libvirt/docs/architecture.html

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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /><link rel="stylesheet" type="text/css" href="libvirt.css" /><link rel="SHORTCUT ICON" href="/32favicon.png" /><title>libvirt architecture</title></head><body><div id="container"><div id="intro"><div id="adjustments"></div><div id="pageHeader"></div><div id="content2"><h1 class="style1">libvirt architecture</h1><p> Currently libvirt supports 2 kind of virtualization, and its internal
structure 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" id="Xen">Libvirt Xen support</a></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 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>
  <li>when used as non-root libvirt connect to a proxy daemon running
      as root and providing read-only support</li>
</ul><p>The library will usually interact 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 then fork a libvirt_proxy program
running as root and providing read_only access to the API, this is then
only useful for reporting and monitoring.</p><h3><a name="QEmu" id="QEmu">Libvirt QEmu and KVM support</a></h3><p>The model for QEmu and KVM is completely similar, basically KVM is
based on QEmu for the process controlling a new domain, only small details
differs between the two. In both case the libvirt API is provided
by a controlling process forked by libvirt in the background and
which launch and control the QEmu or KVM process. That program called
libvirt_qemud talks though a specific protocol to the library, and 
connects to the console of the QEmu process in order to control and
report on its status. Libvirt tries to expose all the emulations
models 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" id="drivers">the driver based architecture</a></h3><p>As the previous section explains, libvirt can communicate using different
channels with the Xen hypervisor, and is also able to use different kind
of hypervisor. To simplify the internal design, code, ease
maintainance and simplify the support of other virtualization engine the
internals have been structured as one core component, the libvirt.c module
acting as a front-end for the library API and a set of hypvisor drivers
defining a common set of routines. That way the Xen Daemon accces, the Xen
Store one, the Hypervisor hypercall are all isolated in separate C modules
implementing at least a subset of the common operations defined by the
drivers present in driver.h. The driver architecture is used to add support
for other virtualization engines and </p><ul><li>xend_internal: implements the driver functions though the Xen
  Daemon.</li>
  <li>xs_internal: implements the subset of the driver availble though the
    Xen Store.</li>
  <li>xen_internal: provide the implementation of the functions possible via
    direct Xen hypervisor access.</li>
  <li>proxy_internal: provide read-only Xen access via a proxy, the proxy
    code 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 KVM 
    virtualization engines. It also uses a qemud/ specific daemon which
    interracts with the QEmu process to implement libvirt API.</li>
  <li>test: this is a test driver useful for regression tests of the
    front-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 possible
though the Xen Daemon, in that case the driver entry points are initialized to
NULL.</p><p></p></div></div><div class="linkList2"><div class="llinks2"><h3 class="links2"><span>main menu</span></h3><ul><li><a href="index.html">Home</a></li><li><a href="news.html">Releases</a></li><li><a href="intro.html">Introduction</a></li><li><a href="architecture.html">libvirt architecture</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="format.html">XML Format</a></li><li><a href="python.html">Binding for Python</a></li><li><a href="errors.html">Handling of errors</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="html/index.html">API Menu</a></li><li><a href="examples/index.html">C code examples</a></li><li><a href="ChangeLog.html">Recent Changes</a></li></ul></div><div class="llinks2"><h3 class="links2"><span>related links</span></h3><ul><li><a href="https://www.redhat.com/archives/libvir-list/">Mail archive</a></li><li><a