass storage device based on file. The optional format option is used rather than detecting the format.

qemu-kvm [...] -usbdevice

Pass through the host device (identified by bus.addr).


Serial converter to a host character device.


Emulates a braille device using BrlAPI to display the braille output.


Emulates a network adapter that supports CDC ethernet and RNDIS protocols.


Emulates a virtual USB mouse. This option overrides the default PS/2 mouse emulation. The following example shows the hardware status of a mouse on VM Guest started with qemu-kvm [...] -usbdevice mouse:

tux@venus:~> hwinfo --mouse
20: USB 00.0: 10503 USB Mouse
[Created at usb.122]
UDI: /org/freedesktop/Hal/devices/usb_device_627_1_1_if0
Hardware Class: mouse
Model: "Adomax QEMU USB Mouse"
Hotplug: USB
Vendor: usb 0x0627 "Adomax Technology Co., Ltd"
Device: usb 0x0001 "QEMU USB Mouse"

Emulates a pointer device that uses absolute coordinates (such as touchscreen). This option overrides the default PS/2 mouse emulation. The tablet device is useful if you are viewing VM Guest via the VNC protocol. See Section 14.5, Viewing a VM Guest with VNC for more information.

USB Pass-Through

To assign an existing host USB device to a VM Guest, you need to find out its host bus and device ID.

tux@vmhost:~> lsusb
Bus 002 Device 005: ID 12d1:1406 Huawei Technologies Co., Ltd. E1750

In the above example, we want to assign a USB stick connected to the host's USB bus number 2 with device number 5. Now run the VM Guest with the following additional options:

qemu-kvm [...] -usb -device usb-host,hostbus=2,hostaddr=5

After the guest is booted, check that the assigned USB device is present on it.

tux@vmguest:~> lsusb
Bus 001 Device 002: ID 12d1:1406 Huawei Technologies Co., Ltd. E1750

NOTE: The guest operating system must take care of mounting the assigned USB device so that it is accessible for the user.

14.3.4 PCI Pass-Through

PCI Pass-Through is a technique to give your VM Guest exclusive access to a PCI device.

NOTE: To make use of PCI Pass-Through, your motherboard chipset, BIOS, and CPU must have support for AMD's IOMMU (or VT-d in Intel speak) virtualization technology. To make sure that your computer supports this feature, ask your supplier specifically to deliver a system that supports PCI Pass-Through.

NOTE: Assignment of graphics cards is not supported by SUSE.

Configuring PCI Pass-Through

  1. Make sure that CONFIG_DMAR_DEFAULT_ON is set in the host's running Kernel:

    grep CONFIG_DMAR_DEFAULT_ON /boot/config-`uname -r`

    If this option is not set, edit your boot loader configuration and add intel_iommu=on (Intel machines) or iommu=pt iommu=1 (AMD machines). Then reboot the host machine.

  2. Check that IOMMU is actively enabled and recognized on the host. Run dmesg | grep -e DMAR -e IOMMU on Intel machines, or dmesg | grep AMD-Vi on AMD machines. If you get no output, check carefully if your hardware supports IOMMU (VT-d) and check that it has been enabled in BIOS.

  3. Identify the host PCI device to assign to the guest.

    tux@vmhost:~> lspci -nn
    [...] 00:1b.0 Audio device [0403]: Intel Corporation 82801H (ICH8 Family) \
    HD Audio Controller [8086:284b] (rev 02) [...]

    Note down the device (00:1b.0) and vendor (8086:284b) ID.

  4. Unbind the device from host Kernel driver and bind it to the PCI stub driver.

    tux@vmhost:~> modprobe pci_stub
    tux@vmhost:~> echo "8086 284b" > /sys/bus/pci/drivers/pci-stub/new_id
    tux@vmhost:~> echo "0000:00:1b.0" > /sys/bus/pci/devices/0000:00:1b.0/driver/unbind
    tux@vmhost:~> echo "0000:00:1b.0" > /sys/bus/pci/drivers/pci-stub/bind
  5. Now run the VM Guest with the PCI device assigned.

    qemu-kvm [...] -device pci-assign,host=00:1b.0

NOTE: If the PCI device shares IRQ with other devices, it cannot be assigned to a VM Guest.

KVM also supports PCI device hot-plugging to a VM Guest. To achieve this, you need to switch to a QEMU monitor (see Section 16.0, Administrating Virtual Machines with QEMU Monitor for more information) and issue the following commands:

  • hot add:

    device_add pci-assign,host=00:1b.0,id=new_pci_device
  • hot remove:

    device_del new_pci_device

14.3.5 Character Devices

Use -chardev to create a new character device. The option uses the following general syntax:

qemu-kvm [...] -chardev backend_type,id=id_string

where backend_type can be one of null, socket, udp, msmouse, vc, file, pipe, console, serial, pty, stdio, braille, tty, or parport. All character devices must have a unique identification string up to 127 characters long. It is used to identify the device in other related directives. For the complete description of all back-end's suboptions, see the manual page (man 1 qemu-kvm). A brief description of the available back-ends follows:


Creates an empty device which outputs no data and drops any data it receives.


Connects to QEMU's process standard input and standard output.


Creates a two-way stream socket. If path is specified, a Unix socket is created:

qemu-kvm [...] -chardev \

The server suboption specifies that the socket is a listening socket.

If port is specified, a TCP socket is created:

qemu-kvm [...] -chardev \

The command creates a local listening (server) TCP socket on port 7777. QEMU will not block waiting for a client to connect to the listening port (nowait).


Sends all network traffic from VM Guest to a remote host over the UDP protocol.

qemu-kvm [...] -chardev udp,id=udp_fwd,host=mercury.example.com,port=7777

The command binds port 7777 on the remote host mercury.example.com and sends VM Guest network traffic there.


Creates a new QEMU text console. You can optionally specify the dimensions of the virtual console:

qemu-kvm [...] -chardev vc,id=vc1,width=640,height=480 -mon chardev=vc1

The command creates a new virtual console called vc1 of the specified size, and connects the QEMU monitor to it.


Logs all traffic from VM Guest to a file on VM Host Server. The path is required and will be created if it does not exist.

qemu-kvm [...] -chardev file,id=qemu_log1,path=/var/log/qemu/guest1.log

By default QEMU creates a set of character devices for serial and parallel ports, and a special console for QEMU monitor. You can, however, create your own character devices and use them for just mentioned purposes. The following options will help you:

-serial char_dev

Redirects the VM Guest's virtual serial port to a character device char_dev on VM Host Server. By default, it is a virtual console (vc) in graphical mode, and stdio in non-graphical mode. The -serial understands many suboptions. See the manual page man 1 qemu-kvm for their complete list.

You can emulate up to 4 serial ports. Use -serial none to disable all serial ports.

-parallel device

Redirects the VM Guest's parallel port to a device. This option supports the same devices as -serial.

HINT: With SUSE Linux Enterprise Server as a VM Host Server, you can directly use the hardware parallel port devices /dev/parportN where N is the number of the port.

You can emulate up to 3 parallel ports. Use -parallel none to disable all parallel ports.

-monitor char_dev

Redirects the QEMU monitor to a character device char_dev on VM Host Server. This option supports the same devices as -serial. By default, it is a virtual console (vc) in a graphical mode, and stdio in non-graphical mode.

For a complete list of available character devices back-ends, see the man page (man 1 qemu-kvm).