libvirt/docs/drvqemu.rst

QEMU/KVM/HVF hypervisor driver

The libvirt KVM/QEMU driver can manage any QEMU emulator from version 4.2.0 or later.

It supports multiple QEMU accelerators: software emulation also known as TCG, hardware-assisted virtualization on Linux with KVM and hardware-assisted virtualization on macOS with Hypervisor.framework (since 8.1.0).

Project Links

• The KVM Linux hypervisor
• The QEMU emulator
• Hypervisor.framework reference

Deployment pre-requisites

• QEMU emulators: The driver will probe /usr/bin for the presence of qemu, qemu-system-x86_64, qemu-system-microblaze, qemu-system-microblazeel, qemu-system-mips,qemu-system-mipsel, qemu-system-sparc,qemu-system-ppc. The results of this can be seen from the capabilities XML output.
• KVM hypervisor: The driver will probe /usr/bin for the presence of qemu-kvm and /dev/kvm device node. If both are found, then KVM fully virtualized, hardware accelerated guests will be available.
• Hypervisor.framework (HVF): The driver will probe sysctl for the presence of Hypervisor.framework. If it is found it will be possible to create hardware accelerated guests.

Connections to QEMU driver

The libvirt QEMU driver is a multi-instance driver, providing a single system wide privileged driver (the "system" instance), and per-user unprivileged drivers (the "session" instance). The URI driver protocol is "qemu". Some example connection URIs for the libvirt driver are:

qemu:///session                      (local access to per-user instance)
qemu+unix:///session                 (local access to per-user instance)

qemu:///system                       (local access to system instance)
qemu+unix:///system                  (local access to system instance)
qemu://example.com/system            (remote access, TLS/x509)
qemu+tcp://example.com/system        (remote access, SASl/Kerberos)
qemu+ssh://root@example.com/system   (remote access, SSH tunnelled)

Embedded driver

Since 6.1.0 the QEMU driver has experimental support for operating in an embedded mode. In this scenario, rather than connecting to the libvirtd daemon, the QEMU driver runs in the client application process directly. To use this the client application must have registered & be running an instance of the event loop. To open the driver in embedded mode the app use the new URI path and specify a virtual root directory under which the driver will create content. The path to the root directory must be absolute. Passing a relative path results in an error.

qemu:///embed?root=/some/dir

Broadly speaking the range of functionality is intended to be on a par with that seen when using the traditional system or session libvirt connections to QEMU. The features will of course differ depending on whether the application using the embedded driver is running privileged or unprivileged. For example PCI device assignment or TAP based networking are only available when running privileged. While the embedded mode is still classed as experimental some features may change their default settings between releases.

By default if the application uses any APIs associated with secondary drivers, these will result in a connection being opened to the corresponding driver in libvirtd. For example, this allows a virtual machine from the embedded QEMU to connect its NIC to a virtual network or connect its disk to a storage volume. Some of the secondary drivers will also be able to support running in embedded mode. Currently this is supported by the secrets driver, to allow for use of VMs with encrypted disks

Directory tree

Under the specified root directory the following locations will be used

/some/dir
  |
  +- log
  |   |
  |   +- qemu
  |   +- swtpm
  |
  +- etc
  |   |
  |   +- qemu
  |   +- pki
  |       |
  |       +- qemu
  |
  +- run
  |   |
  |   +- qemu
  |   +- swtpm
  |
  +- cache
  |   |
  |   +- qemu
  |
  +- lib
      |
      +- qemu
      +- swtpm

Note that UNIX domain sockets used for QEMU virtual machines had a maximum filename length of 108 characters. Bear this in mind when picking a root directory to avoid risk of exhausting the filename space. The application is responsible for recursively purging the contents of this directory tree once they no longer require a connection, though it can also be left intact for reuse when opening a future connection.

API usage with event loop

To use the QEMU driver in embedded mode the application must register an event loop with libvirt. Many of the QEMU driver API calls will rely on the event loop processing data. With this in mind, applications must NEVER invoke API calls from the event loop thread itself, only other threads. Not following this rule will lead to deadlocks in the API. This restriction was lifted starting from 6.2.0 release, when QMP processing moved to a dedicated thread. However, it is important to let the event loop run after each API call, even the ones made from the event loop thread itself.

