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<h1 align="center">Libvirt the virtualization API</h1>

<h1>Note: this is the flat content of the <a
href="index.html">website</a></h1>

<h1 style="text-align: center">libvirt</h1>

<h3>what is <span class="style1">libvirt?</span></h3>

<p>Libvirt is a C toolkit to interact with the virtualization capabilitiesof
recent versions of Linux (and other OSes). It is free software availableunder
the <a href="http://www.opensource.org/licenses/lgpl-license.html">GNULesser
General Public License</a>. Virtualization of the Linux OperatingSystem means
the ability to run multiple instances of Operating Systemsconcurently on a
single hardware system where the basic resources are drivenby a Linux
instance. The library aim at providing long term stable C APIinitially for
the <a
href="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/index.html">Xenparavirtualization</a>but
should be able to integrate other virtualizationmechanisms if needed.</p>

<h2><a name="News">Releases</a></h2>

<p>Here is the list of official releases, however since it is early on in
thedevelopment of libvirt, it is preferable when possible to just use the <a
href="downloads.html">CVS version or snapshot</a>, contact the mailing
listand check the <a href="ChangeLog.html">ChangeLog</a>to gauge
progresses.</p>

<h3>0.2.0: Feb 14 2007</h3>
<ul>
  <li>Various internal cleanups (Mark McLoughlin, Richard Jones,Daniel
    Berrange, Karel Zak)</li>
  <li>Bug fixes: avoid a crash in connect (Daniel Berrange), virsh
    argsparsing (Richard Jones)</li>
  <li>Add support for QEmu and KVM virtualization (Daniel Berrange)</li>
  <li>Add support for network configuration (Mark McLoughlin)</li>
  <li>Minor improvements: regression testing (Daniel Berrange),localization
    string updates</li>
</ul>

<h3>0.1.11: Jan 22 2007</h3>
<ul>
  <li>Finish XML &lt;-&gt; XM config files support</li>
  <li>Remove memory leak when freeing virConf objects</li>
  <li>Finishing inactive domain support (Daniel Berrange)</li>
  <li>Added a Relax-NG schemas to check XML instances</li>
</ul>

<h3>0.1.10: Dec 20 2006</h3>
<ul>
  <li>more localizations</li>
  <li>bug fixes: VCPU info breakages on xen 3.0.3, xenDaemonListDomains
    buffer overflow (Daniel Berrange), reference count bug when creating Xen
    domains (Daniel Berrange).</li>
  <li>improvements: support graphic framebuffer for Xen paravirt (Daniel
    Berrange), VNC listen IP range support (Daniel Berrange), support for
    default Xen config files and inactive domains of 3.0.4 (Daniel
  Berrange).</li>
</ul>

<h3>0.1.9: Nov 29 2006</h3>
<ul>
  <li>python bindings: release interpeter lock when calling C (Daniel
    Berrange)</li>
  <li>don't raise HTTP error when looking informations for a domain</li>
  <li>some refactoring to use the driver for all entry points</li>
  <li>better error reporting (Daniel Berrange)</li>
  <li>fix OS reporting when running as non-root</li>
  <li>provide XML parsing errors</li>
  <li>extension of the test framework (Daniel Berrange)</li>
  <li>fix the reconnect regression test</li>
  <li>python bindings: Domain instances now link to the Connect to avoid
    garbage collection and disconnect</li>
  <li>separate the notion of maximum memory and current use at the XML
  level</li>
  <li>Fix a memory leak (Daniel Berrange)</li>
  <li>add support for shareable drives</li>
  <li>add support for non-bridge style networking configs for guests(Daniel
    Berrange)</li>
  <li>python bindings: fix unsigned long marshalling (Daniel Berrange)</li>
  <li>new config APIs virConfNew() and virConfSetValue() to build configs
    from scratch</li>
  <li>hot plug device support based on Michel Ponceau patch</li>
  <li>added support for inactive domains, new APIs, various associated
    cleanup (Daniel Berrange)</li>
  <li>special device model for HVM guests (Daniel Berrange)</li>
  <li>add API to dump core of domains (but requires a patched xend)</li>
  <li>pygrub bootloader informations take over &lt;os&gt; informations</li>
  <li>updated the localization strings</li>
</ul>

<h3>0.1.8: Oct 16 2006</h3>
<ul>
  <li>Bug for system with page size != 4k</li>
  <li>vcpu number initialization (Philippe Berthault)</li>
  <li>don't label crashed domains as shut off (Peter Vetere)</li>
  <li>fix virsh man page (Noriko Mizumoto)</li>
  <li>blktapdd support for alternate drivers like blktap (Daniel
  Berrange)</li>
  <li>memory leak fixes (xend interface and XML parsing) (Daniel
  Berrange)</li>
  <li>compile fix</li>
  <li>mlock/munlock size fixes (Daniel Berrange)</li>
  <li>improve error reporting</li>
</ul>

<h3>0.1.7: Sep 29 2006</h3>
<ul>
  <li>fix a memory bug on getting vcpu informations from xend (Daniel
    Berrange)</li>
  <li>fix another problem in the hypercalls change in Xen
    changeset86d26e6ec89b when getting domain informations (Daniel
  Berrange)</li>
</ul>

<h3>0.1.6: Sep 22 2006</h3>
<ul>
  <li>Support for localization of strings using gettext (Daniel Berrange)</li>
  <li>Support for new Xen-3.0.3 cdrom and disk configuration (Daniel
  Berrange)</li>
  <li>Support for setting VNC port when creating domains with newxend config
    files (Daniel Berrange)</li>
  <li>Fix bug when running against xen-3.0.2 hypercalls (Jim Fehlig)</li>
  <li>Fix reconnection problem when talking directly to http xend</li>
</ul>

<h3>0.1.5: Sep 5 2006</h3>
<ul>
  <li>Support for new hypercalls change in Xen changeset 86d26e6ec89b</li>
  <li>bug fixes: virParseUUID() was wrong, netwoking for paravirt
    guestsi(Daniel Berrange), virsh on non-existent domains (Daniel
    Berrange),string cast bug when handling error in python (Pete Vetere),
    HTTP500 xend error code handling (Pete Vetere and Daniel Berrange)</li>
  <li>improvements: test suite for SEXPR &lt;-&gt; XML format conversions
    (DanielBerrange), virsh output regression suite (Daniel Berrange), new
    environvariable VIRSH_DEFAULT_CONNECT_URI for the default URI when
    connecting(Daniel Berrange), graphical console support for paravirt
    guests(Jeremy Katz), parsing of simple Xen config files (with Daniel
    Berrange),early work on defined (not running) domains (Daniel
    Berrange),virsh output improvement (Daniel Berrange</li>
</ul>

<h3>0.1.4: Aug 16 2006</h3>
<ul>
  <li>bug fixes: spec file fix (Mark McLoughlin), error report problem
    (withHugh Brock), long integer in Python bindings (with Daniel Berrange),
    XMLgeneration bug for CDRom (Daniel Berrange), bug whem using number()
    XPathfunction (Mark McLoughlin), fix python detection code, remove
    duplicateinitialization errors (Daniel Berrange)</li>
  <li>improvements: UUID in XML description (Peter Vetere), proxy
    codecleanup, virtual CPU and affinity support + virsh support
    (MichelPonceau, Philippe Berthault, Daniel Berrange), port and tty
    informationsfor console in XML (Daniel Berrange), added XML dump to
    driver and proxysupport (Daniel Berrange), extention of boot options with
    support forfloppy and cdrom (Daniel Berrange), features block in XML to
    report/askPAE, ACPI, APIC for HVM domains (Daniel Berrange), fail
    saide-effectoperations when using read-only connection, large
    improvements to testdriver (Daniel Berrange)</li>
  <li>documentation: spelling (Daniel Berrange), test driver examples.</li>
</ul>

