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).
The following sub-articles provide detailed instructions on QEMU configurations and options:
- QEMU/Bridge with Wifi Routing
- QEMU/KVM_IPv6_Support - IPv6 support in QEMU/KVM.
- Linux guest - Describes the configuration steps needed to setup a virtualized Linux guest with QEMU.
- Usage options - Contains common configuration options used with QEMU (graphics/display, networking, RAM, storage, processor, etc).
- OS2WarpV3 guest - Describes the configuration steps needed to setup a virtualized OS2WarpVs=3 guest with QEMU.
- Windows guest - Describes the configuration steps needed to setup a virtualized Windows guest with QEMU.
BIOS and UEFI firmware
In order to utilize KVM either Vt-x (vmx) or AMD-V (svm) 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 virtualization support issue the following command:
grep --color -E "vmx|svm" /proc/cpuinfo
For a period manufacturers were shipping with virtualization turned off by default in the system's firmware. Note that toggling this feature in the firmware 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.
Described below are the basic requirements for KVM kernel configuration. A more complete and up-to-date list can be found at the KVM Tuning Kernel page.
Different guest (virtualized) OS may require additional kernel options. These are covered in the corresponding #Usage section pages.
General setup ---> Timers subsystem ---> <*> High Resolution Timer Support
[*] Virtualization ---> <*> Kernel-based Virtual Machine (KVM) support
This includes support for ARM64 processors.
[*] Virtualization ---> <M> KVM for Intel processors support
[*] 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.
Accelerated networking, required for
vhost-net USE flag (recommend):
Device Drivers ---> [*] VHOST drivers ---> <*> Host kernel accelerator for virtio net
[*] Virtualization ---> <*> Host kernel accelerator for virtio net
Device Drivers ---> [*] Network device support ---> [*] Network core driver support <*> Universal TUN/TAP device driver support
Needed for 802.1d Ethernet bridging:
[*] Networking support ---> Networking options ---> <*> The IPv6 protocol <*> 802.1d Ethernet Bridging
Intel VT-g (integrated graphics adapter virtualization)
Mediated device passthrough for Intel GPUs (Broadwell and newer) .
Device Drivers ---> <*> VFIO Non-Privileged userspace driver framework <*> Mediated device driver framework Graphics Support ---> <*> Intel 8xx/9xx/G3x/G4x/HD Graphics [*] Enable Intel GVT-g graphics virtualization host support <*> Enable KVM host support Intel GVT-g graphics virtualization
Review the possible USE flags for QEMU:
USE flags for app-emulation/qemu QEMU + Kernel-based Virtual Machine userland tools
||Adds support for braille displays using brltty|
||Enables support for Linux's Async IO|
||Enable alsa output for sound emulation|
||Enable eBPF support for RSS implementation.|
||Enable bzip2 compression support|
||Enable disassembly support with dev-libs/capstone|
||Support ISOs / -cdrom directives via HTTP or HTTPS.|
||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|
||Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally|
||Enables firmware device tree support|
||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)|
||Enables FUSE block device export|
||Enables GlusterFS cluster fileystem via sys-cluster/glusterfs|
||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.|
||Add support for x11-libs/gtk+ (The GIMP Toolkit)|
||Enable Infiniband RDMA transport support|
||Enable efficient I/O via sys-libs/liburing.|
||Enable direct iSCSI support via net-libs/libiscsi instead of indirectly via the Linux block layer that sys-block/open-iscsi does.|
||Add support for the JACK Audio Connection Kit|
||Enable jemalloc allocator support|
||Enable jpeg image support for the VNC console server|
||Support Linux keyrings via sys-apps/keyutils|
||Enable support for lzo compression|
||Enable multipath persistent reservation passthrough via sys-fs/multipath-tools.|
||Enable the ncurses-based console|
||Enable NFS support|
||Add Native Language Support (using gettext - GNU locale utilities)|
||Enable NUMA support|
