Introduction to qemu

qemu is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V).

This package is known to build and work properly using an LFS 12.1 platform.

Package Information

  • Download (HTTP): https://download.qemu.org/qemu-8.2.1.tar.xz

  • Download MD5 sum: bda54248d773be2599df66f8995f10e1

  • Download size: 124 MB

  • Estimated disk space required: 2.2 GB (373 MB installed)

  • Estimated build time: 1.3 SBU (add 0.8 SBU for tests, both using parallelism=4)

Qemu Dependencies


GLib-2.78.4 and Pixman-0.43.2



pipewire-1.0.3 or PulseAudio-17.0 (can be used instead of alsa-lib), BlueZ-5.72, cURL-8.6.0, Cyrus SASL-2.1.28, Fuse-3.16.2, GnuTLS-3.8.3, GTK+-3.24.41, keyutils-1.6.3, libaio-0.3.113, libusb-1.0.27, libgcrypt-1.10.3, libjpeg-turbo-3.0.1, libseccomp-2.5.5, libssh2-1.11.0, libpng-1.6.42, libtasn1-4.19.0, Linux-PAM-1.6.0, LZO-2.10, Nettle-3.9.1, Mesa-24.0.1, VTE-0.74.2, capstone, ceph, daxctl, JACK, glusterfs, libbpf, libcacard, libcap-ng, libdw, libfdt, libiscsi, libnfs, libpmem, libssh, libu2f-emu, lzfse, netmap, numactl, rdma-core, SELinux, snappy, spice, usbredir, and VDE

Optional (Runtime)


Optional (for building the documentation)




This optional dependencies list is not comprehensive. See the output of ./configure --help for a more complete list.

Editor Notes: https://wiki.linuxfromscratch.org/blfs/wiki/qemu

KVM Prerequisites

Before building qemu, check to see if your processor supports Virtualization Technology (VT):

grep -E '^flags.*(vmx|svm)' /proc/cpuinfo

If you get any output, you have VT technology (vmx for Intel processors and svm for AMD processors). You then need to go into your system BIOS and ensure it is enabled. After enabling, reboot back to your LFS instance.

Kernel Configuration

Enable the following options in the kernel configuration and recompile the kernel if necessary:

[*] Virtualization --->                                         [VIRTUALIZATION]
  <*/M>   Kernel-based Virtual Machine (KVM) support                       [KVM]
  # Enable the option for your CPU:
  < /*/M>   KVM for Intel (and compatible) processors support        [KVM_INTEL]
  < /*/M>   KVM for AMD processors support                             [KVM_AMD]

The Intel or AMD settings are not both required, but the one matching your system processor is required.

To use the bridge network device, as explained below, check that bridge-utils-1.7.1 is installed and the following options in the kernel configuration are enabled:

[*] Networking support --->                                                [NET]
  Networking options --->
    <*/M> 802.1d Ethernet Bridging                                      [BRIDGE]

Device Drivers --->
  [*] Network device support --->                                   [NETDEVICES]
    [*]   Network core driver support                                 [NET_CORE]
    <*/M>   Universal TUN/TAP device driver support                        [TUN]

Installation of qemu

The udev rule of LFS only allows the root user, the users owning a local login session supported by the optional runtime dependency Systemd-255, or the users in the kvm group to use the KVM device. As the root user, add any non-root users that might use the KVM device either without Systemd-255 installed or remotely (via a SSH connection) to the kvm group:

usermod -a -G kvm <username>

Install qemu by running the following commands:



Qemu is capable of running many targets. The build process is also capable of building multiple targets at one time in a comma delimited list assigned to --target-list. Run ./configure --help to get a complete list of available targets.

if [ $(uname -m) = i686 ]; then

mkdir -vp build &&
cd        build &&

../configure --prefix=/usr               \
             --sysconfdir=/etc           \
             --localstatedir=/var        \
             --target-list=$QEMU_ARCH    \
             --audio-drv-list=alsa       \
             --disable-pa                \
             --enable-slirp              \
             --docdir=/usr/share/doc/qemu-8.2.1 &&

unset QEMU_ARCH &&


qemu uses ninja as a subprocess when building. To run the tests, issue: ninja test. Two tests, bios-tables-test and migration-test, are known to fail.