<h3>0.1.3: Jul 11 2006</h3>
<ul>
  <li>bugfixes: build as non-root, fix xend access when root, handling
    ofempty XML elements (Mark McLoughlin), XML serialization and parsing
    fixes(Mark McLoughlin), allow to create domains without disk
    (MarkMcLoughlin),</li>
  <li>improvement: xenDaemonLookupByID from O(n^2) to O(n) (Daniel
    Berrange),support for fully virtualized guest (Jim Fehlig, DV, Mark
    McLoughlin)</li>
  <li>documentation: augmented to cover hvm domains</li>
</ul>

<h3>0.1.2: Jul 3 2006</h3>
<ul>
  <li>headers include paths fixup</li>
  <li>proxy mechanism for unpriviledged read-only access by httpu</li>
</ul>

<h3>0.1.1: Jun 21 2006</h3>
<ul>
  <li>building fixes: ncurses fallback (Jim Fehlig), VPATH builds (Daniel
    P.Berrange)</li>
  <li>driver cleanups: new entry points, cleanup of libvirt.c (with Daniel
    P.Berrange)</li>
  <li>Cope with API change introduced in Xen changeset 10277</li>
  <li>new test driver for regression checks (Daniel P. Berrange)</li>
  <li>improvements: added UUID to XML serialization, buffer usage (KarelZak),
    --connect argument to virsh (Daniel P. Berrange),</li>
  <li>bug fixes: uninitialized memory access in error reporting,
    S-Exprparsing (Jim Fehlig, Jeremy Katz), virConnectOpen bug, remove a
    TODO inxs_internal.c</li>
  <li>documentation: Python examples (David Lutterkort), new Perl bindingURL,
    man page update (Karel Zak)</li>
</ul>

<h3>0.1.0: Apr 10 2006</h3>
<ul>
  <li>building fixes: --with-xen-distdir option (Ronald Aigner), out of
    treebuild and pkginfo cflag fix (Daniel Berrange)</li>
  <li>enhancement and fixes of the XML description format (David
    Lutterkortand Jim Fehlig)</li>
  <li>new APIs: for Node information and Reboot</li>
  <li>internal code cleanup: refactoring internals into a driver model,
    moreerror handling, structure sharing, thread safety and ref counting</li>
  <li>bug fixes: error message (Jim Meyering), error allocation in virsh
    (JimMeyering), virDomainLookupByID (Jim Fehlig),</li>
  <li>documentation: updates on architecture, and format, typo fix
    (JimMeyering)</li>
  <li>bindings: exception handling in examples (Jim Meyering), perl ones
    outof tree (Daniel Berrange)</li>
  <li>virsh: more options, create, nodeinfo (Karel Zak), renaming of
    someoptions (Karel Zak), use stderr only for errors (Karel Zak), man
    page(Andrew Puch)</li>
</ul>

<h3>0.0.6: Feb 28 2006</h3>
<ul>
  <li>add UUID lookup and extract API</li>
  <li>add error handling APIs both synchronous and asynchronous</li>
  <li>added minimal hook for error handling at the python level, improved
    thepython bindings</li>
  <li>augment the documentation and tests to cover error handling</li>
</ul>

<h3>0.0.5: Feb 23 2006</h3>
<ul>
  <li>Added XML description parsing, dependance to libxml2, implemented
    thecreation API virDomainCreateLinux()</li>
  <li>new APIs to lookup and name domain by UUID</li>
  <li>fixed the XML dump when using the Xend access</li>
  <li>Fixed a few more problem related to the name change</li>
  <li>Adding regression tests in python and examples in C</li>
  <li>web site improvement, extended the documentation to cover the XMLformat
    and Python API</li>
  <li>Added devhelp help for Gnome/Gtk programmers</li>
</ul>

<h3>0.0.4: Feb 10 2006</h3>
<ul>
  <li>Fix various bugs introduced in the name change</li>
</ul>

<h3>0.0.3: Feb 9 2006</h3>
<ul>
  <li>Switch name from from 'libvir' to libvirt</li>
  <li>Starting infrastructure to add code examples</li>
  <li>Update of python bindings for completeness</li>
</ul>

<h3>0.0.2: Jan 29 2006</h3>
<ul>
  <li>Update of the documentation, web site redesign (Diana Fong)</li>
  <li>integration of HTTP xend RPC based on libxend by Anthony Liquori
    formost operations</li>
  <li>Adding Save and Restore APIs</li>
  <li>extended the virsh command line tool (Karel Zak)</li>
  <li>remove xenstore transactions (Anthony Liguori)</li>
  <li>fix the Python bindings bug when domain and connections where freed</li>
</ul>

<h3>0.0.1: Dec 19 2005</h3>
<ul>
  <li>First release</li>
  <li>Basic management of existing Xen domains</li>
  <li>Minimal autogenerated Python bindings</li>
</ul>

<h2><a name="Introducti">Introduction</a></h2>

<p>Libvirt is a C toolkit to interact with the virtualization capabilities
ofrecent versions of Linux (and other OSes), but libvirt won't try to
provideall possible interfaces for interacting with the virtualization
features.</p>

<p>To avoid ambiguity about the terms used here here are the definitions
forsome of the specific concepts used in libvirt documentation:</p>
<ul>
  <li>a <strong>node</strong>is a single physical machine</li>
  <li>an <strong>hypervisor</strong>is a layer of software allowing
    tovirtualize a node in a set of virtual machines with possibly
    differentconfigurations than the node itself</li>
  <li>a <strong>domain</strong>is an instance of an operating system
    runningon a virtualized machine provided by the hypervisor</li>
</ul>

<p style="text-align: center"><img
alt="Hypervisor and domains running on a node" src="node.gif"></p>

<p>Now we can define the goal of libvirt: to provide the lowest
possiblegeneric and stable layer to manage domains on a node.</p>

<p>This implies the following:</p>
<ul>
  <li>the API is not targetted to a single virtualization environment,
    itcurrently supports Xen and QEmu/KVM. This also implies that some
    veryspecific capabilities which are not generic enough may not be
    provided aslibvirt APIs</li>
  <li>the API should allow to do efficiently and cleanly all the
    operationsneeded to manage domains on a node</li>
  <li>the API will not try to provide hight level multi-nodes
    managementfeatures like load balancing, though they could be implemented
    on top oflibvirt</li>
  <li>stability of the API is a big concern, libvirt should
    isolateapplications from the frequent changes expected at the lower level
    of thevirtualization framework</li>
</ul>

<p>So libvirt should be a building block for higher level management toolsand
for applications focusing on virtualization of a single node (the
onlyexception being domain migration between node capabilities which may need
tobe added at the libvirt level). Where possible libvirt should be
extendableto be able to provide the same API for remote nodes, however this
is not thecase at the moment, the code currently handle only local node
accesses(extension for remote access support is being worked on, see<a
href="bugs.html">the mailing list</a>discussions about it).</p>

<h2><a name="architecture">libvirt architecture</a></h2>

<p>Currently libvirt supports 2 kind of virtualization, and its
internalstructure is based on a driver model which simplifies adding new
engines:</p>
<ul>
  <li><a href="#Xen">Xen hypervisor</a></li>
  <li><a href="#QEmu">QEmu and KVM based virtualization</a></li>
  <li><a href="#drivers">the driver architecture</a></li>
</ul>

