ja:libvirt zh-hans:Libvirt

Libvirt is a collection of software that provides a convenient way to manage virtual machines and other virtualization functionality, such as storage and network interface management. These software pieces include a long term stable C API, a daemon (libvirtd), and a command line utility (virsh). A primary goal of libvirt is to provide a single way to manage multiple different virtualization providers/hypervisors, such as the KVM/QEMU, Xen, LXC, OpenVZ or VirtualBox hypervisors (among others).

Some of the major libvirt features are:

• Virtual machine management: Various domain lifecycle operations such as start, stop, pause, save, restore, and migrate. Hotplug operations for many device types including disk and network interfaces, memory, and CPUs.

• Remote machine support: All libvirt functionality is accessible on any machine running the libvirt daemon, including remote machines. A variety of network transports are supported for connecting remotely, with the simplest being SSH, which requires no extra explicit configuration.

• Storage management: Any host running the libvirt daemon can be used to manage various types of storage: create file images of various formats (qcow2, vmdk, raw, ...), mount NFS shares, enumerate existing LVM volume groups, create new LVM volume groups and logical volumes, partition raw disk devices, mount iSCSI shares, and much more.

• Network interface management: Any host running the libvirt daemon can be used to manage physical and logical network interfaces. Enumerate existing interfaces, as well as configure (and create) interfaces, bridges, vlans, and bond devices.

• Virtual NAT and Route based networking: Any host running the libvirt daemon can manage and create virtual networks. Libvirt virtual networks use firewall rules to act as a router, providing virtual machines transparent access to the host machines network.

Installation

Because of its daemon/client architecture, libvirt needs only be installed on the machine which will host the virtualized system. Note that the server and client can be the same physical machine.

Server

Install the libvirt package, as well as at least one hypervisor:

• The libvirt KVM/QEMU driver is the primary libvirt driver and if KVM is enabled, fully virtualized, hardware accelerated guests will be available. See the QEMU article for more information.

Other supported hypervisors include LXC, VirtualBox and Xen. See the respective articles for installation instructions. With respect to libvirtd installation note:

• The libvirt LXC driver has no dependency on the LXC userspace tools provided by lxc, therefore there is no need to install the package if planning on using the driver. libvirtd needs to be running to use libvirt-lxc connection.

• Xen support is available, but not by default (27356). You need to use the ABS to modify libvirt's PKGBUILD and build it without the -Ddriver_libxl=disabled option.

For network connectivity, install:

• dnsmasq for the default NAT/DHCP networking.

• openbsd-netcat for remote management over SSH.

Other optional dependencies may provide desired or extended features, such as dmidecode for DMI system info support. Install the ones you may need as dependencies after reading pacman's output for libvirt.

Client

The client is the user interface that will be used to manage and access the virtual machines.

A list of libvirt-compatible software can be found here.

Configuration

Libvirt can manage QEMU virtual machines in two modes, system and sessionhttps://libvirt.org/drvqemu.html#connections-to-qemu-driverhttps://wiki.libvirt.org/FAQ.html#what-is-the-difference-between-qemu-system-and-qemu-session-which-one-should-i-use:

• system URIs connect to the libvirtd daemon running as root which is launched at system startup. Virtual machines created and run using 'system' are usually launched as root, unless configured otherwise (for example in /etc/libvirt/qemu.conf)

• session URIs launch a libvirtd instance as your local user, and all virtual machines are run with local user permissions.

Regarding their pros and cons:

• Virtual machine auto-starting on host boot only works for 'system', and the root libvirtd instance has necessary permissions to use proper networking via bridges or virtual networks. qemu:///system is generally what tools like virt-manager default to.

• qemu:///session has a serious drawback: since the libvirtd instance does not have sufficient privileges, the only out of the box network option is qemu's usermode networking, which has non-obvious limitations, so its usage is discouraged (more info on qemu networking options)

• The benefit of qemu:///session is that permission issues vanish: disk images can easily be stored in $HOME, serial PTYs are owned by the user, etc.

For system-level administration (i.e. global settings and image-volume location), libvirt minimally requires setting up authorization, and starting the daemon.

For user-session administration, daemon setup and configuration is not required; however, authorization is limited to local abilities; the front-end will launch a local instance of the libvirtd daemon.

Set up authentication

From libvirt: Connection authentication:

1. The libvirt daemon allows the administrator to choose the authentication mechanisms used for client connections on each network socket independently. This is primarily controlled via the libvirt daemon master config file in /etc/libvirt/libvirtd.conf. Each of the libvirt sockets can have its authentication mechanism configured independently. There is currently a choice of none, polkit and sasl.

Using libvirt group

The easiest way to ensure your user has access to libvirt daemon is to add member to libvirt user group.

Members of the libvirt group have password-less access to the RW daemon socket by default.

Using polkit

Because libvirt pulls polkit as a dependency during installation, polkit is used as the default value for the unix_sock_auth parameter (source). File-based permissions remain nevertheless available.

The libvirt daemon provides two polkit actions in /usr/share/polkit-1/actions/org.libvirt.unix.policy:

• org.libvirt.unix.manage for full management access (RW daemon socket), and

• org.libvirt.unix.monitor for monitoring only access (read-only socket).

