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QEMU

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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.

Note
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).

Installation

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.

Kernel

Activate the following kernel options:

KERNEL 
[*] 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
Warning
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):

KERNEL 
[*] 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

+aio Enables support for Linux's Async IO
+curl Support ISOs / -cdrom directives via HTTP or HTTPS.
+doc Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally
+fdt Enables firmware device tree support
+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)
+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.
+jpeg Enable jpeg image support for the VNC console server
+oss Add support for OSS (Open Sound System)
+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.
+png Enable png image support for the VNC console server
+seccomp Enable seccomp (secure computing mode) to perform system call filtering at runtime to increase security of programs
+slirp Enable TCP/IP in hypervisor via net-libs/libslirp
+vhost-net Enable accelerated networking using vhost-net, see https://www.linux-kvm.org/page/VhostNet
+vnc Enable VNC (remote desktop viewer) support
accessibility Adds support for braille displays using brltty
alsa Enable alsa output for sound emulation
bpf Enable eBPF support for RSS implementation.
bzip2 Enable bzip2 compression support
capstone Enable disassembly support with dev-libs/capstone
debug Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
fuse Enables FUSE block device export
glusterfs Enables GlusterFS cluster fileystem via sys-cluster/glusterfs
gtk Add support for x11-libs/gtk+ (The GIMP Toolkit)
infiniband Enable Infiniband RDMA transport support
io-uring Enable the use of io_uring for efficient asynchronous IO and system requests
iscsi Enable direct iSCSI support via net-libs/libiscsi instead of indirectly via the Linux block layer that sys-block/open-iscsi does.
jack Add support for the JACK Audio Connection Kit
jemalloc Use dev-libs/jemalloc for memory management
keyutils Support Linux keyrings via sys-apps/keyutils
lzo Enable support for lzo compression
multipath Enable multipath persistent reservation passthrough via sys-fs/multipath-tools.
ncurses Enable the ncurses-based console
nfs Enable NFS support
nls Add Native Language Support (using gettext - GNU locale utilities)
numa Enable NUMA support
opengl Add support for OpenGL (3D graphics)
pam Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip
pipewire Enable pipewire output for sound emulation
plugins Enable qemu plugin API via shared library loading.
pulseaudio Enable pulseaudio output for sound emulation
python Add optional support/bindings for the Python language
rbd Enable rados block device backend support, see https://docs.ceph.com/en/mimic/rbd/qemu-rbd/
sasl Add support for the Simple Authentication and Security Layer
sdl Enable the SDL-based console
sdl-image SDL Image support for icons
selinux  !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur
smartcard Enable smartcard support
snappy Enable support for Snappy compression (as implemented in app-arch/snappy)
spice Enable Spice protocol support via app-emulation/spice
ssh Enable SSH based block device support via net-libs/libssh2
static Build the User and Software MMU (system) targets as well as tools as static binaries
static-user Build the User targets as static binaries
systemtap Enable SystemTap/DTrace tracing
test Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)
udev Enable virtual/udev integration (device discovery, power and storage device support, etc)
usb Enable USB passthrough via dev-libs/libusb
usbredir Use sys-apps/usbredir to redirect USB devices to another machine over TCP
vde Enable VDE-based networking
virgl Enable experimental Virgil 3d (virtual software GPU)
virtfs Enable VirtFS via virtio-9p-pci / fsdev. See https://wiki.qemu.org/Documentation/9psetup
vte Enable terminal support (x11-libs/vte) in the GTK+ interface
xattr Add support for getting and setting POSIX extended attributes, through sys-apps/attr. Requisite for the virtfs backend.
xdp Enable support for XDP through net-libs/xdp-tools
xen Enables support for Xen backends
zstd Enable support for ZSTD compression
Data provided by the Gentoo Package Database · Last update: 2025-02-27 09:31 More information about USE flags

Note
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.

USE_EXPAND

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"
QEMU_USER_TARGETS="x86_64"

Otherwise you can always edit the package.use file.

FILE package.use
qemu_softmmu_targets_m68k qemu_user_targets_x86_64

Emerge

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

root #emerge --ask app-emulation/qemu

Configuration

Networking

For Networking configuration, see the networking options documentation.

IPv6

For IPv6 networking see the IPv6 subarticle.

Permissions

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.

virt-manager

Note
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.

Note
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.

Usage

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.

Troubleshooting

"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
tun

See also

External resources

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