Location of configuration files

The QEMU driver comes with sane default values. However, during its initialization it reads a configuration file which offers system administrator or an user to override some of that default. The location of the file depends on the connection URI, as follows:

qemu:///system	/etc/libvirt/qemu.conf
qemu:///session	$XDG_CONFIG_HOME/libvirt/qemu.conf
qemu:///embed	$rootdir/etc/qemu.conf

If $XDG_CONFIG_HOME is not set in the environment, it defaults to $HOME/.config. For the embed URI the $rootdir represents the specified root directory from the connection URI.

Please note, that it is very likely that the only qemu.conf file that will exist after installing libvirt is the /etc/libvirt/qemu.conf, if users of the session daemon or the embed driver want to override a built in value, then they need to create the file before connecting to the respective URI.

Driver security architecture

There are multiple layers to security in the QEMU driver, allowing for flexibility in the use of QEMU based virtual machines.

Driver instances

As explained above there are two ways to access the QEMU driver in libvirt. The "qemu:///session" family of URIs connect to a libvirtd instance running as the same user/group ID as the client application. Thus the QEMU instances spawned from this driver will share the same privileges as the client application. The intended use case for this driver is desktop virtualization, with virtual machines storing their disk images in the user's home directory and being managed from the local desktop login session.

The "qemu:///system" family of URIs connect to a libvirtd instance running as the privileged system account 'root'. Thus the QEMU instances spawned from this driver may have much higher privileges than the client application managing them. The intended use case for this driver is server virtualization, where the virtual machines may need to be connected to host resources (block, PCI, USB, network devices) whose access requires elevated privileges.

POSIX users/groups

In the "session" instance, the POSIX users/groups model restricts QEMU virtual machines (and libvirtd in general) to only have access to resources with the same user/group ID as the client application. There is no finer level of configuration possible for the "session" instances.

In the "system" instance, libvirt releases from 0.7.0 onwards allow control over the user/group that the QEMU virtual machines are run as. A build of libvirt with no configuration parameters set will still run QEMU processes as root:root. It is possible to change this default by using the --with-qemu-user=$USERNAME and --with-qemu-group=$GROUPNAME arguments to 'configure' during build. It is strongly recommended that vendors build with both of these arguments set to 'qemu'. Regardless of this build time default, administrators can set a per-host default setting in the /etc/libvirt/qemu.conf configuration file via the user=$USERNAME and group=$GROUPNAME parameters. When a non-root user or group is configured, the libvirt QEMU driver will change uid/gid to match immediately before executing the QEMU binary for a virtual machine.

If QEMU virtual machines from the "system" instance are being run as non-root, there will be greater restrictions on what host resources the QEMU process will be able to access. The libvirtd daemon will attempt to manage permissions on resources to minimise the likelihood of unintentional security denials, but the administrator / application developer must be aware of some of the consequences / restrictions.

• The directories /var/run/libvirt/qemu/, /var/lib/libvirt/qemu/ and /var/cache/libvirt/qemu/ must all have their ownership set to match the user / group ID that QEMU guests will be run as. If the vendor has set a non-root user/group for the QEMU driver at build time, the permissions should be set automatically at install time. If a host administrator customizes user/group in /etc/libvirt/qemu.conf, they will need to manually set the ownership on these directories.

• When attaching USB and PCI devices to a QEMU guest, QEMU will need to access files in /dev/bus/usb and /sys/bus/pci/devices respectively. The libvirtd daemon will automatically set the ownership on specific devices that are assigned to a guest at start time. There should not be any need for administrator changes in this respect.

• Any files/devices used as guest disk images must be accessible to the user/group ID that QEMU guests are configured to run as. The libvirtd daemon will automatically set the ownership of the file/device path to the correct user/group ID. Applications / administrators must be aware though that the parent directory permissions may still deny access. The directories containing disk images must either have their ownership set to match the user/group configured for QEMU, or their UNIX file permissions must have the 'execute/search' bit enabled for 'others'.