The default policy for the RW daemon socket will require to authenticate as an admin. This is akin to sudo auth, but does not require that the client application ultimately run as root. Default policy will still allow any application to connect to the RO socket.

Arch defaults to consider anybody in the wheel group as an administrator: this is defined in /usr/share/polkit-1/rules.d/50-default.rules (see Polkit#Administrator identities). Therefore there is no need to create a new group and rule file if your user is a member of the wheel group: upon connection to the RW socket (e.g. via virt-manager) you will be prompted for your user's password.

You may change the group authorized to access the RW daemon socket. As an example, to authorize the mykvm group, create the following file:

/* Allow users in mykvm group to manage the libvirt
daemon without authentication */
polkit.addRule(function(action, subject) {
    if (action.id == "org.libvirt.unix.manage" &&
        subject.isInGroup("mykvm")) {
            return polkit.Result.YES;
    }
});

/etc/polkit-1/rules.d/50-libvirt.rules

Then add yourself to the mykvm group and relogin. Replace mykvm with any group of your preference just make sure it exists and that your user is a member of it (see Users and groups for more information).

Do not forget to relogin for group changes to take effect.

Authenticate with file-based permissions

To define file-based permissions for users in the libvirt group to manage virtual machines, uncomment and define:

#unix_sock_group = "libvirt"
#unix_sock_ro_perms = "0777"  # set to 0770 to deny non-group libvirt users
#unix_sock_rw_perms = "0770"
#auth_unix_ro = "none"
#auth_unix_rw = "none"

/etc/libvirt/libvirtd.conf

While some guides mention changed permissions of certain libvirt directories to ease management, keep in mind permissions are lost on package update. To edit these system directories, root user is expected.

Daemon

Start both libvirtd.service and virtlogd.service. Optionally enable libvirtd.service (which will also enable virtlogd.socket and virtlockd.socket units, so there is NO need to also enable virtlogd.service).

Another possibility is to only start/enable libvirtd.socket and virtlogd.socket for on-demand socket activation.

Unencrypt TCP/IP sockets

Edit /etc/libvirt/libvirtd.conf:

listen_tls = 0
listen_tcp = 1
auth_tcp="none"

/etc/libvirt/libvirtd.conf

It is also necessary to start the server in listening mode by editing /etc/conf.d/libvirtd:

LIBVIRTD_ARGS="--listen"

/etc/conf.d/libvirtd

Access virtual machines using their hostnames

For host access to guests on non-isolated, bridged networks, enable the libvirt and/or libvirt_guest NSS modules provided by libvirt. For the comparison of the two modules and technical details, see libvirt documentation.

Add desired modules in nsswitch.conf(5):

hosts: files libvirt libvirt_guest dns myhostname

/etc/nsswitch.conf

Test

To test if libvirt is working properly on a system level:

$ virsh -c qemu:///system

To test if libvirt is working properly for a user-session:

$ virsh -c qemu:///session

Management

Libvirt management is done mostly with three tools: virt-manager (GUI), virsh, and guestfish (which is part of libguestfs).

virsh

The virsh program is for managing guest domains (virtual machines) and works well for scripting, virtualization administration. Though most virsh commands require root privileges to run due to the communication channels used to talk to the hypervisor, typical management, creation, and running of domains (like that done with VirtualBox) can be done as a regular user.

The virsh program includes an interactive terminal that can be entered if no commands are passed (options are allowed though): virsh. The interactive terminal has support for tab completion.

From the command line:

$ virsh [option] <command> [argument]...

From the interactive terminal:

virsh # <command> [argument]...

Help is available:

$ virsh help [option*] or [group-keyword*]

Storage pools

A pool is a location where storage volumes can be kept. What libvirt defines as volumes others may define as "virtual disks" or "virtual machine images". Pool locations may be a directory, a network filesystem, or partition (this includes a LVM). Pools can be toggled active or inactive and allocated for space.

On the system-level, /var/lib/libvirt/images/ will be activated by default; on a user-session, virt-manager creates $XDG_DATA_HOME/images.

Print active and inactive storage pools:

$ virsh pool-list --all

Create a new pool using virsh

If one wanted to add a storage pool, here are examples of the command form, adding a directory, and adding a LVM volume:

$ virsh pool-define-as name type [source-host] [source-path] [source-dev] [source-name] [<target>] [--source-format format]
$ virsh pool-define-as poolname dir - - - - /home/username/.local/libvirt/images
$ virsh pool-define-as poolname fs - -  /dev/vg0/images - mntpoint

The above command defines the information for the pool, to build it:

$ virsh pool-build     poolname
$ virsh pool-start     poolname
$ virsh pool-autostart poolname

To remove it:

$ virsh pool-undefine  poolname

Create a new pool using virt-manager

First, connect to a hypervisor (e.g. QEMU/KVM system, or user-session). Then, right-click on a connection and select Details; select the Storage tab, push the + button on the lower-left, and follow the wizard.

Storage volumes

Once the pool has been created, volumes can be created inside the pool. If building a new domain (virtual machine), this step can be skipped as a volume can be created in the domain creation process.