<h3><a name="Xen">Libvirt Xen support</a></h3>

<p>When running in a Xen environment, programs using libvirt have to
executein "Domain 0", which is the primary Linux OS loaded on the machine.
That OSkernel provides most if not all of the actual drivers used by the set
ofdomains. It also runs the Xen Store, a database of informations shared by
thehypervisor, the kernels, the drivers and the xen daemon. Xend. The xen
daemonsupervise the control and execution of the sets of domains. The
hypervisor,drivers, kernels and daemons communicate though a shared system
busimplemented in the hypervisor. The figure below tries to provide a view
ofthis environment:</p>
<img src="architecture.gif" alt="The Xen architecture">

<p>The library can be initialized in 2 ways depending on the level
ofpriviledge of the embedding program. If it runs with root
access,virConnectOpen() can be used, it will use different ways to connect
tothe Xen infrastructure:</p>
<ul>
  <li>a connection to the Xen Daemon though an HTTP RPC layer</li>
  <li>a read/write connection to the Xen Store</li>
  <li>use Xen Hypervisor calls</li>
  <li>when used as non-root libvirt connect to a proxy daemon runningas root
    and providing read-only support</li>
</ul>

<p>The library will usually interact with the Xen daemon for any
operationchanging the state of the system, but for performance and accuracy
reasonsmay talk directly to the hypervisor when gathering state informations
atleast when possible (i.e. when the running program using libvirt has
rootpriviledge access).</p>

<p>If it runs without root access virConnectOpenReadOnly() should be used
toconnect to initialize the library. It will then fork a libvirt_proxy
programrunning as root and providing read_only access to the API, this is
thenonly useful for reporting and monitoring.</p>

<h3><a name="QEmu">Libvirt QEmu and KVM support</a></h3>

<p>The model for QEmu and KVM is completely similar, basically KVM isbased on
QEmu for the process controlling a new domain, only small detailsdiffers
between the two. In both case the libvirt API is providedby a controlling
process forked by libvirt in the background andwhich launch and control the
QEmu or KVM process. That program calledlibvirt_qemud talks though a specific
protocol to the library, andconnects to the console of the QEmu process in
order to control andreport on its status. Libvirt tries to expose all the
emulationsmodels of QEmu, the selection is done when creating the new
domain,by specifying the architecture and machine type targetted.</p>

<p>The code controlling the QEmu process is available in
the<code>qemud/</code>subdirectory.</p>

<h3><a name="drivers">the driver based architecture</a></h3>

<p>As the previous section explains, libvirt can communicate using
differentchannels with the Xen hypervisor, and is also able to use different
kindof hypervisor. To simplify the internal design, code, easemaintainance
and simplify the support of other virtualization engine theinternals have
been structured as one core component, the libvirt.c moduleacting as a
front-end for the library API and a set of hypvisor driversdefining a common
set of routines. That way the Xen Daemon accces, the XenStore one, the
Hypervisor hypercall are all isolated in separate C modulesimplementing at
least a subset of the common operations defined by thedrivers present in
driver.h. The driver architecture is used to add supportfor other
virtualization engines and</p>
<ul>
  <li>xend_internal: implements the driver functions though the
  XenDaemon.</li>
  <li>xs_internal: implements the subset of the driver availble though theXen
    Store.</li>
  <li>xen_internal: provide the implementation of the functions possible
    viadirect Xen hypervisor access.</li>
  <li>proxy_internal: provide read-only Xen access via a proxy, the proxycode
    is in the <code>proxy/</code>sub directory.</li>
  <li>xm_internal: provide support for Xen defined but not running
  domains.</li>
  <li>qemu_internal: implement the driver functions for QEmu and
    KVMvirtualization engines. It also uses a qemud/ specific daemon
    whichinterracts with the QEmu process to implement libvirt API.</li>
  <li>test: this is a test driver useful for regression tests of thefront-end
    part of libvirt.</li>
</ul>

<p>Note that a given driver may only implement a subset of those
functions,for example saving a Xen domain state to disk and restoring it is
only possiblethough the Xen Daemon, in that case the driver entry points are
initialized toNULL.</p>

<p></p>

<h2><a name="Downloads">Downloads</a></h2>

<p>The latest versions of libvirt can be found on the  <a
href="ftp://libvirt.org/libvirt/">libvirt.org</a>server ( <a
href="http://libvirt.org/sources/">HTTP</a>, <a
href="ftp://libvirt.org/libvirt/">FTP</a>). You will find there the
releasedversions as well as <a
href="http://libvirt.org/sources/libvirt-cvs-snapshot.tar.gz">snapshottarballs</a>updated
from CVS head every hour</p>

<p>Anonymous <a href="http://ximbiot.com/cvs/cvshome/docs/">CVS</a>is
alsoavailable, first register onto the server:</p>

<p><code>cvs -d :pserver:anoncvs@libvirt.org:2401/data/cvs login</code></p>

<p>it will request a password, enter <strong>anoncvs</strong>. Then you
cancheckout the development tree with:</p>

<p><code>cvs -d :pserver:anoncvs@libvirt.org:2401/data/cvs
colibvirt</code></p>

<p>Use ./autogen.sh to configure the local checkout, then
<code>make</code>and <code>make install</code>, as usual. All normal cvs
commands are nowavailable except commiting to the base.</p>

<h2><a name="Format">XML Formats</a></h2>

<p>This section describes the XML formats used mostly to represent domains,
there are variations on the format based on the kind of domains run and the
options used to launch them:</p>
<ul>
  <li><a href="#Normal1">Normal paravirtualized Xen domains</a></li>
  <li><a href="#Fully1">Fully virtualized Xen domains</a></li>
  <li><a href="#KVM1">KVM domains</a></li>
  <li><a href="#Net1">Networking options for QEmu and KVM</a></li>
  <li><a href="#QEmu1">QEmu domains</a></li>
  <li><a href="#Capa1">Discovering virtualization capabilities</a></li>
</ul>

<p>The formats try as much as possible to follow the same structure and reuse
elements and attributes where it makes sense.</p>

<h3 id="Normal"><a name="Normal1" id="Normal1">Normal paravirtualized
Xendomains</a>:</h3>

<p>The library use an XML format to describe domains, as input to <a
href="html/libvirt-libvirt.html#virDomainCreateLinux">virDomainCreateLinux()</a>and
as the output of <a
href="html/libvirt-libvirt.html#virDomainGetXMLDesc">virDomainGetXMLDesc()</a>,the
following is an example of the format as returned by the shell
command<code>virsh xmldump fc4</code>, where fc4 was one of the running
domains:</p>
<pre>&lt;domain type='xen' <span style="color: #0071FF; background-color: #FFFFFF">id='18'</span>&gt;
  &lt;name&gt;fc4&lt;/name&gt;
  <span style="color: #00B200; background-color: #FFFFFF">&lt;os&gt;
    &lt;type&gt;linux&lt;/type&gt;
    &lt;kernel&gt;/boot/vmlinuz-2.6.15-1.43_FC5guest&lt;/kernel&gt;
    &lt;initrd&gt;/boot/initrd-2.6.15-1.43_FC5guest.img&lt;/initrd&gt;
    &lt;root&gt;/dev/sda1&lt;/root&gt;
    &lt;cmdline&gt; ro selinux=0 3&lt;/cmdline&gt;
  &lt;/os&gt;</span>
  &lt;memory&gt;131072&lt;/memory&gt;
  &lt;vcpu&gt;1&lt;/vcpu&gt;
  &lt;devices&gt;
    <span style="color: #FF0080; background-color: #FFFFFF">&lt;disk type='file'&gt;
      &lt;source file='/u/fc4.img'/&gt;
      &lt;target dev='sda1'/&gt;
    &lt;/disk&gt;</span>
    <span style="color: #0000FF; background-color: #FFFFFF">&lt;interface type='bridge'&gt;
      &lt;source bridge='xenbr0'/&gt;
      &lt;mac address='</span><span style="color: #0000FF; background-color: #FFFFFF"></span><span style="color: #0000FF; background-color: #FFFFFF">aa:00:00:00:00:11'/&gt;
      &lt;script path='/etc/xen/scripts/vif-bridge'/&gt;
    &lt;/interface&gt;</span>
    <span style="color: #FF8000; background-color: #FFFFFF">&lt;console tty='/dev/pts/5'/&gt;</span>
  &lt;/devices&gt;
&lt;/domain&gt;</pre>