href="https://bugzilla.redhat.com/bugzilla/buglist.cgi?product=Fedora+Core&amp;component=libvirt&amp;bug_status=NEW&amp;bug_status=ASSIGNED&amp;bug_status=REOPENED&amp;bug_status=MODIFIED&amp;short_desc_type=allwordssubstr&amp;short_desc=&amp;long_desc_type=allwordssubstr">Open bugs</a></li><li><a href="http://virt-manager.et.redhat.com/">virt-manager</a></li><li><a href="http://search.cpan.org/~danberr/Sys-Virt-0.1.0/">Perl bindings</a></li><li><a href="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/index.html">Xen project</a></li><li><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="12" value="Search..." /><input name="submit" type="submit" value="Go" /></form></li><li><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-90x34.gif" alt="Made with Libxml2 Logo" /></a></li></ul><p class="credits">Graphics and design by <a href="mail:dfong@redhat.com">Diana Fong</a></p></div></div><div id="bottom"><p class="p1"></p></div></div></body></html>
* docs/*: extended the documentation Daniel 2005-12-09 13:04:05 +00:00			`<?xml version="1.0" encoding="ISO-8859-1"?>`
			`<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">`
* src/virsh.c: fix virshStrdup to not crash if NULL is passed. Daniel 2007-04-26 10:20:57 +00:00			<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /><link rel="stylesheet" type="text/css" href="libvirt.css" /><link rel="SHORTCUT ICON" href="/32favicon.png" /><title>libvirt architecture</title></head><body><div id="container"><div id="intro"><div id="adjustments"></div><div id="pageHeader"></div><div id="content2"><h1 class="style1">libvirt architecture</h1><p> Currently libvirt supports 2 kind of virtualization, and its internal
			`structure 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>`
* docs/*.html: repair the damages made by Amaya on the content of documentation following feedback from Simon Hernandez Daniel 2007-03-22 15:17:31 +00:00			`</ul><h3><a name="Xen" id="Xen">Libvirt Xen support</a></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,`
* src/virsh.c: fix virshStrdup to not crash if NULL is passed. Daniel 2007-04-26 10:20:57 +00:00			`virConnectOpen() can be used, it will use different ways to connect to`
* docs/*.html: repair the damages made by Amaya on the content of documentation following feedback from Simon Hernandez Daniel 2007-03-22 15:17:31 +00:00			`the Xen infrastructure:</p><ul><li>a connection to the Xen Daemon though an HTTP RPC layer</li>`
* doc/*: started to augment and update the documentation Daniel 2006-01-22 16:27:09 +00:00			`<li>a read/write connection to the Xen Store</li>`
			`<li>use Xen Hypervisor calls</li>`
* docs/*.html: repair the damages made by Amaya on the content of documentation following feedback from Simon Hernandez Daniel 2007-03-22 15:17:31 +00:00			`<li>when used as non-root libvirt connect to a proxy daemon running`
			`as root and providing read-only support</li>`
			`</ul><p>The library will usually interact 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`
* src/virsh.c: fix virshStrdup to not crash if NULL is passed. Daniel 2007-04-26 10:20:57 +00:00			`connect to initialize the library. It will then fork a libvirt_proxy program`
			`running as root and providing read_only access to the API, this is then`
			`only useful for reporting and monitoring.</p><h3><a name="QEmu" id="QEmu">Libvirt QEmu and KVM support</a></h3><p>The model for QEmu and KVM is completely similar, basically KVM is`
			`based on QEmu for the process controlling a new domain, only small details`
			`differs between the two. In both case the libvirt API is provided`
			`by a controlling process forked by libvirt in the background and`
			`which launch and control the QEmu or KVM process. That program called`
			`libvirt_qemud talks though a specific protocol to the library, and`
			`connects to the console of the QEmu process in order to control and`
			`report on its status. Libvirt tries to expose all the emulations`
			`models 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" id="drivers">the driver based architecture</a></h3><p>As the previous section explains, libvirt can communicate using different`
			`channels with the Xen hypervisor, and is also able to use different kind`
			`of hypervisor. To simplify the internal design, code, ease`