  The simplest option is the latter one, of just enabling the 'execute/search' bit. For any directory to be used for storing disk images, this can be achieved by running the following command on the directory itself, and any parent directories

  chmod o+x /path/to/directory

  In particular note that if using the "system" instance and attempting to store disk images in a user home directory, the default permissions on $HOME are typically too restrictive to allow access.

The libvirt maintainers strongly recommend against running QEMU as the root user/group. This should not be required in most supported usage scenarios, as libvirt will generally do the right thing to grant QEMU access to files it is permitted to use when it is running non-root.

Linux process capabilities

In versions of libvirt prior to 6.0.0, even if QEMU was configured to run as the root user / group, libvirt would strip all process capabilities. This meant that QEMU could only read/write files owned by root, or with open permissions. In reality, stripping capabilities did not have any security benefit, as it was trivial to get commands to run in another context with full capabilities, for example, by creating a cronjob.

Thus since 6.0.0, if QEMU is running as root, it will keep all process capabilities. Behaviour when QEMU is running non-root is unchanged, it still has no capabilities.

SELinux basic confinement

The basic SELinux protection for QEMU virtual machines is intended to protect the host OS from a compromised virtual machine process. There is no protection between guests.

In the basic model, all QEMU virtual machines run under the confined domain root:system_r:qemu_t. It is required that any disk image assigned to a QEMU virtual machine is labelled with system_u:object_r:virt_image_t. In a default deployment, package vendors/distributor will typically ensure that the directory /var/lib/libvirt/images has this label, such that any disk images created in this directory will automatically inherit the correct labelling. If attempting to use disk images in another location, the user/administrator must ensure the directory has be given this requisite label. Likewise physical block devices must be labelled system_u:object_r:virt_image_t.

Not all filesystems allow for labelling of individual files. In particular NFS, VFat and NTFS have no support for labelling. In these cases administrators must use the 'context' option when mounting the filesystem to set the default label to system_u:object_r:virt_image_t. In the case of NFS, there is an alternative option, of enabling the virt_use_nfs SELinux boolean.

SELinux sVirt confinement

The SELinux sVirt protection for QEMU virtual machines builds to the basic level of protection, to also allow individual guests to be protected from each other.

In the sVirt model, each QEMU virtual machine runs under its own confined domain, which is based on system_u:system_r:svirt_t:s0 with a unique category appended, eg, system_u:system_r:svirt_t:s0:c34,c44. The rules are setup such that a domain can only access files which are labelled with the matching category level, eg system_u:object_r:svirt_image_t:s0:c34,c44. This prevents one QEMU process accessing any file resources that are prevent to another QEMU process.

There are two ways of assigning labels to virtual machines under sVirt. In the default setup, if sVirt is enabled, guests will get an automatically assigned unique label each time they are booted. The libvirtd daemon will also automatically relabel exclusive access disk images to match this label. Disks that are marked as <shared> will get a generic label system_u:system_r:svirt_image_t:s0 allowing all guests read/write access them, while disks marked as <readonly> will get a generic label system_u:system_r:svirt_content_t:s0 which allows all guests read-only access.

With statically assigned labels, the application should include the desired guest and file labels in the XML at time of creating the guest with libvirt. In this scenario the application is responsible for ensuring the disk images & similar resources are suitably labelled to match, libvirtd will not attempt any relabelling.

If the sVirt security model is active, then the node capabilities XML will include its details. If a virtual machine is currently protected by the security model, then the guest XML will include its assigned labels. If enabled at compile time, the sVirt security model will always be activated if SELinux is available on the host OS. To disable sVirt, and revert to the basic level of SELinux protection (host protection only), the /etc/libvirt/qemu.conf file can be used to change the setting to security_driver="none"

AppArmor sVirt confinement

When using basic AppArmor protection for the libvirtd daemon and QEMU virtual machines, the intention is to protect the host OS from a compromised virtual machine process. There is no protection between guests.