<p>The root element must be called <code>domain</code>with no namespace,
the<code>type</code>attribute indicates the kind of hypervisor used, 'xen'
isthe default value. The <code>id</code>attribute gives the domain id
atruntime (not however that this may change, for example if the domain is
savedto disk and restored). The domain has a few children whose order is
notsignificant:</p>
<ul>
  <li>name: the domain name, preferably ASCII based</li>
  <li>memory: the maximum memory allocated to the domain in kilobytes</li>
  <li>vcpu: the number of virtual cpu configured for the domain</li>
  <li>os: a block describing the Operating System, its content will
    bedependant on the OS type
    <ul>
      <li>type: indicate the OS type, always linux at this point</li>
      <li>kernel: path to the kernel on the Domain 0 filesystem</li>
      <li>initrd: an optional path for the init ramdisk on the Domain
        0filesystem</li>
      <li>cmdline: optional command line to the kernel</li>
      <li>root: the root filesystem from the guest viewpoint, it may bepassed
        as part of the cmdline content too</li>
    </ul>
  </li>
  <li>devices: a list of <code>disk</code>,
    <code>interface</code>and<code>console</code>descriptions in no special
    order</li>
</ul>

<p>The format of the devices and their type may grow over time, but
thefollowing should be sufficient for basic use:</p>

<p>A <code>disk</code>device indicates a block device, it can have twovalues
for the type attribute either 'file' or 'block' corresponding to the 2options
availble at the Xen layer. It has two mandatory children, and oneoptional one
in no specific order:</p>
<ul>
  <li>source with a file attribute containing the path in Domain 0 to thefile
    or a dev attribute if using a block device, containing the devicename
    ('hda5' or '/dev/hda5')</li>
  <li>target indicates in a dev attribute the device where it is mapped inthe
    guest</li>
  <li>readonly an optional empty element indicating the device
  isread-only</li>
</ul>

<p>An <code>interface</code>element describes a network device mapped on
theguest, it also has a type whose value is currently 'bridge', it also have
anumber of children in no specific order:</p>
<ul>
  <li>source: indicating the bridge name</li>
  <li>mac: the optional mac address provided in the address attribute</li>
  <li>ip: the optional IP address provided in the address attribute</li>
  <li>script: the script used to bridge the interfcae in the Domain 0</li>
  <li>target: and optional target indicating the device name.</li>
</ul>

<p>A <code>console</code>element describes a serial console connection tothe
guest. It has no children, and a single attribute
<code>tty</code>whichprovides the path to the Pseudo TTY on which the guest
console can beaccessed</p>

<p>Life cycle actions for the domain can also be expressed in the XML
format,they drive what should be happening if the domain crashes, is rebooted
or ispoweroff. There is various actions possible when this happen:</p>
<ul>
  <li>destroy: The domain is cleaned up (that's the default normal
    processingin Xen)</li>
  <li>restart: A new domain is started in place of the old one with the
    sameconfiguration parameters</li>
  <li>preserve: The domain will remain in memory until it is
    destroyedmanually, it won't be running but allows for post-mortem
    debugging</li>
  <li>rename-restart: a variant of the previous one but where the old
    domainis renamed before being saved to allow a restart</li>
</ul>

<p>The following could be used for a Xen production system:</p>
<pre>&lt;domain&gt;
  ...
  &lt;on_reboot&gt;restart&lt;/on_reboot&gt;
  &lt;on_poweroff&gt;destroy&lt;/on_poweroff&gt;
  &lt;on_crash&gt;rename-restart&lt;/on_crash&gt;
  ...
&lt;/domain&gt;</pre>

<p>While the format may be extended in various ways as support for
morehypervisor types and features are added, it is expected that this core
subsetwill remain functional in spite of the evolution of the library.</p>

<h3 id="Fully"><a name="Fully1" id="Fully1">Fully virtualized
guests</a>(added in 0.1.3):</h3>

<p>Here is an example of a domain description used to start a
fullyvirtualized (a.k.a. HVM) Xen domain. This requires hardware
virtualizationsupport at the processor level but allows to run unmodified
operatingsystems:</p>
<pre>&lt;domain type='xen' id='3'&gt;
  &lt;name&gt;fv0&lt;/name&gt;
  &lt;uuid&gt;4dea22b31d52d8f32516782e98ab3fa0&lt;/uuid&gt;
  &lt;os&gt;
    <span style="color: #0000E5; background-color: #FFFFFF">&lt;type&gt;hvm&lt;/type&gt;</span>
    <span style="color: #0000E5; background-color: #FFFFFF">&lt;loader&gt;/usr/lib/xen/boot/hvmloader&lt;/loader&gt;</span>
    <span style="color: #0000E5; background-color: #FFFFFF">&lt;boot dev='hd'/&gt;</span>
  &lt;/os&gt;
  &lt;memory&gt;524288&lt;/memory&gt;
  &lt;vcpu&gt;1&lt;/vcpu&gt;
  &lt;on_poweroff&gt;destroy&lt;/on_poweroff&gt;
  &lt;on_reboot&gt;restart&lt;/on_reboot&gt;
  &lt;on_crash&gt;restart&lt;/on_crash&gt;
  &lt;features&gt;
     <span style="color: #E50000; background-color: #FFFFFF">&lt;pae/&gt;
     &lt;acpi/&gt;
     &lt;apic/&gt;</span>
  &lt;/features&gt;
  &lt;devices&gt;
    <span style="color: #0000E5; background-color: #FFFFFF">&lt;emulator&gt;/usr/lib/xen/bin/qemu-dm&lt;/emulator&gt;</span>
    &lt;interface type='bridge'&gt;
      &lt;source bridge='xenbr0'/&gt;
      &lt;mac address='00:16:3e:5d:c7:9e'/&gt;
      &lt;script path='vif-bridge'/&gt;
    &lt;/interface&gt;
    &lt;disk type='file'&gt;
      &lt;source file='/root/fv0'/&gt;
      &lt;target <span style="color: #0000E5; background-color: #FFFFFF">dev='hda'</span>/&gt;
    &lt;/disk&gt;
    &lt;disk type='file' <span style="color: #0000E5; background-color: #FFFFFF">device='cdrom'</span>&gt;
      &lt;source file='/root/fc5-x86_64-boot.iso'/&gt;
      &lt;target <span style="color: #0000E5; background-color: #FFFFFF">dev='hdc'</span>/&gt;
      &lt;readonly/&gt;
    &lt;/disk&gt;
    &lt;disk type='file' <span style="color: #0000E5; background-color: #FFFFFF">device='floppy'</span>&gt;
      &lt;source file='/root/fd.img'/&gt;
      &lt;target <span style="color: #0000E5; background-color: #FFFFFF">dev='fda'</span>/&gt;
    &lt;/disk&gt;
    <span style="color: #0000E5; background-color: #FFFFFF">&lt;graphics type='vnc' port='5904'/&gt;</span>
  &lt;/devices&gt;
&lt;/domain&gt;</pre>

