Libvirt

libvirt is an abstraction layer and a daemon for managing virtual machines -- remote or locally, using multiple virtualization backends (QEMU/KVM, VirtualBox, Xen, etc).

This entry doesn't try to cover everything about libvirt, just the things that were not intuitive at first or not well documented.

Installing

Currently libvirt and tools are only available from AUR. If you are unfamiliar with how to install AUR packages, see: Arch User Repository.

For servers you need the libvirt package from AUR and urlgrabber, qemu-kvm, dnsmasq and bridge-utils from Arch repositories.

For GUI management tools you also need all of the following from AUR: virtviewer virtinst virt-manager

Configuration

Run daemon

To run the libvirt daemon:

sudo /etc/rc.d/libvirtd start

If you want to start it at boot, edit "/etc/rc.conf" and add libvirtd to the DAEMONS= line.

Polkit authentication

Note:  ??? I never got this to work. If you know how to do it, please edit this section

To allow yourself to manage VMs as non-root, run this on the server:

sudo polkit-auth --user $USERNAME --grant org.libvirt.unix.manage

Alternatively you can only grant the monitoring rights with org.libvirt.unix.monitor

Unix File-based Permissions

Note: This is an alternative to Polkit authentication.

If you wish to use unix file-based permissions to allow some non-root users to use libvirt, you can modify the config files.

First you will need to create the libvirt group and add any users you want to have access to libvirt to that group.

sudo groupadd libvirt
sudo gpasswd -a [user] libvirt

Any users that are currently logged in will need to log out and back in to update their groups. Alternately the user can use the following command in the shell they will be launching libvirt from to update the group:

newgrp libvirt

Then you can either enable permissions-based access by uncommenting the following line on the PKGBUILD for libvirt before running makepkg:

#  patch -Np1 -i "$srcdir"/unixperms.patch || return 1

or you can make the changes to your permissions and config files by hand. Uncomment the following lines in the file /etc/libvirt/libvirtd.conf (they are not all in the same location in the file):

#unix_sock_group = "libvirt"
#unix_sock_ro_perms = "0777"
#unix_sock_rw_perms = "0770"
#auth_unix_ro = "none"
#auth_unix_rw = "none"

You may also wish to change unix_sock_ro_perms from "0777" to "0770" to disallow read-only access to people who are not members of the libvirt group.

Enable KVM acceleration for QEMU

Note: KVM will conflict with VirtualBox. You cannot use KVM and VirtualBox at the same time.

Running virtual machines with the usual QEMU emulation, without KVM, will be painfully slow. You definitely want to enable KVM support if your CPU supports it. To find out, run the following:

egrep '^flags.*(vmx|svm)' /proc/cpuinfo

To enable KVM, you need to load the kvm-amd or kvm-intel kernel module depending on your CPU. Run modprobe:

sudo modprobe kvm-amd

Usually you would also add it to the MODULES= line in "/etc/rc.conf"

If KVM is not working, you will find the following message in your "/var/log/libvirt/qemu/VIRTNAME.log"

Could not initialize KVM, will disable KVM support

More info is available from the official KVM FAQ

Usage

Installing new VM

To create a new VM, you need some sort of installation media, which is usually a plain .iso file. Copy it to the "/var/lib/libvirt/images" directory (alternatively you can create a new storage pool directory in virt-manager and copy it there)

Then run virt-manager, connect to the server, right click on the connection and choose New. Choose a name, and select Local install media. Just continue with the wizard.

On the 4th step, you may want to uncheck Allocate entire disk now -- this way you will save space when your VM isn't using all of its disk. However, this can cause increased fragmentation of the disk.

On the 5th step, open Advanced options and make sure that Virt Type is set to kvm. If the kvm choice is not available, see section Enable KVM acceleration for QEMU above.

Creating a storage pool in virt-manager

First, connect to an existing server. Once you're there, right click and choose Details. Go to Storage and press the + icon at the lower left. Then just follow the wizard. :)

Using VirtualBox with virt-manager

Note: VirtualBox support in libvirt is not quite stable yet and may cause your libvirtd to crash. Usually this is harmless and everything will be back once you restart the daemon.

virt-manager does not let you to add any VirtualBox connections from the GUI. However, you can launch it from the command line:

virt-manager -c vbox:///system

Or if you want to manage a remote system over SSH:

virt-manager -c vbox+ssh://username@host/system

Manage a remote system over SSH

The nc utility is needed for remote management over SSH

pacman -S openbsd-netcat
ln -s /usr/bin/nc.openbsd /usr/bin/nc

To connect to the remote system using virsh :

virsh -c qemu+ssh://username@host/system

If something goes wrong, you can get some logs using :

LIBVIRT_DEBUG=1 virsh -c qemu+ssh://username@host/system

To display the graphical console for a virtual machine :

virt-viewer --connect qemu+ssh://username@host/system myvm

To display the virtual machine desktop management tool :

virt-manager -c qemu+ssh://username@host/system