||Add support for OpenGL (3D graphics)|
||Add support for OSS (Open Sound System)|
||Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip|
||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.|
||Enable pipewire output for sound emulation|
||Enable qemu plugin API via shared library loading.|
||Enable png image support for the VNC console server|
||Enable pulseaudio output for sound emulation|
||Add optional support/bindings for the Python language|
||Enable rados block device backend support, see https://docs.ceph.com/en/mimic/rbd/qemu-rbd/|
||Add support for the Simple Authentication and Security Layer|
||Enable the SDL-based console|
||SDL Image support for icons|
||Enable seccomp (secure computing mode) to perform system call filtering at runtime to increase security of programs|
||!!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur|
||Enable TCP/IP in hypervisor via net-libs/libslirp|
||Enable smartcard support|
||Enable support for Snappy compression (as implemented in app-arch/snappy)|
||Enable Spice protocol support via app-emulation/spice|
||Enable SSH based block device support via net-libs/libssh2|
||Build the User targets as static binaries|
||Enable SystemTAP/DTrace tracing|
||Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)|
||Enable virtual/udev integration (device discovery, power and storage device support, etc)|
||Enable USB passthrough via dev-libs/libusb|
||Use sys-apps/usbredir to redirect USB devices to another machine over TCP|
||Enable VDE-based networking|
||Enable accelerated networking using vhost-net, see https://www.linux-kvm.org/page/VhostNet|
||Enable experimental Virgil 3d (virtual software GPU)|
||Enable VirtFS via virtio-9p-pci / fsdev. See https://wiki.qemu.org/Documentation/9psetup|
||Enable VNC (remote desktop viewer) support|
||Enable terminal support (x11-libs/vte) in the GTK+ interface|
||Add support for getting and setting POSIX extended attributes, through sys-apps/attr. Requisite for the virtfs backend.|
||Enables support for Xen backends|
||Enable support for ZSTD compression|
More than one USE flag (
spice) can be enabled for graphical output. If graphics are desired it is generally recommended to enable more than one graphical USE flag.
If virt-manager is going to be used, be sure to enable the
spiceUSE flags on the qemu package for correct operation.
Additional ebuild configuration frobs are provided as the USE_EXPAND variables QEMU_USER_TARGETS and QEMU_SOFTMMU_TARGETS. See app-emulation/qemu for a list of all the available targets (there are a heck of a lot of them; most of them are very obscure and may be ignored; leaving these variables at their default values will disable almost everything which is probably just fine for most users).
For each target specified, a qemu executable will be built. A
softmmu target is the standard qemu use-case of emulating an entire system (like VirtualBox or VMWare, but with optional support for emulating CPU hardware along with peripherals).
user targets execute user-mode code only; the (somewhat shockingly ambitious) purpose of these targets is to "magically" allow importing user-space linux ELF binaries from a different architecture into the native system (that is, they are like multilib, without the awkward need for a software stack or CPU capable of running it).
In order to enable QEMU_USER_TARGETS and QEMU_SOFTMMU_TARGETS we can edit the variables globally in /etc/portage/make.conf, i.e.:
QEMU_SOFTMMU_TARGETS="arm x86_64 sparc" QEMU_USER_TARGETS="x86_64"
Or, the /etc/portage/package.use file(s) can be modified. Two equivalent syntaxes are available: traditional USE flag syntax, i.e.:
app-emulation/qemu qemu_softmmu_targets_arm qemu_softmmu_targets_x86_64 qemu_softmmu_targets_sparc app-emulation/qemu qemu_user_targets_x86_64
Another alternative is to use the newer sexy USE_EXPAND-specific syntax:
app-emulation/qemu QEMU_SOFTMMU_TARGETS: arm x86_64 sparc QEMU_USER_TARGETS: x86_64
After reviewing and adding any desired USE flags, emerge app-emulation/qemu:
emerge --ask app-emulation/qemu
Qemu can be used in two ways, with GUI front ends and through the command line. The configuration of QEMU depends on which method is employed.