<p>There is a few things to notice specifically for HVM domains:</p>
<ul>
  <li>the optional <code>&lt;features&gt;</code>block is used to
    enablecertain guest CPU / system features. For HVM guests the
    followingfeatures are defined:
    <ul>
      <li><code>pae</code>- enable PAE memory addressing</li>
      <li><code>apic</code>- enable IO APIC</li>
      <li><code>acpi</code>- enable ACPI bios</li>
    </ul>
  </li>
  <li>the <code>&lt;os&gt;</code>block description is very different, firstit
    indicates that the type is 'hvm' for hardware virtualization, theninstead
    of a kernel, boot and command line arguments, it points to an osboot
    loader which will extract the boot informations from the boot
    devicespecified in a separate boot element. The <code>dev</code>attribute
    onthe <code>boot</code>tag can be one of:
    <ul>
      <li><code>fd</code>- boot from first floppy device</li>
      <li><code>hd</code>- boot from first harddisk device</li>
      <li><code>cdrom</code>- boot from first cdrom device</li>
    </ul>
  </li>
  <li>the <code>&lt;devices&gt;</code>section includes an emulator
    entrypointing to an additional program in charge of emulating the
  devices</li>
  <li>the disk entry indicates in the dev target section that the
    emulationfor the drive is the first IDE disk device hda. The list of
    device namessupported is dependant on the Hypervisor, but for Xen it can
    be any IDEdevice <code>hda</code>-<code>hdd</code>, or a floppy
    device<code>fda</code>, <code>fdb</code>. The
    <code>&lt;disk&gt;</code>elementalso supports a 'device' attribute to
    indicate what kinda of hardware toemulate. The following values are
    supported:
    <ul>
      <li><code>floppy</code>- a floppy disk controller</li>
      <li><code>disk</code>- a generic hard drive (the default itomitted)</li>
      <li><code>cdrom</code>- a CDROM device</li>
    </ul>
    For Xen 3.0.2 and earlier a CDROM device can only be emulated on
    the<code>hdc</code>channel, while for 3.0.3 and later, it can be
    emulatedon any IDE channel.</li>
  <li>the <code>&lt;devices&gt;</code>section also include at least oneentry
    for the graphic device used to render the os. Currently there isjust 2
    types possible 'vnc' or 'sdl'. If the type is 'vnc', then anadditional
    <code>port</code>attribute will be present indicating the TCPport on
    which the VNC server is accepting client connections.</li>
</ul>

<p>It is likely that the HVM description gets additional optional elementsand
attributes as the support for fully virtualized domain expands,especially for
the variety of devices emulated and the graphic supportoptions offered.</p>

<h3><a name="KVM1">KVM domain (added in 0.2.0)</a></h3>

<p>Support for the <a href="http://kvm.qumranet.com/">KVM virtualization</a>
is provided in recent Linux kernels (2.6.20 and onward). This requires
specific hardware with acceleration support and the availability of the
special version of the <a
href="http://fabrice.bellard.free.fr/qemu/">QEmu</a> binary. Since this
relies on QEmu for the machine emulation like fully virtualized guests the
XML description is quite similar, here is a simple example:</p>
<pre>&lt;domain <span style="color: #FF0000; background-color: #FFFFFF">type='kvm'</span>&gt;
  &lt;name&gt;demo2&lt;/name&gt;
  &lt;uuid&gt;4dea24b3-1d52-d8f3-2516-782e98a23fa0&lt;/uuid&gt;
  &lt;memory&gt;131072&lt;/memory&gt;
  &lt;vcpu&gt;1&lt;/vcpu&gt;
  &lt;os&gt;
    &lt;type&gt;hvm&lt;/type&gt;
  &lt;/os&gt;
  &lt;devices&gt;
    <span style="color: #FF0000; background-color: #FFFFFF">&lt;emulator&gt;/home/user/usr/kvm-devel/bin/qemu-system-x86_64&lt;/emulator&gt;</span>
    &lt;disk type='file' device='disk'&gt;
      &lt;source file='/home/user/fedora/diskboot.img'/&gt;
      &lt;target dev='hda'/&gt;
    &lt;/disk&gt;
    &lt;interface <span style="color: #FF0000; background-color: #FFFFFF">type='user'</span>&gt;
      &lt;mac address='24:42:53:21:52:45'/&gt;
    &lt;/interface&gt;
    &lt;graphics type='vnc' port='-1'/&gt;
  &lt;/devices&gt;
&lt;/domain&gt;</pre>

<p>The specific points to note if using KVM are:</p>
<ul>
  <li>the top level domain element carries a type of 'kvm'</li>
  <li>the &lt;devices&gt; emulator points to the special qemu binary required
    for KVM</li>
  <li>networking interface definitions definitions are somewhat different due
    to a different model from Xen see below</li>
</ul>

<p>except those points the options should be quite similar to Xen HVM
ones.</p>

<h3><a name="Net1">Networking options for QEmu and KVM (added in 0.2.0)</a></h3>

<p>The networking support in the QEmu and KVM case is more flexible, and
support a variety of options:</p>
<ol>
  <li>Userspace SLIRP stack
    <p>Provides a virtual LAN with NAT to the outside world. The virtual
    network has DHCP &amp; DNS services and will give the guest VM addresses
    starting from <code>10.0.2.15</code>. The default router will be
    <code>10.0.2.2</code> and the DNS server will be <code>10.0.2.3</code>.
    This networking is the only option for unprivileged users who need their
    VMs to have outgoing access. Example configs are:</p>
    <pre>&lt;interface type='user'/&gt;</pre>
    <pre>
&lt;interface type='user'&gt;                                                  
  &lt;mac address="11:22:33:44:55:66:/&gt;                                     
&lt;/interface&gt;
    </pre>
  </li>
  <li>Virtual network
    <p>Provides a virtual network using a bridge device in the host.
    Depending on the virtual network configuration, the network may be
    totally isolated,NAT'ing to aan explicit network device, or NAT'ing to
    the default route. DHCP and DNS are provided on the virtual network in
    all cases and the IP range can be determined by examining the virtual
    network config with '<code>virsh net-dumpxml &lt;network
    name&gt;</code>'. There is one virtual network called'default' setup out
    of the box which does NAT'ing to the default route and has an IP range of
    <code>192.168.22.0/255.255.255.0</code>. Each guest will have an
    associated tun device created with a name of vnetN, which can also be
    overriden with the &lt;target&gt; element. Example configs are:</p>
    <pre>&lt;interface type='network'&gt;
  &lt;source network='default'/&gt;
&lt;/interface&gt;

&lt;interface type='network'&gt;
  &lt;source network='default'/&gt;
  &lt;target dev='vnet7'/&gt;
  &lt;mac address="11:22:33:44:55:66:/&gt;
&lt;/interface&gt;
    </pre>
  </li>
  <li>Bridge to to LAN
    <p>Provides a bridge from the VM directly onto the LAN. This assumes
    there is a bridge device on the host which has one or more of the hosts
    physical NICs enslaved. The guest VM will have an associated tun device
    created with a name of vnetN, which can also be overriden with the
    &lt;target&gt; element. The tun device will be enslaved to the bridge.
    The IP range / network configuration is whatever is used on the LAN. This
    provides the guest VM full incoming &amp; outgoing net access just like a
    physical machine. Examples include:</p>
    <pre>&lt;interface type='bridge'&gt;
 &lt;source dev='br0'/&gt;
&lt;/interface&gt;

&lt;interface type='bridge'&gt;
  &lt;source dev='br0'/&gt;
  &lt;target dev='vnet7'/&gt;
  &lt;mac address="11:22:33:44:55:66:/&gt;
&lt;/interface&gt;       &lt;interface type='bridge'&gt;
         &lt;source dev='br0'/&gt;
         &lt;target dev='vnet7'/&gt;
         &lt;mac address="11:22:33:44:55:66:/&gt;
       &lt;/interface&gt;</pre>
  </li>
  <li>Generic connection to LAN
    <p>Provides a means for the administrator to execute an arbitrary script
    to connect the guest's network to the LAN. The guest will have a tun
    device created with a name of vnetN, which can also be overriden with the
    &lt;target&gt; element. After creating the tun device a shell script will
    be run which is expected to do whatever host network integration is
    required. By default this script is called /etc/qemu-ifup but can be
    overriden.</p>
    <pre>&lt;interface type='ethernet'/&gt;

