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).
- 1 Installation
- 2 Configuration
- 3 Usage
- 4 Troubleshooting
- 5 See also
- 6 External resources
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:
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
[*] 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.
vhost-net USE flag (recommend):
[*] 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
python USE flag is needed for 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:
File systems ---> <*> The Extended 4 (ext4) filesystem [*] Ext4 Security Labels
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||local|
||Enables support for Linux's Async IO||local|
||Enable alsa output for sound emulation||local|
||Enable Bluetooth Support||global|
||Enable disassembly support with dev-libs/capstone||local|
||Support ISOs / -cdrom directives vis HTTP or HTTPS.||local|
||Enables firmware device tree support||local|
||Enables GlusterFS cluster fileystem via sys-cluster/glusterfs||local|
||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|
||Use gtk-2 instead of gtk-3||local|
||Enable direct iSCSI support via net-libs/libiscsi instead of indirectly via the Linux block layer that sys-block/open-iscsi does.||local|
||Enable jpeg image support for the VNC console server||local|
||Enable support for lzo compression||global|
||Enable the ncurses-based console||local|
||Enable NFS support||local|
||Enable NUMA support||local|
||Add support for OpenGL (3D graphics)||global|
||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|
||Enable png image support for the VNC console server||local|
||Enable pulseaudio output for sound emulation||local|
||Enable rados block device backend support, see http://ceph.newdream.net/wiki/QEMU-RBD||local|
||Enable the SDL-based console||local|
||Use libsdl2 instead of libsdl||local|
||Enable support for snappy compression||local|
||Enable Spice protocol support via app-emulation/spice||local|
||Enable SSH based block device support via net-libs/libssh2||local|
||Build the User and Software MMU (system) targets as well as tools as static binaries||local|
||Build the User targets as static binaries||local|
||Enable SystemTAP/DTrace tracing||local|
||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|
||Enable USB passthrough via dev-libs/libusb||local|
||Use sys-apps/usbredir to redirect USB devices to another machine over TCP||local|
||Enable VDE-based networking||local|
||Enable accelerated networking using vhost-net, see http://www.linux-kvm.org/page/VhostNet||local|
||Enable experimental Virgil 3d (virtual software GPU)||local|
||Enable VirtFS via virtio-9p-pci / fsdev. See http://wiki.qemu.org/Documentation/9psetup||local|
||Enable VNC (remote desktop viewer) support||global|
||Enable terminal support (x11-libs/vte) in the GTK+ interface||local|
||Add support for getting and setting POSIX extended attributes, through sys-apps/attr. Requisite for the virtfs backend.||local|
||Enables support for Xen backends||local|
||Support xfsctl() notification and syncing for XFS backed virtual disks.||local|
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.
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.
QEMU_SOFTMMU_TARGETS="arm x86_64 sparc" QEMU_USER_TARGETS="x86_64"
Otherwise you can always edit the package.use file.
After reviewing and adding any desired USE flags, emerge app-emulation/qemu:
emerge --ask app-emulation/qemu
Basic networking support can be configured "out of the box" by creating a vlan with the following options.
-net nic,vlan=0 -net user,vlan=0
As of QEMU 0.12 the `-net` method is considered obsolete, although it continues to work.
A more advanced networking concept is outlined below, which enables guest access to an external network and also works with both wired and wireless adapters on the host. If desired, a DHCP server can also be setup on the host to allow for dynamic guest IP configurations. There are many different tutorials available online to further understand these concepts.
This example networking configuration needs some extra software installed:
emerge --ask net-firewall/iptables
Creating TUN/TAP device
This allows the guest to communicate with the bridge. QEMU's default group is
kvm, ensure that the correct group is given permissions to control the TAP. Enabling promiscuous mode (
promisc) for the adapter might be unnecessary.
ip tuntap add dev tap0 mode tap group kvm
ip link set dev tap0 up promisc on
ip addr add 0.0.0.0 dev tap0
Create network bridge
Creating a network bridge seems necessary, even if only 1 guest is configured. Create the bridge and add each TAP to it. Spanning tree protocol (
stp) is disabled because there is only 1 bridge.
ip link add br0 type bridge
ip link set br0 up
ip link set tap0 master br0
echo 0 > /sys/class/net/br0/bridge/stp_state
ip addr add 10.0.1.1/24 dev br0
Packet forwarding and NAT
Allows for proper packet routing (be sure to replace
eth1 with an appropriate network interface name):
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i br0 -o eth1 -j ACCEPT
The following should be added to the configuration:
-net nic,vlan=0 -net tap,ifname=tap0,script=no,downscript=no
After starting the guest, the IP should be configured to be on the vlan and the gateway should be the IP given to the bridge. The exact process will vary based on OS.
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):
gpasswd -a <username> kvm
To make life easier, there are multiple user-friendly front ends to QEMU:
|AQEMU||app-emulation/aqemu||https://sourceforge.net/projects/aqemu/||Graphical interface for QEMU and KVM emulators, using Qt4.|
|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.|
To run as a normal user, after emerging, ensure they are in the libvirt group:
usermod -a -G libvirt <user>
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.
- http://www.linux-kvm.org/page/KvmOnGentoo - The Gentoo article on the KVM wiki
- http://wiki.qemu.org/Main_Page - The Official QEMU wiki
- http://www.tuxradar.com/content/howto-linux-and-windows-virtualization-kvm-and-qemu - A TuxRadar howto.