The AppArmor sVirt protection for QEMU virtual machines builds on this basic level of protection, to also allow individual guests to be protected from each other.

In the sVirt model, if a profile is loaded for the libvirtd daemon, then each qemu:///system QEMU virtual machine will have a profile created for it when the virtual machine is started if one does not already exist. This generated profile uses a profile name based on the UUID of the QEMU virtual machine and contains rules allowing access to only the files it needs to run, such as its disks, pid file and log files. Just before the QEMU virtual machine is started, the libvirtd daemon will change into this unique profile, preventing the QEMU process from accessing any file resources that are present in another QEMU process or the host machine.

The AppArmor sVirt implementation is flexible in that it allows an administrator to customize the template file in /etc/apparmor.d/libvirt/TEMPLATE for site-specific access for all newly created QEMU virtual machines. Also, when a new profile is generated, two files are created: /etc/apparmor.d/libvirt/libvirt-<uuid> and /etc/apparmor.d/libvirt/libvirt-<uuid>.files. The former can be fine-tuned by the administrator to allow custom access for this particular QEMU virtual machine, and the latter will be updated appropriately when required file access changes, such as when a disk is added. This flexibility allows for situations such as having one virtual machine in complain mode with all others in enforce mode.

While users can define their own AppArmor profile scheme, a typical configuration will include a profile for /usr/sbin/libvirtd, /usr/lib/libvirt/virt-aa-helper or /usr/libexec/virt-aa-helper(a helper program which the libvirtd daemon uses instead of manipulating AppArmor directly), and an abstraction to be included by /etc/apparmor.d/libvirt/TEMPLATE (typically /etc/apparmor.d/abstractions/libvirt-qemu). An example profile scheme can be found in the examples/apparmor directory of the source distribution.

If the sVirt security model is active, then the node capabilities XML will include its details. If a virtual machine is currently protected by the security model, then the guest XML will include its assigned profile name. If enabled at compile time, the sVirt security model will be activated if AppArmor is available on the host OS and a profile for the libvirtd daemon is loaded when libvirtd is started. To disable sVirt, and revert to the basic level of AppArmor protection (host protection only), the /etc/libvirt/qemu.conf file can be used to change the setting to security_driver="none".

Cgroups device ACLs

Linux kernels have a capability known as "cgroups" which is used for resource management. It is implemented via a number of "controllers", each controller covering a specific task/functional area. One of the available controllers is the "devices" controller, which is able to setup access control lists of block/character devices that a cgroup should be allowed to access. If the "devices" controller is mounted on a host, then libvirt will automatically create a dedicated cgroup for each QEMU virtual machine and setup the device access control list so that the QEMU process can only access shared devices, and explicitly assigned disks images backed by block devices.

The list of shared devices a guest is allowed access to is

/dev/null, /dev/full, /dev/zero,
/dev/random, /dev/urandom,
/dev/ptmx, /dev/kvm,

In the event of unanticipated needs arising, this can be customized via the /etc/libvirt/qemu.conf file. To mount the cgroups device controller, the following command should be run as root, prior to starting libvirtd

mkdir /dev/cgroup
mount -t cgroup none /dev/cgroup -o devices

libvirt will then place each virtual machine in a cgroup at /dev/cgroup/libvirt/qemu/$VMNAME/

Live migration compatibility

Many factors can affect the ability to live migrate a guest between a pair of hosts. It is critical that when QEMU is started on the destination host, the exposed guest machine ABI matches what was exposed by the existing QEMU process on the source host. To facilitate this, when libvirt receives a guest configuration document, it will attempt to expand any features that were not specified, to ensure a stable guest machine ABI. Mostly this involves adding address information to all devices, and adding controllers to attach the devices to.

Certain features that affect the guest ABI, however, may only be known at the time the guest is started and can be influenced by features of the host OS and its hardware. This means that even if the guest XML configuration is the same, it may still be impossible to migrate the guest between two hosts.