&lt;interface type='ethernet'&gt;
  &lt;target dev='vnet7'/&gt;
  &lt;script path='/etc/qemu-ifup-mynet'/&gt;
&lt;/interface&gt;</pre>
  </li>
  <li>Multicast tunnel
    <p>A multicast group is setup to represent a virtual network. Any VMs
    whose network devices are in the same multicast group can talk to each
    other even across hosts. This mode is also available to unprivileged
    users. There is no default DNS or DHCP support and no outgoing network
    access. To provide outgoing network access, one of the VMs should have a
    2nd NIC which is connected to one of the first 4 network types and do the
    appropriate routing. The multicast protocol is compatible with that used
    by user mode linux guests too. The source address used must be from the
    multicast address block.</p>
    <pre>&lt;interface type='mcast'&gt;
  &lt;source address='230.0.0.1' port='5558'/&gt;
&lt;/interface&gt;</pre>
  </li>
  <li>TCP tunnel
    <p>A TCP client/server architecture provides a virtual network. One VM
    provides the server end of the netowrk, all other VMS are configured as
    clients. All network traffic is routed between the VMs via the server.
    This mode is also available to unprivileged users. There is no default
    DNS or DHCP support and no outgoing network access. To provide outgoing
    network access, one of the VMs should have a 2nd NIC which is connected
    to one of the first 4 network types and do the appropriate routing.</p>
    <p>Example server config:</p>
    <pre>&lt;interface type='server'&gt;
  &lt;source address='192.168.0.1' port='5558'/&gt;
&lt;/interface&gt;</pre>
    <p>Example client config:</p>
    <pre>&lt;interface type='client'&gt;
  &lt;source address='192.168.0.1' port='5558'/&gt;
&lt;/interface&gt;</pre>
  </li>
</ol>

<p>To be noted, options 2, 3, 4 are also supported by Xen VMs, so it is
possible to use these configs to have networking with both Xen &amp;
QEMU/KVMs connected to each other.</p>

<h3>Q<a name="QEmu1">Emu domain (added in 0.2.0)</a></h3>

<p>Libvirt support for KVM and QEmu is the same code base with only minor
changes. The configuration is as a result nearly identical, the only changes
are related to QEmu ability to emulate <a
href="http://www.qemu.org/status.html">various CPU type and hardware
platforms</a>, and kqemu support (QEmu own kernel accelerator when the
emulated CPU is i686 as well as the target machine):</p>
<pre>&lt;domain <span style="color: #FF0000; background-color: #FFFFFF">type='qemu'</span>&gt;
  &lt;name&gt;QEmu-fedora-i686&lt;/name&gt;
  &lt;uuid&gt;c7a5fdbd-cdaf-9455-926a-d65c16db1809&lt;/uuid&gt;
  &lt;memory&gt;219200&lt;/memory&gt;
  &lt;currentMemory&gt;219200&lt;/currentMemory&gt;
  &lt;vcpu&gt;2&lt;/vcpu&gt;
  &lt;os&gt;
    <span style="color: #FF0000; background-color: #FFFFFF">&lt;type arch='i686' machine='pc'&gt;hvm&lt;/type&gt;</span>
    &lt;boot dev='cdrom'/&gt;
  &lt;/os&gt;
  &lt;devices&gt;
    <span style="color: #FF0000; background-color: #FFFFFF">&lt;emulator&gt;/usr/bin/qemu&lt;/emulator&gt;</span>
    &lt;disk type='file' device='cdrom'&gt;
      &lt;source file='/home/user/boot.iso'/&gt;
      &lt;target dev='hdc'/&gt;
      &lt;readonly/&gt;
    &lt;/disk&gt;
    &lt;disk type='file' device='disk'&gt;
      &lt;source file='/home/user/fedora.img'/&gt;
      &lt;target dev='hda'/&gt;
    &lt;/disk&gt;
    &lt;interface type='network'&gt;
      &lt;source name='default'/&gt;
    &lt;/interface&gt;
    &lt;graphics type='vnc' port='-1'/&gt;
  &lt;/devices&gt;
&lt;/domain&gt;</pre>

<p>The difference here are:</p>
<ul>
  <li>the value of type on top-level domain, it's 'qemu' or kqemu if asking
    for <a href="http://www.qemu.org/kqemu-tech.html">kernel assisted
    acceleration</a></li>
  <li>the os type block defines the architecture to be emulated, and
    optionally the machine type, see the discovery API below</li>
  <li>the emulator string must point to the right emulator for that
    architecture</li>
</ul>

<h3><a name="Capa1">Discovering virtualization capabilities (Added in 0.2.1)</a></h3>

<p>As new virtualization engine support gets added to libvirt, and to handle
cases like QEmu supporting a variety of emulations, a query interface has
been added in 0.2.1 allowing to list the set of supported virtualization
capabilities on the host:</p>
<pre>    char * virConnectGetCapabilities (virConnectPtr conn);</pre>

<p>The value returned is an XML document listing the virtualization
capabilities of the host and virtualization engine to which
<code>@conn</code> is connected. One can test it using <code>virsh</code>
command line tool command '<code>capabilities</code>', it dumps the XML
associated to the current connection. For example in the case of a 64 bits
machine with hardware virtualization capabilities enabled in the chip and
BIOS you will see</p>
<pre>&lt;capabilities&gt;
  <span style="color: #E50000; background-color: #FFFFFF">&lt;host&gt;
    &lt;cpu&gt;
      &lt;arch&gt;x86_64&lt;/arch&gt;
      &lt;features&gt;
        &lt;vmx/&gt;
      &lt;/features&gt;
    &lt;/cpu&gt;
  &lt;/host&gt;</span>

  &lt;!-- xen-3.0-x86_64 --&gt;
  <span style="color: #0000E5; background-color: #FFFFFF">&lt;guest&gt;
    &lt;os_type&gt;xen&lt;/os_type&gt;
    &lt;arch name="x86_64"&gt;
      &lt;wordsize&gt;64&lt;/wordsize&gt;
      &lt;domain type="xen"&gt;&lt;/domain&gt;
      &lt;emulator&gt;/usr/lib64/xen/bin/qemu-dm&lt;/emulator&gt;
    &lt;/arch&gt;
    &lt;features&gt;
    &lt;/features&gt;
  &lt;/guest&gt;</span>

  &lt;!-- hvm-3.0-x86_32 --&gt;
  <span style="color: #00B200; background-color: #FFFFFF">&lt;guest&gt;
    &lt;os_type&gt;hvm&lt;/os_type&gt;
    &lt;arch name="i686"&gt;
      &lt;wordsize&gt;32&lt;/wordsize&gt;
      &lt;domain type="xen"&gt;&lt;/domain&gt;
      &lt;emulator&gt;/usr/lib/xen/bin/qemu-dm&lt;/emulator&gt;
      &lt;machine&gt;pc&lt;/machine&gt;
      &lt;machine&gt;isapc&lt;/machine&gt;
      &lt;loader&gt;/usr/lib/xen/boot/hvmloader&lt;/loader&gt;
    &lt;/arch&gt;
    &lt;features&gt;
    &lt;/features&gt;
  &lt;/guest&gt;</span>
  ...
&lt;/capabilities&gt;</pre>

