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    <h1>libvirt internals</h1>

    <p>
      This section provides documents useful to those working on the libvirt
      internals, adding new public APIs, new hypervisor drivers or extending
      the libvirtd daemon code.
    </p>

    <ul>
      <li>Introduction to basic rules and guidelines for
      <a href="hacking.html">hacking</a> on libvirt code</li>
      <li>Guide to adding <a href="api_extension.html">public APIs</a></li>
      <li>Approach for <a href="internals/command.html">spawning commands</a>
      from libvirt driver code</li>
      <li>The libvirt <a href="internals/rpc.html">RPC infrastructure</a></li>
      <li>The <a href="internals/locking.html">Resource Lock Manager</a></li>
    </ul>

    <p>Before adding new code it will be important to get a basic understanding
    of the many elements involved with making any call or change to the libvirt
    code. The architecture <a href="goals.html">goals</a> must be adhered to
    when submitting new code. Understanding the many places that need to be
    touched and the interactions between various subsystems within libvirt
    will directly correlate to the ability to be successful in getting new
    code accepted.</p>
    <p>The following diagram depicts code flow from a client application, in
    this case the libvirt provided <code>virsh</code> command through the
    various layers to elicit a response from some chosen hypervisor.
    </p>

    <p class="image">
      <img alt="virConnectOpen calling sequence"
           src="libvirt-virConnect-example.png"/>
    </p>
    <ul>
        <li>"virsh -c qemu:///system list --all"
        <p>After the virsh code processes the input arguments, it eventually
        will make a call to open the connection using a default set of
        authentication credentials (virConnectAuthDefault). </p></li>
        <li>virConnectOpenAuth()
        <p>Each of the virConnectOpen APIs will first call virInitialize() and
        then revector through the local "do_open():" call.</p>
          <ul>
            <li>virInitialize()
            <p>Calls the registration API for each of the drivers with
            client-side only capabilities and then call the remoteRegister()
            API last. This ensures the virDriverTab[] tries local drivers
            first before using the remote driver.</p></li>
            <li>Loop through virDriverTab[] entries trying to call their
            respective "open" entry point (in our case remoteOpen())</li>
            <li>After successful return from the virDriverTab[] open()
            API, attempt to find and open other drivers (network, interface,
            storage, etc.)</li>
          </ul>
        </li>
        <li>remoteOpen()
        <p>After a couple of URI checks, a call to doRemoteOpen() is made</p>
          <ul>
            <li>Determine network transport and host/port to use from URI
            <p>The transport will be either tls, unix, ssh, libssh2, ext,
            or tcp with the default of tls. Decode the host/port if provided
            or default to "localhost".</p></li>
            <li>virNetClientRegisterAsyncIO()
            <p>Register an I/O callback mechanism to get returned data via
            virNetClientIncomingEvent()</p></li>
            <li>"call(...REMOTE_PROC_OPEN...)"
            <p>Eventually routes into virNetClientProgramCall() which will
            call virNetClientSendWithReply() and eventually uses
            virNetClientIO()to send the message to libvirtd and
            then waits for a response using virNetClientIOEventLoop()</p></li>
            <li>virNetClientIncomingEvent()
            <p>Receives the returned packet and processes through
            virNetClientIOUpdateCallback()</p></li>
          </ul>
        </li>
        <li>libvirtd Daemon
        <p></p>
          <ul>
              <li>Daemon Startup
              <p>The daemon initialization processing will declare itself
              as a daemon via a virNetDaemonNew() call, then creates new server
              using virNetServerNew() and adds that server to the main daemon
              struct with virNetDaemonAddServer() call.  It will then use
              virDriverLoadModule() to find/load all known drivers,
              set up an RPC server program using the <code>remoteProcs[]</code>
              table via a virNetServerProgramNew() call. The table is the
              corollary to the <code>remote_procedure</code> enum list in
              the client. It lists all the functions to be called in
              the same order. Once RPC is set up, networking server sockets
              are opened, the various driver state initialization routines
              are run from the <code>virStateDriverTab[]</code>, the network
              links are enabled, and the daemon waits for work.</p></li>
              <li>RPC
              <p>When a message is received, the <code>remoteProcs[]</code>
              table is referenced for the 'REMOTE_PROC_OPEN' call entry.
              This results in remoteDispatchOpen() being called via the
              virNetServerProgramDispatchCall().</p></li>
              <li>remoteDispatchOpen()
              <p>The API will read the argument passed picking out the
              <code>name</code> of the driver to be opened. The code
              will then call virConnectOpen() or virConnectOpenReadOnly()
              depending on the argument <code>flags</code>.</p></li>
              <li>virConnectOpen() or virConnectOpenReadOnly()
              <p>Just like the client except that upon entry the URI
              is what was passed from the client and will be found
              and opened to process the data.</p>
              <p>The returned structure data is returned via the
              virNetServer interfaces to the remote driver which then
              returns it to the client application.</p></li>
          </ul>
        </li>
    </ul>
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