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QEMU (Quick EMUlator) is a generic, open source hardware emulator and virtualization suite. Often it is used in conjunction with acceleration in the form of a Type-I hypervisor such as KVM (Kernel-based Virtual Machine) or Xen. If no accelerator is used, QEMU will run entirely in user-space using its built in binary translator TCG (Tiny Code Generator). Using QEMU without an accelerator is relatively inefficient and slow.

This article typically uses KVM as the accelerator of choice due to its GPL licensing and availability. Without KVM nearly all commands described here will still work (unless KVM specific).


BIOS and UEFI firmware

In order to utilize KVM either Vt-x or AMD-V must be supported by the processor. Vt-x or AMD-V are Intel and AMD's respective technologies for permitting multiple operating systems to concurrently execute operations on the processors.

To inspect hardware for visualization support issue the following command:

user $grep --color -E "vmx|svm" /proc/cpuinfo

For a period manufacturers were shipping with virtualization turned off by default in the system BIOS. Note that changing this feature in the BIOS may actually require full removal of power from the system to take effect. If restarting the system does not work try shutting down, unplugging the system and pressing the power button in an unplugged state to discharge any residual energy from the power supply unit (PSU). Reapply power to the system to verify success.

If KVM support is available there should be a "kvm" device listed at /dev/kvm. This will take effect after the system has booted to a KVM enabled kernel.


Activate the following kernel options:

[*] Virtualization  --->
    <*>   Kernel-based Virtual Machine (KVM) support
KERNEL Enable KVM support for Intel processors (CONFIG_KVM_INTEL)
[*] Virtualization  --->
    <M>   KVM for Intel processors support
KERNEL Enable KVM support for AMD processors (CONFIG_KVM_AMD)
[*] Virtualization  --->
    <M>   KVM for AMD processors support
If both "KVM for Intel processors support" and "KVM for AMD processors support" are set as built into the kernel (*) an error message will appear from kprint from early boot. Since the system has only one type processor (Intel or AMD) enabling one or both options as modules (M) will make the error message disappear.

Needed for vhost-net USE flag (recommend):

[*] Virtualization  --->
    <*>   Host kernel accelerator for virtio net
KERNEL Optional advanced networking support
Device Drivers  --->
    [*] Network device support  --->
        [*]   Network core driver support
        <*>   Universal TUN/TAP device driver support

Needed for 802.1d Ethernet bridging:

KERNEL Enabling 802.1d Ethernet Bridging support
[*] Networking support  --->
        Networking options  --->
            <*> The IPv6 protocol
            <*> 802.1d Ethernet Bridging

python USE flag is needed for file capabilities support:

KERNEL Enabling Linux file capabilities support
Kernel hacking  --->
        Compile-time checks and compiler options  --->
            [*] Debug Filesystem

When using the ext4 filesystem, enable the filecaps USE flag if stats support is needed:

KERNEL Enabling ext4 kvm_stat support
File systems  --->
    <*> The Extended 4 (ext4) filesystem
    [*]   Ext4 Security Labels

USE flags

Review the possible USE flags for QEMU:

USE flags for app-emulation/qemu QEMU + Kernel-based Virtual Machine userland tools

accessibility Adds support for braille displays using brltty local
aio Enables support for Linux's Async IO local
alsa Enable alsa output for sound emulation local
bzip2 Use the bzlib compression library global
capstone Enable disassembly support with dev-libs/capstone local
curl Support ISOs / -cdrom directives vis HTTP or HTTPS. local
fdt Enables firmware device tree support local
filecaps Use Linux file capabilities to control privilege rather than set*id (this is orthogonal to USE=caps which uses capabilities at runtime e.g. libcap) global
glusterfs Enables GlusterFS cluster fileystem via sys-cluster/glusterfs local
gnutls Enable TLS support for the VNC console server. For 1.4 and newer this also enables WebSocket support. For 2.0 through 2.3 also enables disk quorum support. local
iscsi Enable direct iSCSI support via net-libs/libiscsi instead of indirectly via the Linux block layer that sys-block/open-iscsi does. local
jpeg Enable jpeg image support for the VNC console server local
lzo Enable support for lzo compression global
ncurses Enable the ncurses-based console local
nfs Enable NFS support local
numa Enable NUMA support local
pin-upstream-blobs Pin the versions of BIOS firmware to the version included in the upstream release. This is needed to sanely support migration/suspend/resume/snapshotting/etc... of instances. When the blobs are different, random corruption/bugs/crashes/etc... may be observed. local
png Enable png image support for the VNC console server local
pulseaudio Enable pulseaudio output for sound emulation local
python Add optional support/bindings for the Python language global
rbd Enable rados block device backend support, see http://ceph.newdream.net/wiki/QEMU-RBD local
sdl Enable the SDL-based console local
snappy Enable support for snappy compression local
spice Enable Spice protocol support via app-emulation/spice local
ssh Enable SSH based block device support via net-libs/libssh2 local
static Build the User and Software MMU (system) targets as well as tools as static binaries local
static-user Build the User targets as static binaries local
systemtap Enable SystemTAP/DTrace tracing local
tci Enable the TCG Interpreter which can speed up or slowdown workloads depending on the host and guest CPUs being emulated. In the future it will be a runtime option but for now its compile time. local
usb Enable USB passthrough via dev-libs/libusb local
usbredir Use sys-apps/usbredir to redirect USB devices to another machine over TCP local
vde Enable VDE-based networking local
vhost-net Enable accelerated networking using vhost-net, see http://www.linux-kvm.org/page/VhostNet local
virgl Enable experimental Virgil 3d (virtual software GPU) local
virtfs Enable VirtFS via virtio-9p-pci / fsdev. See http://wiki.qemu.org/Documentation/9psetup local
vnc Enable VNC (remote desktop viewer) support global
vte Enable terminal support (x11-libs/vte) in the GTK+ interface local
xattr Add support for getting and setting POSIX extended attributes, through sys-apps/attr. Requisite for the virtfs backend. local
xen Enables support for Xen backends local
xfs Support xfsctl() notification and syncing for XFS backed virtual disks. local

