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<li class="breadcrumb-item active">The virtual channel subsystem</li>
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<section id="the-virtual-channel-subsystem">
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<h1>The virtual channel subsystem<a class="headerlink" href="#the-virtual-channel-subsystem" title="Link to this heading"></a></h1>
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<p>QEMU implements a virtual channel subsystem with subchannels, (mostly
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functionless) channel paths, and channel devices (virtio-ccw, 3270, and
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devices passed via vfio-ccw). It supports multiple subchannel sets (MSS) and
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multiple channel subsystems extended (MCSS-E).</p>
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<p>All channel devices support the <code class="docutils literal notranslate"><span class="pre">devno</span></code> property, which takes a parameter
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in the form <code class="docutils literal notranslate"><span class="pre"><cssid>.<ssid>.<device</span> <span class="pre">number></span></code>.</p>
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<p>The default channel subsystem image id (<code class="docutils literal notranslate"><span class="pre"><cssid></span></code>) is <code class="docutils literal notranslate"><span class="pre">0xfe</span></code>. Devices in
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there will show up in channel subsystem image <code class="docutils literal notranslate"><span class="pre">0</span></code> to guests that do not
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enable MCSS-E. Note that devices with a different cssid will not be visible
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if the guest OS does not enable MCSS-E (which is true for all supported guest
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operating systems today).</p>
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<p>Supported values for the subchannel set id (<code class="docutils literal notranslate"><span class="pre"><ssid></span></code>) range from <code class="docutils literal notranslate"><span class="pre">0-3</span></code>.
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Devices with a ssid that is not <code class="docutils literal notranslate"><span class="pre">0</span></code> will not be visible if the guest OS
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does not enable MSS (any Linux version that supports virtio also enables MSS).
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Any device may be put into any subchannel set, there is no restriction by
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device type.</p>
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<p>The device number can range from <code class="docutils literal notranslate"><span class="pre">0-0xffff</span></code>.</p>
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<p>If the <code class="docutils literal notranslate"><span class="pre">devno</span></code> property is not specified for a device, QEMU will choose the
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next free device number in subchannel set 0, skipping to the next subchannel
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set if no more device numbers are free.</p>
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<p>QEMU places a device at the first free subchannel in the specified subchannel
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set. If a device is hotunplugged and later replugged, it may appear at a
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different subchannel. (This is similar to how z/VM works.)</p>
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<section id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Link to this heading"></a></h2>
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<ul>
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<li><p>a virtio-net device, cssid/ssid/devno automatically assigned:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">device</span> <span class="n">virtio</span><span class="o">-</span><span class="n">net</span><span class="o">-</span><span class="n">ccw</span>
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</pre></div>
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</div>
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<p>In a Linux guest (without default devices and no other devices specified
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prior to this one), this will show up as <code class="docutils literal notranslate"><span class="pre">0.0.0000</span></code> under subchannel
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<code class="docutils literal notranslate"><span class="pre">0.0.0000</span></code>.</p>
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<p>The auto-assigned-properties in QEMU (as seen via e.g. <code class="docutils literal notranslate"><span class="pre">info</span> <span class="pre">qtree</span></code>)
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would be <code class="docutils literal notranslate"><span class="pre">dev_id</span> <span class="pre">=</span> <span class="pre">"fe.0.0000"</span></code> and <code class="docutils literal notranslate"><span class="pre">subch_id</span> <span class="pre">=</span> <span class="pre">"fe.0.0000"</span></code>.</p>
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</li>
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<li><p>a virtio-rng device in subchannel set <code class="docutils literal notranslate"><span class="pre">0</span></code>:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">device</span> <span class="n">virtio</span><span class="o">-</span><span class="n">rng</span><span class="o">-</span><span class="n">ccw</span><span class="p">,</span><span class="n">devno</span><span class="o">=</span><span class="n">fe</span><span class="mf">.0.0042</span>
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</pre></div>
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</div>
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<p>If added to the same Linux guest as above, it would show up as <code class="docutils literal notranslate"><span class="pre">0.0.0042</span></code>
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under subchannel <code class="docutils literal notranslate"><span class="pre">0.0.0001</span></code>.</p>
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<p>The properties for the device would be <code class="docutils literal notranslate"><span class="pre">dev_id</span> <span class="pre">=</span> <span class="pre">"fe.0.0042"</span></code> and
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<code class="docutils literal notranslate"><span class="pre">subch_id</span> <span class="pre">=</span> <span class="pre">"fe.0.0001"</span></code>.</p>
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</li>
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<li><p>a virtio-gpu device in subchannel set <code class="docutils literal notranslate"><span class="pre">2</span></code>:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">device</span> <span class="n">virtio</span><span class="o">-</span><span class="n">gpu</span><span class="o">-</span><span class="n">ccw</span><span class="p">,</span><span class="n">devno</span><span class="o">=</span><span class="n">fe</span><span class="mf">.2.1111</span>
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</pre></div>
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</div>
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<p>If added to the same Linux guest as above, it would show up as <code class="docutils literal notranslate"><span class="pre">0.2.1111</span></code>
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under subchannel <code class="docutils literal notranslate"><span class="pre">0.2.0000</span></code>.</p>
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<p>The properties for the device would be <code class="docutils literal notranslate"><span class="pre">dev_id</span> <span class="pre">=</span> <span class="pre">"fe.2.1111"</span></code> and
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<code class="docutils literal notranslate"><span class="pre">subch_id</span> <span class="pre">=</span> <span class="pre">"fe.2.0000"</span></code>.</p>
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</li>
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<li><p>a virtio-mouse device in a non-standard channel subsystem image:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">device</span> <span class="n">virtio</span><span class="o">-</span><span class="n">mouse</span><span class="o">-</span><span class="n">ccw</span><span class="p">,</span><span class="n">devno</span><span class="o">=</span><span class="mf">2.0.2222</span>
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</pre></div>
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</div>
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<p>This would not show up in a standard Linux guest.</p>
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<p>The properties for the device would be <code class="docutils literal notranslate"><span class="pre">dev_id</span> <span class="pre">=</span> <span class="pre">"2.0.2222"</span></code> and
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<code class="docutils literal notranslate"><span class="pre">subch_id</span> <span class="pre">=</span> <span class="pre">"2.0.0000"</span></code>.</p>
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</li>
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<li><p>a virtio-keyboard device in another non-standard channel subsystem image:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">device</span> <span class="n">virtio</span><span class="o">-</span><span class="n">keyboard</span><span class="o">-</span><span class="n">ccw</span><span class="p">,</span><span class="n">devno</span><span class="o">=</span><span class="mf">0.0.1234</span>
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</pre></div>
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</div>
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<p>This would not show up in a standard Linux guest, either, as <code class="docutils literal notranslate"><span class="pre">0</span></code> is not
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the standard channel subsystem image id.</p>
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<p>The properties for the device would be <code class="docutils literal notranslate"><span class="pre">dev_id</span> <span class="pre">=</span> <span class="pre">"0.0.1234"</span></code> and
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<code class="docutils literal notranslate"><span class="pre">subch_id</span> <span class="pre">=</span> <span class="pre">"0.0.0000"</span></code>.</p>
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</li>
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