Now, as the root user:

make install

Change the permissions and ownership of a helper script, which is needed when using the bridge network device (see below). Again as the root user, issue:



You need to add any users who might use the bridge network device into the kvm group even if Systemd-255 is installed.

chgrp kvm  /usr/libexec/qemu-bridge-helper &&
chmod 4750 /usr/libexec/qemu-bridge-helper


For convenience you may want to create a symbolic link to run the installed program. For instance (as the root user):

ln -sv qemu-system-`uname -m` /usr/bin/qemu

Command Explanations

--audio-drv-list=alsa: This switch sets the audio driver to ALSA. See below for enabling other audio drivers.

--disable-pa: even if pa is not in --audio-drv-list list, the pulseaudio driver is built, unless disabled by this parameter.

--enable-slirp: This switch forces the building system to check for Libslirp-4.7.0. Remove it if you don't need the -netdev user support.

--audio-drv-list=pa --disable-alsa: This switch sets the audio driver to pulseaudio. For other drivers see the --audio-drv-list choices in the output of ./configure --help. The default audio driver is OSS. To enable support for both alsa and pulseaudio, use --audio-drv-list=alsa,pa.

Using Qemu

Since using qemu means using a virtual computer, the steps to set up the virtual machine are in close analogy with those to set up a real computer. You'll need to decide about CPU, memory, disk, USB devices, network card(s), screen size, etc. Once the hardware is decided, you'll have for example to choose how to connect the machine to internet, and/or to install an OS. In the following, we show basic ways of performing those steps. But qemu is much more than this, and it is strongly advised to read the qemu documentation in /usr/share/doc/qemu-8.2.1/qemu-doc.html.



It is standard practice to name the computer running qemu host and the emulated machine running under qemu the guest. We'll use those notations in the following.



The following instructions assume the optional symbolic link, qemu, has been created. Additionally, qemu should be run in a graphical environment. But it is possible to use qemu headless or through SSH. See the documentation for the various possibilities.


A virtual disk may be set up in the following way:

qemu-img create -f qcow2 $VDISK_FILENAME $VDISK_SIZE

The virtual disk size and filename should be adjusted as desired. The actual size of the file will be less than specified, but will expand as needed, so it is safe to put a high value.

Operating System

To install an operating system, download an iso image from your preferred Linux distribution. For the purposes of this example, we'll use Fedora-16-x86_64-Live-LXDE.iso in the current directory. Run the following:

qemu -enable-kvm                           \
     -drive file=$VDISK_FILENAME           \
     -cdrom Fedora-16-x86_64-Live-LXDE.iso \
     -boot d                               \
     -m 1G

Follow the normal installation procedures for the chosen distribution. The -boot option specifies the boot order of drives as a string of drive letters. Valid drive letters are: a, b (floppy 1 and 2), c (first hard disk), d (first CD-ROM). The -m option is the amount of memory to use for the virtual machine. The choice depends on the load of the host. Modern distributions should be comfortable with 1GB. The -enable-kvm option allows hardware acceleration. Without this switch, the emulation is much slower.

Defining the virtual hardware

The virtual machine hardware is defined by the qemu command line. An example command is given below:

qemu -enable-kvm                     \
     -smp 4                          \
     -cpu host                       \
     -m 1G                           \
     -drive file=$VDISK_FILENAME     \
     -cdrom grub-img.iso             \
     -boot order=c,once=d,menu=on    \
     -net nic,netdev=net0            \
     -netdev user,id=net0            \
     -device ac97                    \
     -vga std                        \
     -serial mon:stdio               \
     -name "fedora-16"

Meaning of the command line options

-enable-kvm: enable full KVM virtualization support. On some hardware, it may be necessary to add the undocumented -machine smm=off option in order to enable KVM.

-smp <N>: enable symmetric multiprocessing with <N> CPUs.

-cpu <model>: simulate CPU <model>. the list of supported models can be obtained with -cpu help.

-drive file=<filename>: defines a virtual disk whose image is stored in <filename>.

-cdrom grub-img.iso: defines an iso formatted file to use as a cdrom. Here we use a grub rescue disk, which may turn handy when something goes wrong at boot time.

-boot order=c,once=d,menu=on: defines the boot order for the virtual BIOS.