Create a new volume with virsh

Create volume, list volumes, resize, and delete:

$ virsh vol-create-as      poolname volumename 10GiB --format aw|bochs|raw|qcow|qcow2|vmdk
$ virsh vol-upload  --pool poolname volumename volumepath
$ virsh vol-list           poolname
$ virsh vol-resize  --pool poolname volumename 12GiB
$ virsh vol-delete  --pool poolname volumename
$ virsh vol-dumpxml --pool poolname volumename  # for details.

Domains

Virtual machines are called domains. If working from the command line, use virsh to list, create, pause, shutdown domains, etc. virt-viewer can be used to view domains started with virsh. Creation of domains is typically done either graphically with virt-manager or with virt-install (a command line program installed as part of the virt-install package).

Creating a new domain typically involves using some installation media, such as an .iso from the storage pool or an optical drive.

Print active and inactive domains:

# virsh list --all

Create a new domain using virt-install

For an extremely detailed domain (virtual machine) setup, it is easier to #Create a new domain using virt-manager. However, basics can easily be done with virt-install and still run quite well. Minimum specifications are --name, --memory, guest storage (--disk, --filesystem, or --nodisks), and an install method (generally an .iso or CD). See virt-install(1) for more details and information about unlisted options.

Arch Linux install (two GiB, qcow2 format volume create; user-networking):

$ virt-install  \
  --name arch-linux_testing \
  --memory 1024             \
  --vcpus=2,maxvcpus=4      \
  --cpu host-model                \
  --cdrom $HOME/Downloads/arch-linux_install.iso \
  --osinfo detect=on,name=archlinux \
  --disk size=2,format=qcow2  \
  --network user            \
  --virt-type kvm

Fedora testing (Xen hypervisor, non-default pool, do not originally view):

$ virt-install  \
  --connect xen:///     \
  --name fedora-testing \
  --memory 2048         \
  --vcpus=2             \
  --cpu=host-model            \
  --cdrom /tmp/fedora20_x84-64.iso      \
  --os-type=linux --os-variant=fedora20 \
  --disk pool=testing,size=4            \
  --network bridge=br0                  \
  --graphics=vnc                        \
  --noautoconsole
$ virt-viewer --connect xen:/// fedora-testing

Windows:

$ virt-install \
  --name=windows7           \
  --memory 2048             \
  --cdrom /dev/sr0          \
  --os-variant=win7         \
  --disk /mnt/storage/domains/windows7.qcow2,size=20GiB \
  --network network=vm-net  \
  --graphics spice

Import existing volume:

$ virt-install  \
  --name demo  \
  --memory 512 \
  --disk /home/user/VMs/mydisk.img \
  --import

Create a new domain using virt-manager

First, connect to the hypervisor (e.g. QEMU/KVM system or user session), right click on a connection and select New, and follow the wizard.

• On the fourth step, de-selecting Allocate entire disk now will make setup quicker and can save disk space in the interim; however, it may cause volume fragmentation over time.

• On the fifth step, open Advanced options and make sure that Virt Type is set to kvm (this is usually the preferred method). If additional hardware setup is required, select the Customize configuration before install option.

Manage a domain

Start a domain:

$ virsh start domain
$ virt-viewer --connect qemu:///session domain

Gracefully attempt to shutdown a domain; force off a domain:

$ virsh shutdown domain
$ virsh destroy  domain

Autostart domain on libvirtd start:

$ virsh autostart domain
$ virsh autostart domain --disable

Shutdown domain on host shutdown:

1. Running domains can be automatically suspended/shutdown at host shutdown using the libvirt-guests.service systemd service. This same service will resume/startup the suspended/shutdown domain automatically at host startup. See libvirt-guests(8) for details and service options.

Edit a domain's XML configuration:

$ virsh edit domain

To know more about XML configurations read the XML format section of the libvirt wiki.

Windows requirements

Recent Windows client and server versions have specific requirements for a machine. From Windows 11 onwards, it is necessary to configure the domain for UEFI mode, as it does not support BIOS mode. Windows Server can still be installed in a non-UEFI environment; it is however recommended by Microsoft to use UEFI mode at least for Windows Server 2022 and newer. See #UEFI support for how to achieve this.

Additionally, Windows 11 requires a Trusted Platform Module to be provided to the domain. Again, for Windows Server this is still optional, but recommended. Please note that TPM version 1.2 is not supported from Windows 11 onwards, and the domain has to be set up with a TPM version 2.0. A TPM emulator can be used, as described in #TPM support.

Lastly, Windows recommends using Secure Boot, and libvirt can provide an adequate environment. See #UEFI support and KVM#Secure Boot to view how to enroll the Microsoft keys in the firmware storage of the domain with virt-firmware. Alternatively, this guide can be used to achieve the same thing manually. Secure Boot is not a strict requirement though, and this can be deployed after the domain has been installed as well.

Recent Windows versions, especially Windows 11 and Windows Server 2025, ship a security feature called Virtualization-based security (VBS). This technology uses hardware virtualization itself to isolate parts of the kernel. It is possible to run Windows with VBS in a libvirt domain using QEMU/KVM if nested virtualization is available for a domain, see #Nested virtualization for how to enable it.

Networking

Virtual networks

Virtual networks are used to connect domains to either internal or external networks. The bridge device is used to define the virtual network. Additionally, the forwarding mode is used to define the internal or external networks a domain is able to reach.

Forwarding modes

Some common forwarding modes are listed below:

Using iptables

If iptables is to be used and not nftables, it is necessary to specify this accordingly in the configuration file.

