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158 lines
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HTML
158 lines
8.1 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
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<link rel="stylesheet" type="text/css" href="main.css" />
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<title>libvirt: libvirt architecture</title>
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<meta name="description" content="libvirt, virtualization, virtualization API" />
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</head>
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<body>
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<div id="header">
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<div id="headerLogo"></div>
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<div id="headerSearch">
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<form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><div>
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<input id="query" name="query" type="text" size="12" value="" />
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<input id="submit" name="submit" type="submit" value="Search" />
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</div></form>
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<div id="body">
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<div id="menu">
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<ul class="l0"><li>
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<div>
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<a title="Front page of the libvirt website" class="inactive" href="index.html">Home</a>
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</li><li>
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<a title="Details of new features and bugs fixed in each release" class="inactive" href="news.html">News</a>
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<a title="Get the latest source releases, binary builds and get access to the source repository" class="inactive" href="downloads.html">Downloads</a>
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</li><li>
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<a title="User contributed content" class="inactive" href="http://wiki.libvirt.org">Wiki</a>
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<a title="Frequently asked questions" class="inactive" href="FAQ.html">FAQ</a>
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<a title="How and where to report bugs and request features" class="inactive" href="bugs.html">Bug reports</a>
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<a title="How to contact the developers via email and IRC" class="inactive" href="contact.html">Contact</a>
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<a title="Miscellaneous links of interest related to libvirt" class="inactive" href="relatedlinks.html">Related Links</a>
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<a title="Overview of all content on the website" class="inactive" href="sitemap.html">Sitemap</a>
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</li></ul>
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</div>
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<div id="content">
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<h1>libvirt architecture</h1>
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<p>Currently libvirt supports 2 kind of virtualization, and its
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internal structure is based on a driver model which simplifies adding new
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engines:</p>
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<ul><li>
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<a href="#Xen">Xen hypervisor</a>
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</li><li>
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<a href="#QEmu">QEmu and KVM based virtualization</a>
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</li><li>
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<a href="#drivers">the driver architecture</a>
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</li></ul>
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<h3>
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<a name="Xen" id="Xen">Libvirt Xen support</a>
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</h3>
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<p>When running in a Xen environment, programs using libvirt have to execute
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in "Domain 0", which is the primary Linux OS loaded on the machine. That OS
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kernel provides most if not all of the actual drivers used by the set of
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domains. It also runs the Xen Store, a database of information shared by the
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hypervisor, the kernels, the drivers and the xen daemon. Xend. The xen daemon
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supervise the control and execution of the sets of domains. The hypervisor,
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drivers, kernels and daemons communicate though a shared system bus
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implemented in the hypervisor. The figure below tries to provide a view of
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this environment:</p>
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<img src="architecture.gif" alt="The Xen architecture" />
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<p>The library can be initialized in 2 ways depending on the level of
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privilege of the embedding program. If it runs with root access,
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virConnectOpen() can be used, it will use three different ways to connect to
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the Xen infrastructure:</p>
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<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 running
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as root and providing read-only support</li></ul>
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<p>The library will usually interact with the Xen daemon for any operation
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changing the state of the system, but for performance and accuracy reasons
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may talk directly to the hypervisor when gathering state information at
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least when possible (i.e. when the running program using libvirt has root
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privilege access).</p>
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<p>If it runs without root access virConnectOpenReadOnly() should be used to
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connect to initialize the library. It will then fork a libvirt_proxy
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program running as root and providing read_only access to the API, this is
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then only useful for reporting and monitoring.</p>
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<h3>
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<a name="QEmu" id="QEmu">Libvirt QEmu and KVM support</a>
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</h3>
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<p>The model for QEmu and KVM is completely similar, basically KVM is based
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on QEmu for the process controlling a new domain, only small details differs
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between the two. In both case the libvirt API is provided by a controlling
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process forked by libvirt in the background and which launch and control the
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QEmu or KVM process. That program called libvirt_qemud talks though a specific
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protocol to the library, and connects to the console of the QEmu process in
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order to control and report on its status. Libvirt tries to expose all the
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emulations models of QEmu, the selection is done when creating the new
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domain, by specifying the architecture and machine type targeted.</p>
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<p>The code controlling the QEmu process is available in the
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<code>qemud/</code> directory.</p>
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<h3>
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<a name="drivers" id="drivers">the driver based architecture</a>
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</h3>
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<p>As the previous section explains, libvirt can communicate using different
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channels with the current hypervisor, and should also be able to use
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different kind of hypervisor. To simplify the internal design, code, ease
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maintenance and simplify the support of other virtualization engine the
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internals have been structured as one core component, the libvirt.c module
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acting as a front-end for the library API and a set of hypervisor drivers
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defining a common set of routines. That way the Xen Daemon access, the Xen
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Store one, the Hypervisor hypercall are all isolated in separate C modules
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implementing at least a subset of the common operations defined by the
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drivers present in driver.h:</p>
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<ul><li>xend_internal: implements the driver functions though the Xen
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Daemon</li><li>xs_internal: implements the subset of the driver available though the
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Xen Store</li><li>xen_internal: provide the implementation of the functions possible via
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direct hypervisor access</li><li>proxy_internal: provide read-only Xen access via a proxy, the proxy code
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is in the <code>proxy/</code>directory.</li><li>xm_internal: provide support for Xen defined but not running
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domains.</li><li>qemu_internal: implement the driver functions for QEmu and
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KVM virtualization engines. It also uses a qemud/ specific daemon
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which interacts with the QEmu process to implement libvirt API.</li><li>test: this is a test driver useful for regression tests of the
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front-end part of libvirt.</li></ul>
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<p>Note that a given driver may only implement a subset of those functions,
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(for example saving a Xen domain state to disk and restoring it is only
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possible though the Xen Daemon), in that case the driver entry points for
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unsupported functions are initialized to NULL.</p>
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<p></p>
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</div>
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</div>
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<div id="footer">
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<p id="sponsor">
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Sponsored by:<br /><a href="http://et.redhat.com/"><img src="et.png" alt="Project sponsored by Red Hat Emerging Technology" /></a></p>
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</div>
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</body>
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