-net nic,netdev=<netid>: defines a network card connected to the network device with id <netid>.

-netdev user,id=<netid>: defines the network user device. This is a virtual local network with addresses, where the host has address and acts as a gateway to internet, and with a name server at address, and an smb server at address A builtin DHCP server can allocate addresses between and

-soundhw <model>: defines the soundcard model. The list may be obtained with -soundhw help.

-vga <type>: defines the type of VGA card to emulate. For -vga std, if you are building a Linux kernel for the guest, it's recommended to enable CONFIG_DRM_BOCHS (as a part of the kernel or a kernel module) to drive all the features of the emulated VGA card, and CONFIG_FB to display the Linux console on it. The other <type> values are not tested by the editors and may require additional dependencies.

-serial mon:stdio: sends the serial port of the guest (/dev/ttyS0 on linux guests), multiplexed with the qemu monitor, to the standard input and output of the qemu process.

-name <name>: sets the name of the guest. This name is displayed in the guest window caption. It may be useful if you run several guests at the same time.

-drive if=pflash,format=raw,readonly=on,file=/usr/share/qemu/edk2-x86_64-code.fd: Load a pre-built EDK2 UEFI firmware, instead of the default PC BIOS. Use this option if you want to boot the guest OS with UEFI.

-drive file=<filename>,if=virtio: Provide Virtio interface to the guest kernel for accessing the disk image, instead of simulating a real disk hardware. This can improve disk I/O performance, but it requires a Virtio driver in guest kernel. Use it instead of a plain -drive if the guest kernel supports Virtio. To build a Linux kernel with Virtio support for the guest, use make defconfig && make kvm_guest.config to create an initial kernel configuration with the Virtio drives enabled, then make your customization. And, if the guest kernel is Linux, the virtual disks using Virtio interface will be named vdx in the devtmpfs, instead of sdx.

-net nic,netdev=net0,model=virtio-net-pci: Provide Virtio interface to the guest kernel for accessing the network interface, instead of simulating a real network interface card. This can improve network I/O performance, but it requires a Virtio driver in guest kernel. Use it instead of a plain -net if the guest kernel supports Virtio.

Controlling the Emulated Display

To set the resolution of the emulated display for a Xorg server running in the guest Linux system, read the section called “Fine Tuning Display Settings”.


The above solution for networking allows the guest to access the local network through the host (and possibly to access internet through the local routers), but the converse is not true. Not even the host can access the guest, unless port forwarding is enabled. And in the case several guests are running, they cannot communicate with each other. Other network devices can be used for this purpose. For example, there is the socket device, which allows several guests to share a common virtual network. In the following, we describe in more details how to set up the bridge device, which allows the guests to appear as if connected to the local network. All the commands below should be run as the root user.

Allow the host to forward IP packets:

sysctl -w net.ipv4.ip_forward=1

To make this permanent, add the command to /etc/sysctl.d/60-net-forward.conf:

cat >> /etc/sysctl.d/60-net-forward.conf << EOF

Set up a required configuration file:

install -vdm 755 /etc/qemu &&
echo allow br0 > /etc/qemu/bridge.conf

In the qemu command line above, replace the switch -netdev user,... with -netdev bridge,....


Installed Programs: elf2dmp, qemu (symlink), qemu-edid, qemu-ga, qemu-img, qemu-io, qemu-keymap, qemu-nbd, qemu-pr-helper, qemu-storage-daemon, and qemu-system-<arch>
Installed Library: None
Installed Directories: /usr/share/qemu and /usr/share/doc/qemu-8.2.1 (optional)

Short Description


Converts files from elf to dmp format


is a test tool for the qemu EDID generator


implements support for QMP (QEMU Monitor Protocol) commands and events that terminate and originate respectively within the guest using an agent built as part of QEMU


provides commands to manage QEMU disk images


is a diagnostic and manipulation program for (virtual) memory media. It is still at an early stage of development


generates qemu reverse keymaps from xkb keymaps, which can be used with the qemu "-k" command line switch


exports Qemu disk images using the QEMU Disk Network Block Device (NBD) protocol


Implements the persistent reservation helper for QEMU


allows to modify disk images using the QEMU Monitor Protocol (QMP) without running a VM


is the QEMU PC System emulator