For example:

# default: #firewall_backend  "nftables"
firewall_backend  "iptables"

/etc/libvirt/network.conf

Retrieving a domain IP address

If using the default network and addresses are assigned using DHCP:

$ virsh net-dhcp-leases default

If the domain is using the qemu-guest-agent:

$ virsh domifaddr --source agent domain

Using nftables

When using network type NAT in combination with a simple nftables firewall, you may need to allow forwarding to/from the virtual network interface, and allow DNS/DHCP requests for DHCP clients from the virtual network interface to the host.

The relevant sections of nftables.conf are below:

# ...
table inet filter {
  chain input {
    type filter hook input priority filter
    policy drop
    # ...
    iifname virbr0 udp dport {53, 67} accept comment "allow VM dhcp/dns requests to host"
    # ...
  }

  chain forward {
    type filter hook forward priority filter
    policy drop
    
    iifname virbr0 accept
    oifname virbr0 accept
  }
}

/etc/nftables.conf

Adding an IPv6 address

When adding an IPv6 address through any of the configuration tools, you will likely receive the following error:

Check the host setup: enabling IPv6 forwarding with RA routes without accept_ra set to 2 is likely to cause routes loss. Interfaces to look at: eth0

Fix this by running the following command (replace eth0 with the name of your physical interface):

# sysctl net.ipv6.conf.eth0.accept_ra=2

Port forwarding to domains

Details on how to do this can be found in the libvirt NAT forwarding documentation.

Snapshots

Snapshots take the disk, memory, and device state of a domain at a point-of-time, and save it for future use. They have many uses, from saving a "clean" copy of an OS image to saving a domain's state before a potentially destructive operation. Snapshots are identified with a unique name.

Snapshots are saved within the volume itself and the volume must be the format: qcow2 or raw. Snapshots use deltas in order not to take as much space as a full copy would.

Create a snapshot

Once a snapshot is taken it is saved as a new block device and the original snapshot is taken offline. Snapshots can be chosen from and also merged into another (even without shutting down the domain).

Print a running domain's volumes (running domains can be printed with virsh list):

 Target     Source
 ------------------------------------------------
 vda        /vms/domain.img

# virsh domblklist domain

To see a volume's physical properties:

 image: /vms/domain.img
 file format: qcow2
 virtual size: 50G (53687091200 bytes)
 disk size: 2.1G
 cluster_size: 65536

# qemu-img info /vms/domain.img

Create a disk-only snapshot (the option --atomic will prevent the volume from being modified if snapshot creation fails):

# virsh snapshot-create-as domain snapshot1 --disk-only --atomic

List snapshots:

 Name                 Creation Time             State
 ------------------------------------------------------------
 snapshot1           2012-10-21 17:12:57 -0700 disk-snapshot

# virsh snapshot-list domain

One can then copy the original image with cp --sparse=true or rsync -S and then merge the original back into snapshot:

# virsh blockpull --domain domain --path /vms/domain.snapshot1

domain.snapshot1 becomes a new volume. After this is done the original volume (domain.img and snapshot metadata can be deleted. The virsh blockcommit would work opposite to blockpull but it seems to be currently under development (including snapshot-revert feature, scheduled to be released sometime next year.

Other management

List virtual machines in the system mode:

$ virsh --connect qemu:///system list --all

Connect to a non-default hypervisor:

$ virsh --connect xen:///
virsh # uri
xen:///

Connect to a QEMU hypervisor over SSH; and the same with logging:

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

Connect a graphic console over SSH:

$ virt-viewer  --connect qemu+ssh://username@host/system domain
$ virt-manager --connect qemu+ssh://username@host/system domain

Connect to the system's VirtualBox hypervisor (VirtualBox support in libvirt is not stable yet and may cause libvirtd to crash):

$ virsh --connect vbox:///system

List and dump network configuration:

$ virsh -c qemu:///system net-list --all
$ virsh -c qemu:///system net-dumpxml default

Hooks

Hooks are scripts that are triggered by different events happening while starting and running the libvirt daemon. They can be used to execute commands needed in preparation to launch a guest like setup networks or reserve memory.

The following hooks exists:

• daemon - occasions to trigger: start, shutdown, reload

• qemu - occasions to trigger: prepare, prepare, start, started, stopped, release, migrate, restore, reconnect, attach

• lxc - occasions to trigger: prepare, start, started, stopped, release, reconnect

• libxl - occasions to trigger: prepare, start, started, stopped, release migrate, reconnect

• network - occasions to trigger: start, started, stopped, port-created, updated, port-deleted

See the libvirt Documentation for details about each hook and trigger.

Create a hook

Hooks are represented by scripts located at /etc/libvirt/hooks. If the folder does not exist, you have to create it. Each hook is represented by a script in this folder with the same name (e.g. /etc/libvirt/hooks/qemu) or a sub-folder (e.g. /etc/libvirt/hooks/qemu.d/). The later can contain different scripts, which are all run at the trigger points. The scripts are run like any other scripts, so they need to start with the declaration of the command interpreter to use (e.g. #!/bin/bash) and be executable by the libvirt user.