<p>The fist block (in red) indicates the host hardware capbilities, currently
it is limited to the CPU properties but other information may be available,
it shows the CPU architecture, and the features of the chip (the feature
block is similar to what you will find in a Xen fully virtualized domain
description).</p>

<p>The second block (in blue) indicates the paravirtualization support of the
Xen support, you will see the os_type of xen to indicate a paravirtual
kernel, then architecture informations and potential features.</p>

<p>The third block (in green) gives similar informations but when running a
32 bit OS fully virtualized with Xen using the hvm support.</p>

<p>This section is likely to be updated and augmented in the future, see <a
href="https://www.redhat.com/archives/libvir-list/2007-March/msg00215.html">the
discussion</a> which led to the capabilities format in the mailing-list
archives.</p>

<h2><a name="Python" id="Python">Binding for Python</a></h2>

<p>Libvirt comes with direct support for the Python language (just make
sureyou installed the libvirt-python package if not compiling from sources).
Alsonote that Daniel Berrange provides <a
href="http://search.cpan.org/~danberr/Sys-Virt-0.1.0/">bindings for
Perl</a>too.</p>

<p>The Python binding should be complete and are mostly
automaticallygenerated from the formal description of the API in xml. The
bindings arearticulated around 2 classes <code>virConnect</code>and virDomain
mapping tothe C types. Functions in the C API taking either type as argument
thenbecomes methods for the classes, their name is just stripped from
thevirConnect or virDomain(Get) prefix and the first letter gets converted
tolower case, for example the C functions:</p>

<p><code>int <a
href="html/libvirt-libvirt.html#virConnectNumOfDomains">virConnectNumOfDomains</a>(virConnectPtr
conn);</code></p>

<p><code>int <a
href="html/libvirt-libvirt.html#virDomainSetMaxMemory">virDomainSetMaxMemory</a>(virDomainPtr
domain, unsigned long memory);</code></p>

<p>become</p>

<p><code>virConn::numOfDomains(self)</code></p>

<p><code>virDomain::setMaxMemory(self, memory)</code></p>

<p>This process is fully automated, you can get a summary of the conversionin
the file libvirtclass.txt present in the python dir or in the docs.Thereis a
couple of function who don't map directly to their C counterparts due
tospecificities in their argument conversions:</p>
<ul>
  <li><code><a
    href="html/libvirt-libvirt.html#virConnectListDomains">virConnectListDomains</a></code>is
    replaced by <code>virDomain::listDomainsID(self)</code>which returnsa
    list of the integer ID for the currently running domains</li>
  <li><code><a
    href="html/libvirt-libvirt.html#virDomainGetInfo">virDomainGetInfo</a></code>is
    replaced by <code>virDomain::info()</code>which returns a list of
    <ol>
      <li>state: one of the state values (virDomainState)</li>
      <li>maxMemory: the maximum memory used by the domain</li>
      <li>memory: the current amount of memory used by the domain</li>
      <li>nbVirtCPU: the number of virtual CPU</li>
      <li>cpuTime: the time used by the domain in nanoseconds</li>
    </ol>
  </li>
</ul>

<p>So let's look at a simple example inspired from the
<code>basic.py</code>test found in <code>python/tests/</code>in the source
tree:</p>
<pre>import <span style="color: #0071FF; background-color: #FFFFFF">libvirt</span>
import sys

conn = <span style="color: #0071FF; background-color: #FFFFFF">libvirt</span>.openReadOnly(None)
if conn == None:
    print 'Failed to open connection to the hypervisor'
    sys.exit(1)

try:
    dom0 = conn.<span style="color: #007F00; background-color: #FFFFFF">lookupByName</span>("Domain-0")
except:
    print 'Failed to find the main domain'
    sys.exit(1)

print "Domain 0: id %d running %s" % (dom0.<span style="color: #FF0080; background-color: #FFFFFF">ID</span>(), dom0.<span style="color: #FF0080; background-color: #FFFFFF">OSType</span>())
print dom0.<span style="color: #FF0080; background-color: #FFFFFF">info</span>()</pre>

<p>There is not much to comment about it, it really is a straight mappingfrom
the C API, the only points to notice are:</p>
<ul>
  <li>the import of the module called <code><span
    style="color: #0071FF; background-color: #FFFFFF">libvirt</span></code></li>
  <li>getting a connection to the hypervisor, in that case using
    theopenReadOnly function allows the code to execute as a normal user.</li>
  <li>getting an object representing the Domain 0 using <span
    style="color: #007F00; background-color: #FFFFFF">lookupByName</span></li>
  <li>if the domain is not found a libvirtError exception will be raised</li>
  <li>extracting and printing some informations about the domain usingvarious
    <span
    style="color: #E50073; background-color: #FFFFFF">methods</span>associated
    to the virDomain class.</li>
</ul>

<h2><a name="Errors" id="Errors">Handling of errors</a></h2>

<p>The main goals of libvirt when it comes to error handling are:</p>
<ul>
  <li>provide as much detail as possible</li>
  <li>provide the informations as soon as possible</li>
  <li>dont force the library user into one style of error handling</li>
</ul>

<p>As result the library provide both synchronous, callback based
andasynchronous error reporting. When an error happens in the library code
theerror is logged, allowing to retrieve it later and if the user registered
anerror callback it will be called synchronously. Once the call to libvirt
endsthe error can be detected by the return value and the full information
forthe last logged error can be retrieved.</p>

<p>To avoid as much as prossible troubles with a global variable in
amultithreaded environment, libvirt will associate when possible the errors
tothe current connection they are related to, that way the error is stored in
adynamic structure which can be made thread specific. Error callback can
beset specifically to a connection with</p>

<p>So error handling in the code is the following:</p>
<ol>
  <li>if the error can be associated to a connection for example when
    failingto look up a domain
    <ol>
      <li>if there is a callback associated to the connection set with <a
        href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,call
        it with the error informations</li>
      <li>otherwise if there is a global callback set with <a
        href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>,call
        it with the error information</li>
      <li>otherwise call <a
        href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>which
        is the default error function of the library issuing the erroron
        stderr</li>
      <li>save the error in the connection for later retrieval with <a
        href="html/libvirt-virterror.html#virConnGetLastError">virConnGetLastError</a></li>
    </ol>
  </li>
  <li>otherwise like when failing to create an hypervisor connection:
    <ol>
      <li>if there is a global callback set with <a
        href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>,call
        it with the error information</li>
      <li>otherwise call <a
        href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>which
        is the default error function of the library issuing the erroron
        stderr</li>
      <li>save the error in the connection for later retrieval with <a
        href="html/libvirt-virterror.html#virGetLastError">virGetLastError</a></li>
    </ol>
  </li>
</ol>

<p>In all cases the error informations are provided as a <a
href="html/libvirt-virterror.html#virErrorPtr">virErrorPtr</a>pointer
toread-only structure <a
href="html/libvirt-virterror.html#virError">virError</a>containing
thefollowing fields:</p>
<ul>
  <li>code: an error number from the <a
    href="html/libvirt-virterror.html#virErrorNumber">virErrorNumber</a>enum</li>
  <li>domain: an enum indicating which part of libvirt raised the error see<a
    href="html/libvirt-virterror.html#virErrorDomain">virErrorDomain</a></li>
  <li>level: the error level, usually VIR_ERR_ERROR, though there is room
    forwarnings like VIR_ERR_WARNING</li>
  <li>message: the full human-readable formatted string of the error</li>
  <li>conn: if available a pointer to the <a
    href="html/libvirt-libvirt.html#virConnectPtr">virConnectPtr</a>connection
    to the hypervisor where this happened</li>
  <li>dom: if available a pointer to the <a
    href="html/libvirt-libvirt.html#virDomainPtr">virDomainPtr</a>domaintargetted
    in the operation</li>
</ul>

