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<section id="vhost-user-back-ends">
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<span id="vhost-user"></span><h1>vhost-user back ends<a class="headerlink" href="#vhost-user-back-ends" title="Link to this heading"></a></h1>
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<p>vhost-user back ends are way to service the request of VirtIO devices
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outside of QEMU itself. To do this there are a number of things
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required.</p>
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<section id="vhost-user-device">
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<h2>vhost-user device<a class="headerlink" href="#vhost-user-device" title="Link to this heading"></a></h2>
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<p>These are simple stub devices that ensure the VirtIO device is visible
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to the guest. The code is mostly boilerplate although each device has
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a <code class="docutils literal notranslate"><span class="pre">chardev</span></code> option which specifies the ID of the <code class="docutils literal notranslate"><span class="pre">--chardev</span></code>
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device that connects via a socket to the vhost-user <em>daemon</em>.</p>
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</section>
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<section id="vhost-user-daemon">
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<h2>vhost-user daemon<a class="headerlink" href="#vhost-user-daemon" title="Link to this heading"></a></h2>
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<p>This is a separate process that is connected to by QEMU via a socket
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following the <a class="reference internal" href="../../interop/vhost-user.html#vhost-user-proto"><span class="std std-ref">Vhost-user Protocol</span></a>. There are a number of daemons
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that can be built when enabled by the project although any daemon that
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meets the specification for a given device can be used.</p>
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</section>
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<section id="shared-memory-object">
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<h2>Shared memory object<a class="headerlink" href="#shared-memory-object" title="Link to this heading"></a></h2>
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<p>In order for the daemon to access the VirtIO queues to process the
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requests it needs access to the guest’s address space. This is
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achieved via the <code class="docutils literal notranslate"><span class="pre">memory-backend-file</span></code> or <code class="docutils literal notranslate"><span class="pre">memory-backend-memfd</span></code>
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objects. A reference to a file-descriptor which can access this object
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will be passed via the socket as part of the protocol negotiation.</p>
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<p>Currently the shared memory object needs to match the size of the main
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system memory as defined by the <code class="docutils literal notranslate"><span class="pre">-m</span></code> argument.</p>
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</section>
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<section id="example">
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<h2>Example<a class="headerlink" href="#example" title="Link to this heading"></a></h2>
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<p>First start your daemon.</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ virtio-foo --socket-path=/var/run/foo.sock $OTHER_ARGS
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</pre></div>
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</div>
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<p>Then you start your QEMU instance specifying the device, chardev and
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memory objects.</p>
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<pre class="literal-block">$ qemu-system-x86_64 \
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-m 4096 \
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-chardev socket,id=ba1,path=/var/run/foo.sock \
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-device vhost-user-foo,chardev=ba1,$OTHER_ARGS \
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-object memory-backend-memfd,id=mem,size=4G,share=on \
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-numa node,memdev=mem \
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...</pre>
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</section>
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<p>This documentation is for QEMU version 8.2.2.</p>
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