qemu-8.1.2

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-8.1.2/qemu-doc.html.

Note

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.

Note

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.

Disk

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

VDISK_SIZE=50G
VDISK_FILENAME=vdisk.img
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 10.0.2.0/24, where the host has address 10.0.2.2 and acts as a gateway to internet, and with a name server at address 10.0.2.3, and an smb server at address 10.0.2.4. A builtin DHCP server can allocate addresses between 10.0.2.15 and 10.0.2.31.

-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”.

Networking

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
net.ipv4.ip_forward=1
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,....