Migration CPU model compatibility

The most common problems with migration compatibility surround the use of the guest CPU host-model or host-passthrough modes. Both of these modes attempt to expose the full host CPU featureset to the guest. The host-model mode attempts to expose as many features as possible while retaining the ability to accurately check compatibility between hosts prior to migration running. The host-passthrough mode attempts to expose the host CPU as precisely as possible, but with the cost that it is not possible for libvirt to check compatibility prior to migration.

If using host-model the target host hardware and software deployment must expose a superset of the features of the source host CPU. If using host-passthrough the target host CPU and software deployment must always expose a superset of the features, however, it is further strongly recommended that the source and destination hosts be identical in every way.

In both cases, there are a number of factors that will influence the CPU features available to the guest

• Physical CPU model - the core constraint on what features are available. Check /proc/cpuinfo for CPU model name.
• Firmware revision (BIOS/UEFI/etc) - firmware updates may bundle microcode updates which arbitrarily add or remove CPU features, typically in response to new hardware vulnerabilities. Check dmidecode for details on x86 and aarch64 platforms for firmware version, and /proc/cpuinfo for associated microcode version (if not updated by the OS).
• Firmware settings - certain firmware settings can affect accessibility of features. For example, turning on/off SMT/HT not only affects the number of logical CPUs available to the OS, but can indirectly influence other factors such as the number of performance counters available for use. Check the firmware specific configuration interface.
• Host kernel version - the host kernel software version may have a need to block certain physical CPU features from use in the guest. It can also emulate certain features that may not exist in the silicon, for example, x2apic. Check uname -r output for kernel version.
• Host kernel settings - the kernel command line options can be used to block certain physical CPU features from use in the guest, for example, tsx=off, l1tf=... or nosmt. Check /proc/cmdline and /etc/modprobe.d/*.conf.
• microcode update version - while the firmware will load the initial microcode in to the CPU, the OS may ship packages providing newer microcode updates since these can be deployed on a more timely manner than firmware updates. These updates can arbitrarily load add or remove CPU features. Check /proc/cpuinfo for microcode version.
• QEMU version - even when the kernel supports exposing a CPU feature to the guest, an update in the QEMU emulator version will be required to unlock its usage with a guest, except with host-passthrough. Check the output of $QEMU -version.
• libvirt version - even when the kernel and QEMU support exposing a CPU feature to the guest, an update in the libvirt version will be required to unlock its usage with a guest, except with host-passthrough. Check virsh version.
• Nested virtualization - due to the limitations of nested virtualization, a L1 nested host may not be able to expose the same featureset as a bare metal host, even if everything else is the same.

The virsh capabilities output will provide information on the high level CPU model, its features, microcode version. Most of the time this will provide enough information to know whether the CPUs of two hosts will be compatible. If there are unexpected differences though, checking the above list of influencing factors can reveal where the difference arises from.

Import and export of libvirt domain XML configs

The QEMU driver currently supports a single native config format known as qemu-argv. The data for this format is expected to be a single line first a list of environment variables, then the QEMu binary name, finally followed by the QEMU command line arguments

Converting from QEMU args to domain XML

Note: this operation is deleted as of 5.5.0 and will return an error.

The virsh domxml-from-native provides a way to convert an existing set of QEMU args into a guest description using libvirt Domain XML that can then be used by libvirt. Please note that this command is intended to be used to convert existing qemu guests previously started from the command line to be managed through libvirt. It should not be used a method of creating new guests from scratch. New guests should be created using an application calling the libvirt APIs (see the libvirt applications page for some examples) or by manually crafting XML to pass to virsh.

Converting from domain XML to QEMU args

The virsh domxml-to-native provides a way to convert a guest description using libvirt Domain XML, into a set of QEMU args that would be used by libvirt to start the qemu process.

Note that currently the command line formatted by libvirt is no longer suited for manually running qemu as the configuration expects various resources and open file descriptors passed to the process which are usually prepared by libvirtd as well as certain features being configured via the monitor.

The qemu arguments as returned by virsh domxml-to-native thus are not trivially usable outside of libvirt.