* docs/*.html: repair the damages made by Amaya on the content of documentation following feedback from Simon Hernandez Daniel 2007-03-22 15:17:31 +00:00			`maintainance and simplify the support of other virtualization engine the`
			`internals have been structured as one core component, the libvirt.c module`
			`acting as a front-end for the library API and a set of hypvisor drivers`
			`defining a common set of routines. That way the Xen Daemon accces, the Xen`
			`Store one, the Hypervisor hypercall are all isolated in separate C modules`
			`implementing at least a subset of the common operations defined by the`
* src/virsh.c: fix virshStrdup to not crash if NULL is passed. Daniel 2007-04-26 10:20:57 +00:00			`drivers present in driver.h. The driver architecture is used to add support`
			`for other virtualization engines and </p><ul><li>xend_internal: implements the driver functions though the Xen`
			`Daemon.</li>`
* docs/*.html: repair the damages made by Amaya on the content of documentation following feedback from Simon Hernandez Daniel 2007-03-22 15:17:31 +00:00			`<li>xs_internal: implements the subset of the driver availble though the`
* src/virsh.c: fix virshStrdup to not crash if NULL is passed. Daniel 2007-04-26 10:20:57 +00:00			`Xen Store.</li>`
* docs/*.html: repair the damages made by Amaya on the content of documentation following feedback from Simon Hernandez Daniel 2007-03-22 15:17:31 +00:00			`<li>xen_internal: provide the implementation of the functions possible via`
* src/virsh.c: fix virshStrdup to not crash if NULL is passed. Daniel 2007-04-26 10:20:57 +00:00			`direct Xen hypervisor access.</li>`
			`<li>proxy_internal: provide read-only Xen access via a proxy, the proxy`
			`code 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 KVM`
			`virtualization engines. It also uses a qemud/ specific daemon which`
			`interracts with the QEmu process to implement libvirt API.</li>`
* docs/*.html: repair the damages made by Amaya on the content of documentation following feedback from Simon Hernandez Daniel 2007-03-22 15:17:31 +00:00			`<li>test: this is a test driver useful for regression tests of the`
			`front-end part of libvirt.</li>`
			`</ul><p>Note that a given driver may only implement a subset of those functions,`
* src/virsh.c: fix virshStrdup to not crash if NULL is passed. Daniel 2007-04-26 10:20:57 +00:00			`for example saving a Xen domain state to disk and restoring it is only possible`
			`though the Xen Daemon, in that case the driver entry points are initialized to`
			NULL.</p><p></p></div></div><div class="linkList2"><div class="llinks2"><h3 class="links2"><span>main menu</span></h3><ul><li><a href="index.html">Home</a></li><li><a href="news.html">Releases</a></li><li><a href="intro.html">Introduction</a></li><li><a href="architecture.html">libvirt architecture</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="format.html">XML Format</a></li><li><a href="python.html">Binding for Python</a></li><li><a href="errors.html">Handling of errors</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="html/index.html">API Menu</a></li><li><a href="examples/index.html">C code examples</a></li><li><a href="ChangeLog.html">Recent Changes</a></li></ul></div><div class="llinks2"><h3 class="links2"><span>related links</span></h3><ul><li><a href="https://www.redhat.com/archives/libvir-list/">Mail archive</a></li><li><a href="https://bugzilla.redhat.com/bugzilla/buglist.cgi?product=Fedora+Core&component=libvirt&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=MODIFIED&short_desc_type=allwordssubstr&short_desc=&long_desc_type=allwordssubstr">Open bugs</a></li><li><a href="http://virt-manager.et.redhat.com/">virt-manager</a></li><li><a href="http://search.cpan.org/~danberr/Sys-Virt-0.1.0/">Perl bindings</a></li><li><a href="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/index.html">Xen project</a></li><li><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="12" value="Search..." /><input name="submit" type="submit" value="Go" /></form></li><li><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-90x34.gif" alt="Made with Libxml2 Logo" /></a></li></ul><p class="credits">Graphics and design by <a href="mail:dfong@redhat.com">Diana Fong</a></p></div></div><div id="bottom"><p class="p1"></p></div></div></body></html>