Every time a trigger point is met, the script is run. For example, the daemon script would run at least two times in a start/stop cycle of the system, at start and at shutdown. To run an command only at a given point, you have to implement conditions in the script. To do this, libvirt passes parameters which can be used to identify the current trigger condition.

According to the libvirt documentation these parameters are defined as follows:

• Parameter 1: The name of the object involved in the operation

• Parameter 2: The name of the operation being performed

• Parameter 3: Used if a sub-operation is to be named

• Parameter 4: An extra argument if needed

If one of the arguments is not applicable, a dash is passed.

Example

To run an command every time you start an qemu guest, before any resources are allocated, you can use the qemu hook. At this point, libvirt runs the hooks like this: /etc/libvirt/hooks/qemu <guest_name> prepare begin - The script for this could like this:

#!/bin/bash
guest_name="$1"
libvirt_task="$2"
if [ "$libvirt_task" = "prepare" ]; then
	<run some important code here>
fi

/etc/libvirt/hooks/qemu

If the guest is stopped, the same script would be run, but this time the daemon would start the command like this: /etc/libvirt/hooks/qemu <guest_name> stopped end -

Sharing data between host and guest

Virtio-FS

Sharing files with Virtio-FS lists an overview of the supported options to enable filesharing with the guest.

Set up the memory backend

Memory backends must be allocated before using virtiofs. memfd and file-backed memory backends can be used in system sessions and unprivileged QEMU/KVM user sessions. Hugepages only supports system sessions.

memfd

To use memfd memory backend, you need to add the following domain XML elements:

<domain>
  ...
  <memoryBacking>
    <source type='memfd'/>
    <access mode='shared'/>
  </memoryBacking>
  ...
</domain>

# virsh edit name_of_virtual_machine

file-backed

Add the following domain XML elements:

<domain>
  ...
  <memoryBacking>
    <access mode='shared'/>
  </memoryBacking>
  ...
</domain>

# virsh edit name_of_virtual_machine

You can configure where the backing file is stored with the memory_backing_dir option in /etc/libvirt/qemu.conf or, if you are running a user session, in $XDG_CONFIG_HOME/libvirt/qemu.conf:

memory_backing_dir = "/dev/shm/"

hugepage

First you need to enable hugepages which are used by the virtual machine:

vm.nr_hugepages = nr_hugepages

/etc/sysctl.d/40-hugepage.conf

To determine the number of hugepages needed check the size of the hugepages:

$ grep Hugepagesize /proc/meminfo

The number of hugepages is memory size of virtual machine / Hugepagesize. Add to this value some additional pages. You have to reboot after this step, so that the hugepages are allocated.

Now you have to prepare the configuration of the virtual machine:

<domain>
  ...
  <memoryBacking>
    <hugepages/>
  </memoryBacking>
  ...
  <cpu ...>
    <numa>
      <cell memory='memory size of virtual machine' unit='KiB' memAccess='shared'/>
    </numa>
  </cpu>
  ...
</domain>

# virsh edit name_of_virtual_machine

It is necessary to add the NUMA definition so that the memory access can be declared as shared. id and cpus values for NUMA will be inserted by virsh.

Configure filesystem passthrough

Add the following domain XML elements:

<domain>
...
  <devices>
    ...
    <filesystem type='mount' accessmode='passthrough'>
      <driver type='virtiofs'/>
      <source dir='path/to/folder/on/host'/>
      <target dir='mount_tag'/>
    </filesystem>
    ...
  </devices>
</domain>

# virsh edit name_of_virtual_machine

Replace path/to/folder/on/host with the directory you want to share, and mount_tag with an arbitrary string that will be used to identify the shared file system in the guest.

It should now be possible to mount the folder in the shared machine:

# mount -t virtiofs mount_tag /mnt/mount/path

Add the following fstab entry to mount the folder automatically at boot:

...
mount_tag /mnt/mount/path virtiofs rw,noatime 0 0

/etc/fstab

Mapping user/group IDs in unprivileged mode

By default, the root user (id 0) in the guest is mapped to the current user on the host. Other IDs are mapped to the subordinate user IDs specified in subuid(5) and subgid(5).

You can also configure this mapping manually using idmap tag:

<domain>
...
  <devices>
    ...
    <filesystem type='mount' accessmode='passthrough'>
      <idmap>
        <uid start="2000" target="1000" count="1"/>
        <gid start="2000" target="1000" count="1"/>
      </idmap>
    </filesystem>
    ...
  </devices>
</domain>

# virsh edit name_of_virtual_machine

9p

File system directories can be shared using the 9P protocol. Details are available in QEMU's documentation of 9psetup.

Configure the virtual machine as follows:

<domain>
...
  <devices>
    ...
    <filesystem type="mount" accessmode="mapped">
      
      <target dir="mount_tag"/>
    </filesystem>
  </devices>
</domain>

Boot the guest and mount the shared directory from it using:

# mount -t 9p -o trans=virtio,version=9p2000.L mount_tag /path/to/mount_point/on/guest

See https://docs.kernel.org/filesystems/9p.html for more mount options.