Pass-through of arbitrary qemu commands

Libvirt provides an XML namespace and an optional library libvirt-qemu.so for dealing specifically with qemu. When used correctly, these extensions allow testing specific qemu features that have not yet been ported to the generic libvirt XML and API interfaces. However, they are unsupported, in that the library is not guaranteed to have a stable API, abusing the library or XML may result in inconsistent state the crashes libvirtd, and upgrading either qemu-kvm or libvirtd may break behavior of a domain that was relying on a qemu-specific pass-through. If you find yourself needing to use them to access a particular qemu feature, then please post an RFE to the libvirt mailing list to get that feature incorporated into the stable libvirt XML and API interfaces.

The library provides two API: virDomainQemuMonitorCommand, for sending an arbitrary monitor command (in either HMP or QMP format) to a qemu guest ( Since 0.8.3 ), and virDomainQemuAttach, for registering a qemu domain that was manually started so that it can then be managed by libvirtd ( Since 0.9.4 , removed as of 5.5.0 ).

Additionally, the following XML additions allow fine-tuning of the command line given to qemu when starting a domain ( Since 0.8.3 ). In order to use the XML additions, it is necessary to issue an XML namespace request (the special xmlns:name attribute) that pulls in http://libvirt.org/schemas/domain/qemu/1.0; typically, the namespace is given the name of qemu. With the namespace in place, it is then possible to add an element <qemu:commandline> under domain, with the following sub-elements repeated as often as needed:

qemu:arg

Add an additional command-line argument to the qemu process when starting the domain, given by the value of the attribute value.

qemu:env

Add an additional environment variable to the qemu process when starting the domain, given with the name-value pair recorded in the attributes name and optional value.

Example:

<domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>QEMU-fedora-i686</name>
  <memory>219200</memory>
  <os>
    <type arch='i686' machine='pc'>hvm</type>
  </os>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
  </devices>
  <qemu:commandline>
    <qemu:arg value='-newarg'/>
    <qemu:env name='QEMU_ENV' value='VAL'/>
  </qemu:commandline>
</domain>

QEMU feature configuration for testing

In some cases e.g. when developing a new feature or for testing it may be required to control a given qemu feature (or qemu capability) to test it before it's complete or disable it for debugging purposes. Since 5.5.0 it's possible to use the same special qemu namespace as above (http://libvirt.org/schemas/domain/qemu/1.0) and use <qemu:capabilities> element to add (<qemu:add capability="capname"/>) or remove (<qemu:del capability="capname"/>) capability bits. The naming of the feature bits is the same libvirt uses in the status XML. Note that this feature is meant for experiments only and should _not be used in production.

Example:

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

   [...]

  <qemu:capabilities>
    <qemu:add capability='blockdev'/>
    <qemu:del capability='drive'/>
  </qemu:capabilities>
</domain>

Control of QEMU deprecation warnings

The following knob controls how QEMU behaves towards deprecated commands and arguments used by libvirt:

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

   [...]

  <qemu:deprecation behavior='crash'/>

This setting is meant for developers and CI efforts to make it obvious when libvirt relies on fields which are deprecated so that it can be fixes as soon as possible.

Possible options are:

none

(default) qemu is supposed to accept and output deprecated fields and commands

omit

qemu is instructed to omit deprecated fields on output, behaviour towards fields and commands from libvirtd is not changed

reject

qemu is instructed to report an error if a deprecated command or field is used by libvirtd

crash

qemu crashes when an deprecated command or field is used by libvirtd

For both "reject" and "crash" qemu is instructed to omit any deprecated fields on output.

The "reject" option is less harsh towards the VMs but some code paths ignore errors reported by qemu and thus it may not be obvious that a deprecated command/field was used, thus it's suggested to use the "crash" option instead.

In cases when qemu doesn't support configuring the behaviour this setting is silently ignored to allow testing older qemu versions without having to reconfigure libvirtd.

DO NOT use in production.

Overriding properties of QEMU devices

For development or testing the <qemu:override> tag allows to override specific properties of devices instantiated by libvirt.