More than one USE flag (gtk, ncurses, sdl or spice) can be enabled for graphical output. If graphics are desired it is generally recommended to enable more than one graphical USE flag.


See app-emulation/qemu for a list of possible values for QEMU_USER_TARGETS and QEMU_SOFTMMU_TARGETS.

For each target specified, a qemu executable will be built. Normally, a softmmu target will work; user targets execute user-mode code only - i.e. the goal is to execute a single ELF binary from a different architecture, rather than an entire OS.

In order to enable QEMU_USER_TARGETS and QEMU_SOFTMMU_TARGETS you have to edit the make.conf file.

FILE make.conf
QEMU_SOFTMMU_TARGETS="arm x86_64 sparc"

Otherwise you can always edit the package.use file.

FILE package.use
qemu_softmmu_targets_m68k qemu_user_targets_x86_64


After reviewing and adding any desired USE flags, emerge app-emulation/qemu:

root #emerge --ask app-emulation/qemu



For Networking configuration, see the networking options documentation.


For IPv6 networking see the IPv6 subarticle.


In order to run a KVM accelerated virtual machine without logging as root, add normal users to the kvm group. Replace <username> in the example command below with the appropriate user(s):

root #gpasswd -a <username> kvm

Front ends

To make life easier, there are multiple user-friendly front ends to QEMU:

Name Package Homepage Description
AQEMU app-emulation/aqemu https://sourceforge.net/projects/aqemu/ Graphical interface for QEMU and KVM emulators, using Qt5.
GNOME Boxes gnome-extra/gnome-boxes https://wiki.gnome.org/Apps/Boxes GNOME App to manage virtual and remote machines
libvirt app-emulation/libvirt https://www.libvirt.org/ C toolkit to manipulate virtual machines.
QtEmu app-emulation/qtemu http://qtemu.sourceforge.net/ A graphical user interface for QEMU written in Qt4.
qt-virt-manager app-emulation/qt-virt-manager https://f1ash.github.io/qt-virt-manager/ A graphical user interface for libvirt written in Qt4/5.
virt-manager app-emulation/virt-manager https://virt-manager.org A graphical tool for administering virtual machines.


When using virt-manager the usbredir USE flag must be enabled for qemu to work properly.

To run as a normal user, after emerging, ensure they are in the libvirt group:

root #groupadd libvirt
root #usermod -a -G libvirt <user>

Uncomment the following lines from the libvirtd configuration file:

FILE /etc/libvirt/libvirtd.conf
unix_sock_group = "libvirt"
unix_sock_ro_perms = "0777"
unix_sock_rw_perms = "0770"

Be sure to have the user log out then log in again for the new group settings to be applied.

Issue the following command to restart the libvirtd service under OpenRC:

root #/etc/init.d/libvirtd restart

Issue the following command to restart the libvirtd service under Systemd:

root #systemctl restart libvirtd

virt-manager should now be launchable as a regular user.

If permission denied issues are experienced when loading ISO images user directories (somewhere beneath /home/) then the /var/lib/libvirt/images/ directory can be used to store the images.


The following sub-articles provide instructions on QEMU usage:

  • Usage options - Contains common options used with QEMU.
  • Linux guest - Describes the configuration steps needed to setup Linux to run on QEMU.
  • Windows guest - Describes the configuration steps needed to setup Windows to run on QEMU.
  • OS2WarpV3 guest - Describes the configuration steps needed to setup OS2WarpVs=3 to run on QEMU.


"kvm: already loaded the other module"

Sometimes during the early boot splash the error message "kvm: already loaded the other module" can be seen. This message indicates both the Intel and the AMD kernel virtual machine settings have been enabled in the kernel. To fix this, enable as a module or disable either the Intel or AMD KVM option specific to the system's processor in the kernel configuration. For example, if the system has an Intel processor enable the Intel KVM, then make sure the AMD KVM is set as a module (M) or is disabled (N). The relevant options to enable or disable can be found in the kernel's .config file via the CONFIG_KVM_INTEL and CONFIG_KVM_AMD variables or in the configuration section above.

Creating TUN/TAP device - No such file or directory

Sometimes this error can occur if TUN/TAP support cannot be found in the kernel. To solve this, try loading the driver:

root #modprobe tun

If that works, add this to a file in /etc/modules-load.d/ to load on startup:

FILE /etc/modules-load.d/qemu-modules.conf

See also

External resources