<p>and then extra raw informations about the error which may be initializedto
0 or NULL if unused</p>
<ul>
  <li>str1, str2, str3: string informations, usually str1 is the errormessage
    format</li>
  <li>int1, int2: integer informations</li>
</ul>

<p>So usually, setting up specific error handling with libvirt consist
ofregistering an handler with with <a
href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>orwith
<a
href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,chech
the value of the code value, take appropriate action, if needed letlibvirt
print the error on stderr by calling <a
href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>.For
asynchronous error handing, set such a function doing nothing to avoidthe
error being reported on stderr, and call virConnGetLastError
orvirGetLastError when an API call returned an error value. It can be a
goodidea to use <a
href="html/libvirt-virterror.html#virResetLastError">virResetError</a>or <a
href="html/libvirt-virterror.html#virConnResetLastError">virConnResetLastError</a>once
an error has been processed fully.</p>

<p>At the python level, there only a global reporting callback function
atthis point, see the error.py example about it:</p>
<pre>def handler(ctxt, err):
    global errno

    #print "handler(%s, %s)" % (ctxt, err)
    errno = err

libvirt.registerErrorHandler(handler, 'context') </pre>

<p>the second argument to the registerErrorHandler function is passed as
thefist argument of the callback like in the C version. The error is a
tuplecontaining the same field as a virError in C, but cast to Python.</p>

<h2><a name="FAQ" id="FAQ">FAQ</a></h2>

<p>Table of Contents:</p>
<ul>
  <li><a href="FAQ.html#License">License(s)</a></li>
  <li><a href="FAQ.html#Installati">Installation</a></li>
  <li><a href="FAQ.html#Compilatio">Compilation</a></li>
  <li><a href="FAQ.html#Developer">Developer corner</a></li>
</ul>

<h3><a name="License">License</a>(s)</h3>
<ol>
  <li><em>Licensing Terms for libvirt</em>
    <p>libvirt is released under the <a
    href="http://www.opensource.org/licenses/lgpl-license.html">GNU
    LesserGeneral Public License</a>, see the file COPYING.LIB in the
    distributionfor the precise wording. The only library that libvirt
    depends upon isthe Xen store access library which is also licenced under
    the LGPL.</p>
  </li>
  <li><em>Can I embed libvirt in a proprietary application ?</em>
    <p>Yes. The LGPL allows you to embed libvirt into a
    proprietaryapplication. It would be graceful to send-back bug fixes and
    improvementsas patches for possible incorporation in the main development
    tree. Itwill decrease your maintainance costs anyway if you do so.</p>
  </li>
</ol>

<h3><a name="Installati">Installation</a></h3>
<ol>
  <li><em>Where can I get libvirt</em>?
    <p>The original distribution comes from <a
    href="ftp://libvirt.org/libvirt/">ftp://libvirt.org/libvirt/</a>.</p>
  </li>
  <li><em>I can't install the libvirt/libvirt-devel RPM packages due tofailed
    dependencies</em>
    <p>The most generic solution is to re-fetch the latest src.rpm ,
    andrebuild it locally with</p>
    <p><code>rpm --rebuild libvirt-xxx.src.rpm</code>.</p>
    <p>If everything goes well it will generate two binary rpm packages
    (oneproviding the shared libs and virsh, and the other one, the
    -develpackage, providing includes, static libraries and scripts needed to
    buildapplications with libvirt that you can install locally.</p>
    <p>One can also rebuild the RPMs from a tarball:</p>
    <p><code>rpmbuild -ta libdir-xxx.tar.gz</code></p>
    <p>Or from a configured tree with:</p>
    <p><code>make rpm</code></p>
  </li>
  <li><em>Failure to use the API for non-root users</em>
    <p>Large parts of the API may only be accessible with root
    priviledges,however the read only access to the xenstore data doesnot
    have to beforbidden to user, at least for monitoring purposes. If "virsh
    dominfo"fails to run as an user, change the mode of the xenstore
    read-only socketwith:</p>
    <p><code>chmod 666 /var/run/xenstored/socket_ro</code></p>
    <p>and also make sure that the Xen Daemon is running correctly with
    localHTTP server enabled, this is defined
    in<code>/etc/xen/xend-config.sxp</code>which need the following line to
    beenabled:</p>
    <p><code>(xend-http-server yes)</code></p>
    <p>If needed restart the xend daemon after making the change with
    thefollowing command run as root:</p>
    <p><code>service xend restart</code></p>
  </li>
</ol>

<h3><a name="Compilatio">Compilation</a></h3>
<ol>
  <li><em>What is the process to compile libvirt ?</em>
    <p>As most UNIX libraries libvirt follows the "standard":</p>
    <p><code>gunzip -c libvirt-xxx.tar.gz | tar xvf -</code></p>
    <p><code>cd libvirt-xxxx</code></p>
    <p><code>./configure --help</code></p>
    <p>to see the options, then the compilation/installation proper</p>
    <p><code>./configure [possible options]</code></p>
    <p><code>make</code></p>
    <p><code>make install</code></p>
    <p>At that point you may have to rerun ldconfig or a similar utility
    toupdate your list of installed shared libs.</p>
  </li>
  <li><em>What other libraries are needed to compile/install libvirt ?</em>
    <p>Libvirt requires libxenstore, which is usually provided by the
    xenpackages as well as the public headers to compile against
    libxenstore.</p>
  </li>
  <li><em>I use the CVS version and there is no configure script</em>
    <p>The configure script (and other Makefiles) are generated. Use
    theautogen.sh script to regenerate the configure script and
    Makefiles,like:</p>
    <p><code>./autogen.sh --prefix=/usr --disable-shared</code></p>
  </li>
</ol>

<h3><a name="Developer">Developer</a>corner</h3>
<ol>
  <li><em>Troubles compiling or linking programs using libvirt</em>
    <p>To simplify the process of reusing the library, libvirt comes
    withpkgconfig support, which can be used directly from autoconf support
    orvia the pkg-config command line tool, like:</p>
    <p><code>pkg-config libvirt --libs</code></p>
  </li>
</ol>

<h2><a name="Reporting">Reporting bugs and getting help</a></h2>

<p>There is a mailing-list <a
href="mailto:libvir-list@redhat.com">libvir-list@redhat.com</a>for
libvirt,with an  <a
href="https://www.redhat.com/archives/libvir-list/">on-linearchive</a>.
Please subscribe to this list before posting by visiting the <a
href="https://www.redhat.com/mailman/listinfo/libvir-list">associated
Web</a>page and follow the instructions. Patches with explanations and
provided asattachments are really appreciated and will be discussed on the
mailing list.If possible generate the patches by using cvs diff -u in a CVS
checkout.</p>

<p>We use Red Hat Bugzilla to track bugs to libvirt. If you want to report
abug, please check <a
href="http://bugzilla.redhat.com/bugzilla/buglist.cgi?component=libvirt&amp;component=libvirt-devel&amp;component=libvirt-python&amp;bug_status=ASSIGNED&amp;bug_status=INVESTIGATE&amp;bug_status=NEW&amp;bug_status=REOPENED&amp;bug_status=VERIFIED&amp;short_desc_type=allwordssubstr&amp;short_desc=&amp;long_desc_type=allwordssubstr&amp;long_desc=&amp;Search=Search">the
existing open bugs</a>, then if yours isn't a duplicate ofan existing bug, <a
href="http://bugzilla.redhat.com/bugzilla/enter_bug.cgi?product=Fedora%20Core&amp;component=libvirt">log
a new bug</a>. It may be goodto post to the <a
href="mailto:libvir-list@redhat.com">mailing-list</a>too if the issue looks
serious, thanks !</p>
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