The <qemu:device> sub-element groups overrides for a device identified via the alias attribute. The alias corresponds to the <alias name=''> property of a device. It's strongly recommended to use user-specified aliases for devices with overridden properties.

Sub element <qemu:frontend> encapsulates all overrides of properties for the device frontend and overrides what libvirt formats via -device. Since 8.2.0.

The individual properties are overridden by a <qemu:property> element. The name specifies the name of the property to override. In case when libvirt doesn't configure the property a property with the name is added to the commandline. The type attribute specifies a type of the argument used. The type must correspond with the type that is expected by QEMU. Supported values for the type attribute are: string, number, bool (allowed values for bool are true and false) and remove. The remove type is special and instructs libvirt to remove the property without replacement.

The overrides are applied only to initial device configuration passed to QEMU via the commandline. Later hotplug operations will not apply any modifications.

Configuring override for a device alias which is not used or attempting to remove a device property which is not formatted by libvirt will cause failure to startup the VM.

Note: The libvirt project doesn't guarantee any form of compatibility and stability of devices with overridden properties. The domain is tainted when such configuration is used.

Example:

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

   [...]

  <qemu:override>
    <qemu:device alias='ua-devalias'>
      <qemu:frontend>
        <qemu:property name='propname1' type='string' value='test'/>
        <qemu:property name='propname2' type='unsigned' value='123'/>
        <qemu:property name='propname2' type='signed' value='-123'/>
        <qemu:property name='propname3' type='bool' value='false'/>
        <qemu:property name='propname4' type='remove'/>
      </qemu:frontend>
    </qemu:device>
  </qemu:override>
</domain>

Example domain XML config

QEMU emulated guest on x86_64

<domain type='qemu'>
  <name>QEMU-fedora-i686</name>
  <uuid>c7a5fdbd-cdaf-9455-926a-d65c16db1809</uuid>
  <memory>219200</memory>
  <currentMemory>219200</currentMemory>
  <vcpu>2</vcpu>
  <os>
    <type arch='i686' machine='pc'>hvm</type>
    <boot dev='cdrom'/>
  </os>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
    <disk type='file' device='cdrom'>
      <source file='/home/user/boot.iso'/>
      <target dev='hdc'/>
      <readonly/>
    </disk>
    <disk type='file' device='disk'>
      <source file='/home/user/fedora.img'/>
      <target dev='hda'/>
    </disk>
    <interface type='network'>
      <source network='default'/>
    </interface>
    <graphics type='vnc' port='-1'/>
  </devices>
</domain>

KVM hardware accelerated guest on i686

<domain type='kvm'>
  <name>demo2</name>
  <uuid>4dea24b3-1d52-d8f3-2516-782e98a23fa0</uuid>
  <memory>131072</memory>
  <vcpu>1</vcpu>
  <os>
    <type arch="i686">hvm</type>
  </os>
  <clock sync="localtime"/>
  <devices>
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <source file='/var/lib/libvirt/images/demo2.img'/>
      <target dev='hda'/>
    </disk>
    <interface type='network'>
      <source network='default'/>
      <mac address='24:42:53:21:52:45'/>
    </interface>
    <graphics type='vnc' port='-1' keymap='de'/>
  </devices>
</domain>

HVF hardware accelerated guest on x86_64

<domain type='hvf'>
  <name>hvf-demo</name>
  <uuid>4dea24b3-1d52-d8f3-2516-782e98a23fa0</uuid>
  <memory>131072</memory>
  <vcpu>1</vcpu>
  <os>
    <type arch="x86_64">hvm</type>
  </os>
  <features>
    <acpi/>
  </features>
  <clock sync="localtime"/>
  <devices>
    <emulator>/usr/local/bin/qemu-system-x86_64</emulator>
    <controller type='scsi' index='0' model='virtio-scsi'/>
    <disk type='volume' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source pool='default' volume='myos'/>
      <target bus='scsi' dev='sda'/>
    </disk>
    <interface type='user'>
      <mac address='24:42:53:21:52:45'/>
      <model type='virtio'/>
    </interface>
    <graphics type='vnc' port='-1'/>
  </devices>
</domain>