To mount it at boot, add it to the guest's fstab:

...
mount_tag	/path/to/mount_point/on/guest	9p	trans=virtio,version=9p2000.L	0 0

/etc/fstab

The module for the 9p transport (i.e. 9pnet_virtio for trans=virtio) will not be automatically loaded, so mounting the file system from /etc/fstab will fail and you will encounter an error like 9pnet: Could not find request transport: virtio. The solution is to preload the module during boot:

9pnet_virtio

/etc/modules-load.d/9pnet_virtio.conf

Samba / SMB

An other easy way to share data between guest and host is to use the smb protocol. While performance and latency may not be as good as in the other described ways, its sufficient for simple tasks like transfering simple files like images or documents from and to the guest.

The smb server can be set up directly on either the host, or the guest, for example using Samba, eliminating the need for a dedicated file server. Windows guests have the ability to create smb shares included right after installation (Microsoft Supportpage).

One possible way to access the share under linux (either from the host, or from the guest, depending, where you have installed your server) is to create an entry in your fstab. The samba package is required.

#Accessing a samba share on my vm from the host
//my_vm/my_share /home/archuser/my_vm cifs _netdev,noauto,nofail,user,credentials=/home/archuser/.config/my_vm.key,gid=1000,uid=984 0 0

/etc/fstab

_netdev,noauto,nofail ensures that the share is only mounted when needed without causing issues if the virtual machine is not booted. user,credentials=/home/user/.config/my_vm.key,gid=1000,uid=984 gives you the ability to mount the share on the fly while first accessing it, without needing a password.

UEFI support

Libvirt can support UEFI virtual machines through QEMU and OVMF.

Install the edk2-ovmf package.

Restart libvirtd.service.

Now you are ready to create a UEFI virtual machine. Create a new virtual machine through virt-manager. When you get to the final page of the New VM wizard, do the following:

1. Click Customize configuration before install, then select Finish.

In the Overview screen, change the Firmware field to:

• UEFI x86_64: /usr/share/edk2/x64/OVMF_CODE.4m.fd for x64 UEFI without Secure Boot support,

• UEFI x86_64: /usr/share/edk2/x64/OVMF_CODE.secboot.4m.fd for x64 UEFI with Secure Boot support (without any pre-enrolled certificates).

• Click Apply.

• Click Begin Installation.

See Fedora:Using UEFI with QEMU for more information.

For enrolling keys, virt-firmware can be used, see KVM#Secure Boot.

TPM support

QEMU can provide a Trusted Platform Module to the guest. This can be either a passthrough device, i.e. using a physical TPM in a virtual machines or an emulator. For using an emulator, the swtpm package has to be installed. It is possible to provide TPM versions 1.2 or 2.0 to the guest, but 2.0 requires the domain to use UEFI.

In its simplest form, to provide a useable TPM 2.0 for the guest machine with an emulator, the following block in the domain specification will work:

<tpm model='tpm-crb'>
  <backend type='emulator' version='2.0'/>
  <alias name='tpm0'/>
</tpm>

For additional information and options, please see the libvirt documentation found here.

Hyper-V enlightenments

QEMU supports various Hyper-V compatible enlightenments that improve performance for Windows guests and add some additional features, see the QEMU documentation.

These enlightenments can also be activated through libvirt, depending on the QEMU and libvirt version present. The upstream documentation listing the currently supported enlightenments can be found in this section of the libvirt documentation. In general, they can all be enabled, but doing so will put requirements on the QEMU and libvirt versions used, which can be an issue if domain migration is desired.

Tips and tricks

Using an entire physical disk device inside the virtual machine

You may have a second drive with a different OS (like Windows) on it and may want to gain the ability to also boot it inside a virtual machine. Since the drive access is raw, the OS will perform quite well inside the virtual machine.

Windows virtual machine boot prerequisites

Be sure to install the VirtIO drivers inside the OS on that drive before trying to boot it in the virtual machine. For Win 7 use version 0.1.173-4. Some singular drivers from newer VirtIO builds may be used on Win 7 but you will have to install them manually via the Device Manager. For Win 10 and 11 you can use the latest VirtIO build.

Open the ISO that you downloaded and run virtio-win-guest-tools.exe to install the drivers on your system. For older versions that do not have this binary, run virtio-win-gt-x64.msi instead (or virtio-win-gt-x86.msi for 32-bit systems).

Set up the windows disk interface drivers

You may get a 0x0000007B bluescreen when trying to boot the virtual machine. This means Windows can not access the drive during the early boot stage because the disk interface driver it would need for that is not loaded / is set to start manually.

The solution is to enable these drivers to start at boot.

In HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services, find the folders aliide, amdide, atapi, cmdide, iastor (may not exist), iastorV, intelide, LSI_SAS, msahci, pciide and viaide. Inside each of those, set all their start values to 0 in order to enable them at boot. If your drive is a PCIe NVMe drive, also enable that driver (should it exist).

If you want to use the VirtIO bus to access the drive instead of the SATA bus, you need to install the block device drivers, i.e. vioscsi.sys and viostor.sys. These can be installed via pnputil, on a cmd/PowerShell prompt with admin privileges (assuming the VirtIO ISO you downloaded is mounted at D:):

> pnputil.exe -a -i D:\vioscsi\w11\amd64\*.inf
> pnputil.exe -a -i D:\viostor\w11\amd64\*.inf

Alternatively, they can be installed with the Device Manager or with Driver Store Explorer.

