libvir.html

<html>
<head>
  <meta http-equiv="Content-Type" content="">
  <title>Libvirt the virtualization API</title>
</head>

<body bgcolor="#ffffff">
<h1 align="center">Libvirt the virtualization API</h1>

<h1>Note: this is the flat content of the <a href="index.html">web
site</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 interract with the virtualization capabilities of
recent versions of Linux (and other OSes). It is free software available
under the <a href="http://www.opensource.org/licenses/lgpl-license.html">GNU
Lesser General Public License</a>. Virtualization of the Linux Operating
System means the ability to run multiple instances of Operating Systems
concurently on a single hardware system where the basic resources are driven
by a Linux instance. The library aim at providing long term stable C API
initially for the <a
href="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/index.html">Xen
paravirtualization</a> but should be able to integrate other virtualization
mechanisms 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 the
development of libvirt, it is preferable when possible to just use the <a
href="downloads.html">CVS version or snapshot</a>, contact the mailing list
and check the <a href="ChangeLog.html">ChangeLog</a> to gauge progresses.</p>

<h3>0.0.5: Feb 23 2006</h3>
<ul>
  <li>Added XML description parsing, dependance to libxml2, implemented the
    creation 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 XML
    format 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 for
    most 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 of
recent versions of Linux (and other OSes), but libvirt won't try to provide
all possible interfaces for interacting with the virtualization features.</p>

<p>To avoid ambiguity about the terms used here here are the definitions for
some 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 to
    virtualize a node in a set of virtual machines with possibly different
    configurations than the node itself</li>
  <li>a <strong>domain</strong> is an instance of an operating system running
    on 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 possible
generic and stable layer to manage domains on a node.</p>

<p>This implies the following:</p>
<ul>
  <li>the API should not be targetted to a single virtualization environment
    though Xen is the current default, which also means that some very
    specific capabilities which are not generic enough may not be provided as
    libvirt APIs</li>
  <li>the API should allow to do efficiently and cleanly all the operations
    needed to manage domains on a node</li>
  <li>the API will not try to provide hight level multi-nodes management
    features like load balancing, though they could be implemented on top of
    libvirt</li>
  <li>stability of the API is a big concern, libvirt should isolate
    applications from the frequent changes expected at the lower level of the
    virtualization framework</li>
</ul>

<p>So libvirt should be a building block for higher level management tools
and for applications focusing on virtualization of a single node (the only
exception being domain migration between node capabilities which may need to
be added at the libvirt level). Where possible libvirt should be extendable
to be able to provide the same API for remote nodes, however this is not the
case at the moment, the code currently handle only local node accesses.</p>

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

<h3>This is Xen specific since this is the only hypervisor supported at the
moment</h3>

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

<p>The library can be initialized in 2 ways depending on the level of
priviledge of the embedding program. If it runs with root access,
virConnectOpen() can be used, it will use three different ways to connect to
the 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>
</ul>

<p>The library will usually interract with the Xen daemon for any operation
changing the state of the system, but for performance and accuracy reasons
may talk directly to the hypervisor when gathering state informations at
least when possible (i.e. when the running program using libvirt has root
priviledge access).</p>

<p>If it runs without root access virConnectOpenReadOnly() should be used to
connect to initialize the library. It will try to open the read-only socket
<code>/var/run/xenstored/socket_ro</code> to connect to the Xen Store and
also try to use the RPC to the Xen daemon. In this case use of hypervisor
calls and write to the Xen Store will not be possible, restraining the amount
of APIs available and slowing down information gathering about domains.</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 released
versions as well as <a
href="http://libvirt.org/sources/libvirt-cvs-snapshot.tar.gz">snapshot
tarballs</a> updated from CVS head every hour</p>

<p>Anonymous <a href="http://ximbiot.com/cvs/cvshome/docs/">CVS</a> is also
available, 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 can
checkout the development tree with:</p>

<p><code>cvs -d :pserver:anoncvs@libvirt.org:2401/data/cvs co
libvirt</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 now
available except commiting to the base.</p>

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

<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>
  &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' is
the default value. The <code>id</code> attribute gives the domain id at
runtime (not however that this may change, for example if the domain is saved
to disk and restored). The domain has a few children whose order is not
significant:</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 be
    dependant 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 0
        filesystem</li>
      <li>cmdline: optional command line to the kernel</li>
      <li>root: the root filesystem from the guest viewpoint, it may be
        passed as part of the cmdline content too</li>
    </ul>
  </li>
  <li>devices: a list of <code>disk</code> and <code>interface</code>
    descriptions in no special order</li>
</ul>

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

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

<p>An interface element describes a network device mapped on the guest, it
also has a type whose value is currently 'bridge', it also have a number 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>While the format may be extended in various ways as support for more
hypervisor types and features are added, it is expected that this core subset
will remain functional in spite of the evolution of the library.</p>

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

<p>Libvirt comes with direct support for the Python language (just make sure
you installed the libvirt-python package if not compiling from sources). The
Python binding should be complete and are mostly automatically generated from
the formal description of the API in xml. The bindings are articulated around
2 classes <code>virConnect</code> and virDomain mapping to the C types.
Functions in the C API taking either type as argument then becomes methods
for the classes, their name is just stripped from the virConnect or
virDomain(Get) prefix and the first letter gets converted to lower 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 conversion
in the file libvirtclass.txt present in the python dir or in the docs.There
is a couple of function who don't map directly to their C counterparts due to
specificities 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 returns
    a 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)

dom0 = conn.<span style="color: #007F00; background-color: #FFFFFF">lookupByName</span>("Domain-0")
if dom0 == None:
    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 mapping
from 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 the
    openReadOnly 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>extracting and printing some informations about the domain using
    various <span
    style="color: #E50073; background-color: #FFFFFF">methods</span>
    associated to the virDomain class.</li>
</ul>

<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 Lesser
    General Public License</a>, see the file COPYING.LIB in the distribution
    for the precise wording. The only library that libvirt depends upon is
    the 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 proprietary
    application. It would be graceful to send-back bug fixes and improvements
    as patches for possible incorporation in the main development tree. It
    will 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 to
    failed dependencies</em>
    <p>The most generic solution is to re-fetch the latest src.rpm , and
    rebuild 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 (one
    providing the shared libs and virsh, and the other one, the -devel
    package, providing includes, static libraries and scripts needed to build
    applications 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 be
    forbidden to user, at least for monitoring purposes. If "virsh dinfo"
    fails to run as an user, change the mode of the xenstore read-only socket
    with:</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 local
    HTTP server enabled, this is defined in
    <code>/etc/xen/xend-config.sxp</code> which need the following line to be
    enabled:</p>
    <p><code>(xend-http-server yes)</code></p>
    <p>If needed restart the xend daemon after making the change with the
    following 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 to
    update 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 xen
    packages 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 the
    autogen.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 with
    pkgconfig support, which can be used directly from autoconf support or
    via 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-line
archive</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 as
attachments 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 expect to use <a href="https://bugzilla.redhat.com/">Red Hat
Bugzilla</a> to track bugs for libvirt, though there isn't a libvirt software
module defined yet, in the meantime use the mailing-list, thanks !.</p>
</body>
</html>