To make life easier, there are multiple user-friendly front ends to QEMU:
|AQEMU||https://anyon3.github.io/aqemu.html||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||https://gitlab.com/qtemu/gui||Qt-based front-end for QEMU.|
|qt-virt-manager||https://f1ash.github.io/qt-virt-manager/||A graphical user interface for libvirt written in Qt5.|
|virt-manager||app-emulation/virt-manager||https://virt-manager.org||A graphical tool for administering virtual machines.|
QEMU binaries are used to run the virtualized guest.
qemu-system-x86_64 [options] [disk_image]
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):
gpasswd -a <username> kvm
"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:
If that works, add this to a file in /etc/modules-load.d/ to load on startup:
Configuration does not support video model 'qxl'
This is usually the case if QEMU is not built with the
spice USE flag. To resolve this issue, try to build QEMU with the correct USE flag.
spice to via a package.use file:
Then rebuild the package:
emerge --ask app-emulation/qemu
My qemu has kvm support on some guest architectures
KVM works only for the same architecture. An ARM64 host cannot handle x86_64 instructions.
Invalid context errors on SELinux systems
By default, Libvirt generates a random SELinux MCS label for the QEMU process when it is started. If the loaded SELinux policy does not support MCS categories, the resulting security context will be invalid:
Error starting domain: unable to set socket security context 'system_u:system_r:svirt_t:s0:c123,c456': Invalid argument
kernel: SELinux: Context system_u:object_r:svirt_image_t:s0:c123,c456 is not valid (left unmapped).
The solution is either to switch to one of the policy types which supports MCS categories or manually set the virtual machine's security labels, without MCS categories:
<domain type="kvm"> <name>fedora</name> ... <devices> <disk type="file" device="disk"> <driver name="qemu" type="qcow2"/> <source file="/var/lib/libvirt/images/fedora.qcow2"> <seclabel model='selinux' relabel='yes'> <label>system_u:object_r:svirt_image_t</label> </seclabel> </source> <target dev="vda" bus="virtio"/> <address type="pci" domain="0x0000" bus="0x04" slot="0x00" function="0x0"/> </disk> ... <seclabel type='static' model='selinux' relabel='yes'> <label>system_u:system_r:svirt_t</label> </seclabel> </domain>
Static-user and LTO
GCC will use huge amount of RAM when LTO is enabled on the system while using the
static-user flag, because of this is recommended to disable LTO while compiling in this configuration or use clang if LTO is required. See bug #883419
lto1: internal compiler error: original not compressed with zstd
This is caused by a mismatch of GCC used to compile zlib and glib to the one being used to compile qemu, this can be fixed by rebuilding both before compiling qemu again.
emerge --ask sys-libs/zlib dev-libs/glib
BSOD when booting Windows 10
Create this file:
options kvm ignore_msrs=1
and restart the system.
- Comparison of virtual machines — compares the features of several platform virtual machines.
- Fast Virtio VM — explains a way to build a blazing fast Gentoo VM under KVM using Virtio and mdev.
- GPU passthrough with libvirt qemu kvm — directly present an internal PCI GPU to a virtual machine
- QEMU with Open vSwitch network
- Virtualization — the concept and technique that permits running software in an environment separate from a computer operating system.
- QEMU/QEMU front-ends — user interface application to the QEMU/libvirt API/library.
- Libvirt — a virtualization management toolkit.
- Libvirt/QEMU_networking — details the setup of Gentoo networking by Libvirt for use by guest containers and QEMU-based virtual machines.
- Libvirt/QEMU_guest — covers libvirt and its creation of a virtual machine (VM) for use under the soft-emulation mode QEMU hypervisor Type-2, notably using virsh command.
- Virt-manager — desktop user interface for management of virtual machines and containers through the libvirt library
- Virt-manager/QEMU_guest — QEMU creation of a guest (VM or container)