Find the unique path of your drive

Run ls /dev/disk/by-id/. From the output, pick out the ID of the drive you want to insert into the virtual machine, for example ata-TS512GMTS930L_C199211383. Now add that ID to /dev/disk/by-id/ so you get /dev/disk/by-id/ata-TS512GMTS930L_C199211383. That is the unique path to that drive.

Do not choose an ID that represents a partition on the drive, it must be the one representing the full drive. Here is another example, with the desired drive being nvme-PM9A1_NVMe_Samsung_1024GB__S6H2NF0WC50542:

...

# This is the correct ID to choose
nvme-PM9A1_NVMe_Samsung_1024GB__S6H2NF0WC50542

# The others shown below must not be chosen, 
# as they represent only a single partition
nvme-PM9A1_NVMe_Samsung_1024GB__S6H2NF0WC50542-part1
nvme-PM9A1_NVMe_Samsung_1024GB__S6H2NF0WC50542-part2
nvme-PM9A1_NVMe_Samsung_1024GB__S6H2NF0WC50542-part3
nvme-PM9A1_NVMe_Samsung_1024GB__S6H2NF0WC50542-part4
...

$ ls -l /dev/disk/by-id/

In the example above, the full path used would be /dev/disk/by-id/nvme-PM9A1_NVMe_Samsung_1024GB__S6H2NF0WC50542.

Add the drive in QEMU CLI

In QEMU CLI that would probably be -drive file=/dev/disk/by-id/ata-TS512GMTS930L_C199211383,format=raw,media=disk. Just modify file to be the unique path of your drive.

Add the drive in libvirt

In libvirt XML that translates to

...
    <disk type="block" device="disk">
      <driver name="qemu" type="raw" cache="none" io="native"/>
      <source dev="/dev/disk/by-id/ata-TS512GMTS930L_C199211383"/>
      <target dev="sda" bus="sata"/>
      <address type="drive" controller="0" bus="0" target="0" unit="0"/>
    </disk>
...

$ virsh edit vmname

Just modify source dev to be the unique path of your drive.

Add the drive in virt-manager

When creating a virtual machine, select Import existing drive and just paste that unique path. If you already have the virtual machine, click on Add hardware > Storage > Select or create custom storage > paste the unique path.

Python connectivity code

The libvirt-python package provides a Python API in /usr/lib/python3.x/site-packages/libvirt.py.

General examples are given in /usr/share/doc/libvirt-python-your_libvirt_version/examples/

Unofficial example using qemu-desktop and openssh:

#! /usr/bin/env python3
import socket
import sys
import libvirt

conn = libvirt.open("qemu+ssh://xxx/system")
print("Trying to find node on xxx")
domains = conn.listDomainsID()
for domainID in domains:
    domConnect = conn.lookupByID(domainID)
    if domConnect.name() == 'xxx-node':
        print("Found shared node on xxx with ID {}".format(domainID))
        domServ = domConnect
        break

Advanced Format 4K native disk

To turn a disk into an Advanced Format 4Kn disk, both its physical and logical sector size needs to be set to 4 KiB. For virtio-blk and virtio-scsi this can be done by setting the logical_block_size and physical_block_size options with the <blockio> element. For example:

<domain>
  ...
  <devices>
    ...
    <disk type='file' device='disk'>
      ..
      <blockio logical_block_size='4096' physical_block_size='4096'/>
    </disk>
    ...
  </devices>
</domain>

# virsh edit name_of_virtual_machine

Nested virtualization

libvirt can provide nested virtualization to its domains when running QEMU/KVM. In order to do this, KVM must be configured appropriately, see KVM#Nested virtualization.

The only change to be made in the libvirt configuration is to set the CPU model of the domain to either host-model or host-passthrough.

Commanding QEMU

Libvirt is capable of passing on QEMU command line arguments to the underlying QEMU instance running the virtual machine. This functionality is highly useful when libvirt does not provide QEMU features (yet). For examples, see the entire Intel GVT-g article.

Modify virtual machine XML schema for QEMU

This serves to enable QEMU-specific elements. Change

<domain type='kvm'>

$ virsh edit vmname

to

<domain xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0' type='kvm'>

$ virsh edit vmname

QEMU command line arguments

In libvirt, QEMU command line arguments separated by whitespaces need to be provided separately.

The correct location to insert them is at the end of the <domain> element, i. e. right above the closing </domain> tag.

-display gtk,gl=es,zoom-to-fit=off

Becomes

...
  </devices>
  <qemu:commandline>
    <qemu:arg value="-display"/>
    <qemu:arg value="gtk,gl=es,zoom-to-fit=off"/>
  </qemu:commandline>
</domain>

$ virsh edit vmname

Troubleshooting

PulseAudio on system instance

The PulseAudio daemon normally runs under your regular user account, and will only accept connections from the same user. This can be a problem if QEMU is being run as root through libvirt. To run QEMU as a regular user, edit /etc/libvirt/qemu.conf and set the user option to your username.

user = "dave"

You will also need to tell QEMU to use the PulseAudio backend and identify the server to connect to. Add the following section to your domain configuration using virsh edit.

  <audio id="1" type="pulseaudio" serverName="/run/user/1000/pulse/native">
    <input latency="20000"/>
    <output latency="20000"/>
  </audio>

1000 is your user id. Change it if necessary.

You can omit the latency settings (in microseconds) but using the defaults might result in crackling.

Hypervisor CPU use

Default virtual machine configuration generated by virt-manager may cause rather high (10-20%) CPU use caused by the QEMU process. If you plan to run the virtual machine in headless mode, consider removing some of the unnecessary devices.

Virtual machine cannot be un-paused on virt-manager

If you are using a disk image format such as qcow2 which has a specified virtual capacity, but only stores what is needed, then you need to have space on the host partition for the image to grow. If you see I/O related errors when attempting to start the virtual machine, it is possible that the host partition holding the virtual disk image is full. You can run df -h on the host to verify how much free space is available.

If this is the case, see System maintenance#Clean the filesystem for ways to free up space.

Redirect USB Device is greyed out in virt-manager

If the Redirect USB Device menu item is greyed out, check that the following hardware is configured for the virtual machine:

• A USB Controller.

• One or more USB Redirectors.

If the option is still greyed out even with this hardware present, you can still add your device directly to the VM's hardware list and remove it when you're done.

Error starting domain: Requested operation is not valid

When you try to open a virtual machine this error may pop up. This is because when you try to open a existing virtual machine libvirt tries to search for the default network which is not available. To make it available you have to autostart your network interface so that whenever your restart your computer your network interface is always active. See libvirt networking page.

Look at the name of your network interface with the following command:

# virsh net-list --all

To autostart your network interface:

# virsh net-autostart name_of_the_network

To start your network interface:

# virsh net-start name_of_the_network

Virt Manager Error 'Virt Manager doesn't have search permissions'

Ensure the folder containing your virtual machine files and installation ISO are owned by the libvirt-qemu group

# chown -R $USER:libvirt-qemu /path/to/virtual/machine

Error starting domain: Requested operation is not valid: network 'default' is not active

If for any reason the default network is deactivated, you will not be able to start any guest virtual machines which are configured to use the network. Your first attempt can be simply trying to start the network with virsh.

# virsh net-start default

For additional troubleshooting steps, see https://www.xmodulo.com/network-default-is-not-active.html.

UFW firewall blocking Libvirt

Libvirt requires IP forwarding to be enabled. Run the following commands:

# sudo ufw route allow in on virbr0
# sudo ufw route allow out on virbr0
# sudo ufw allow in on virbr0

In addition to routing rules and policy, you must also setup IP forwarding. This may be done by setting the following in /etc/ufw/sysctl.conf:

# net/ipv4/ip_forward=1
# net/ipv6/conf/default/forwarding=1
# net/ipv6/conf/all/forwarding=1

then restarting the firewall:

# ufw disable
# ufw enable

EGL_NOT_INITIALIZED: render node init failed

This issue commonly affects owners of NVIDIA GPUs with the official drivers, and it prevents enabling OpenGL acceleration with the dGPU's render node selected. This forum thread contains many reports of this problem.

Hopefully Nvidia will fix this in a future driver release, but as of May 11th 2026, it hasn't been fixed yet. Fortunately, there are workarounds.

Switching to the Nouveau driver or doing PCI passthrough via OVMF are good options that should be considered. If there is a GPU from another brand present (e.g. an iGPU on laptops with hybrid graphics), it can be used as the render node instead of the Nvidia GPU. However, if neither of these options are desirable to you, there's another alternative, which was documented here: https://github.com/virt-manager/virt-manager/issues/938

1. Verify if the following udev rules are already present in your system, and add them if not: https://github.com/virt-manager/virt-manager/issues/938#issuecomment-3814628493

SUBSYSTEM=="drm", KERNEL=="renderD*", GROUP="render", MODE="0666"

2. Uncomment the cgroup_device_acl block and add the following devices to it (verify if they exist in your system):

cgroup_device_acl = [
  ...
  "/dev/nvidiactl", "/dev/nvidia0", "/dev/nvidia-modeset", "/dev/nvidia-uvm", "/dev/dri/renderDXYZ"
]

/etc/libvirt/qemu.conf

Find the proper XYZ value that represents the GPU you wish to use. If you know the GPU's PCI address, run ls -l /dev/dri/by-path/ to see the correct value.

Users with older driver versions may need to add seccomp_sandbox = 0 to the configuration file, but this is not recommended and should only be done as a last resource. You can read more about it in QEMU's documentation and Wikipedia.

Restart libvirtd.service after saving your changes to the configuration file.

3. Add the following graphics sections to your VM's XML (adjust them according to your case):

...
<graphics type="spice">
  <listen type="none"/>
  ...
  <gl enable="no"/>
</graphics>
<graphics type="egl-headless">
  <gl rendernode="/dev/dri/by-path/pci-[PCI-ADDRESS]-render"/>
</graphics>

...

<video>
  <model type="virtio" heads="1" primary="yes">
    <acceleration accel3d="yes"/>
  </model>
</video>

$ virsh edit vmname

More information can be found in the Graphics Framebuffers section of the libvirt XML documentation. You can also check this thread on libvirt's GitLab.

Now you should be able to use your Nvidia GPU for OpenGL acceleration. Hopefully this workaround won't be needed in the future.

See also

• Official libvirt web site

• Red Hat Virtualization Deployment and Administration Guide

• Red Hat Virtualization Tuning and Optimization Guide

• Slackware KVM and libvirt

• IBM KVM

• libvirt Networking Handbook