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<li class="toctree-l2 current"><a class="current reference internal" href="#">Virtual CPU hotplug</a><ul>
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<li class="toctree-l3"><a class="reference internal" href="#vcpu-hotplug">vCPU hotplug</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#vcpu-hot-unplug">vCPU hot-unplug</a></li>
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<section id="virtual-cpu-hotplug">
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<h1>Virtual CPU hotplug<a class="headerlink" href="#virtual-cpu-hotplug" title="Link to this heading"></a></h1>
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<p>A complete example of vCPU hotplug (and hot-unplug) using QMP
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<code class="docutils literal notranslate"><span class="pre">device_add</span></code> and <code class="docutils literal notranslate"><span class="pre">device_del</span></code>.</p>
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<section id="vcpu-hotplug">
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<h2>vCPU hotplug<a class="headerlink" href="#vcpu-hotplug" title="Link to this heading"></a></h2>
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<ol class="arabic">
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<li><p>Launch QEMU as follows (note that the “maxcpus” is mandatory to
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allow vCPU hotplug):</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-system-x86_64 -display none -no-user-config -m 2048 \
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-nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \
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-smp 1,maxcpus=2 -cpu IvyBridge-IBRS \
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-qmp unix:/tmp/qmp-sock,server=on,wait=off
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</pre></div>
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</div>
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</li>
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<li><p>Run ‘qmp-shell’ (located in the source tree, under: “scripts/qmp/)
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to connect to the just-launched QEMU:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$> ./qmp-shell -p -v /tmp/qmp-sock
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[...]
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(QEMU)
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</pre></div>
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</div>
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</li>
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<li><p>Find out which CPU types could be plugged, and into which sockets:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">query</span><span class="o">-</span><span class="n">hotpluggable</span><span class="o">-</span><span class="n">cpus</span>
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<span class="p">{</span>
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<span class="s2">"execute"</span><span class="p">:</span> <span class="s2">"query-hotpluggable-cpus"</span><span class="p">,</span>
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<span class="s2">"arguments"</span><span class="p">:</span> <span class="p">{}</span>
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<span class="p">}</span>
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<span class="p">{</span>
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<span class="s2">"return"</span><span class="p">:</span> <span class="p">[</span>
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<span class="p">{</span>
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<span class="s2">"type"</span><span class="p">:</span> <span class="s2">"IvyBridge-IBRS-x86_64-cpu"</span><span class="p">,</span>
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<span class="s2">"vcpus-count"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
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<span class="s2">"props"</span><span class="p">:</span> <span class="p">{</span>
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<span class="s2">"socket-id"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
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<span class="s2">"core-id"</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span>
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<span class="s2">"thread-id"</span><span class="p">:</span> <span class="mi">0</span>
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<span class="p">}</span>
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<span class="p">},</span>
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<span class="p">{</span>
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<span class="s2">"qom-path"</span><span class="p">:</span> <span class="s2">"/machine/unattached/device[0]"</span><span class="p">,</span>
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<span class="s2">"type"</span><span class="p">:</span> <span class="s2">"IvyBridge-IBRS-x86_64-cpu"</span><span class="p">,</span>
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<span class="s2">"vcpus-count"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
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<span class="s2">"props"</span><span class="p">:</span> <span class="p">{</span>
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<span class="s2">"socket-id"</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span>
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<span class="s2">"core-id"</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span>
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<span class="s2">"thread-id"</span><span class="p">:</span> <span class="mi">0</span>
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<span class="p">}</span>
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<span class="p">}</span>
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<span class="p">]</span>
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<span class="p">}</span>
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<span class="p">(</span><span class="n">QEMU</span><span class="p">)</span>
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</pre></div>
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</div>
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</li>
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<li><p>The <code class="docutils literal notranslate"><span class="pre">query-hotpluggable-cpus</span></code> command returns an object for CPUs
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that are present (containing a “qom-path” member) or which may be
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hot-plugged (no “qom-path” member). From its output in step (3), we
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can see that <code class="docutils literal notranslate"><span class="pre">IvyBridge-IBRS-x86_64-cpu</span></code> is present in socket 0,
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while hot-plugging a CPU into socket 1 requires passing the listed
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properties to QMP <code class="docutils literal notranslate"><span class="pre">device_add</span></code>:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">device_add</span> <span class="nb">id</span><span class="o">=</span><span class="n">cpu</span><span class="o">-</span><span class="mi">2</span> <span class="n">driver</span><span class="o">=</span><span class="n">IvyBridge</span><span class="o">-</span><span class="n">IBRS</span><span class="o">-</span><span class="n">x86_64</span><span class="o">-</span><span class="n">cpu</span> <span class="n">socket</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="mi">1</span> <span class="n">core</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="mi">0</span> <span class="n">thread</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="mi">0</span>
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<span class="p">{</span>
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<span class="s2">"execute"</span><span class="p">:</span> <span class="s2">"device_add"</span><span class="p">,</span>
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<span class="s2">"arguments"</span><span class="p">:</span> <span class="p">{</span>
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<span class="s2">"socket-id"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
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<span class="s2">"driver"</span><span class="p">:</span> <span class="s2">"IvyBridge-IBRS-x86_64-cpu"</span><span class="p">,</span>
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<span class="s2">"id"</span><span class="p">:</span> <span class="s2">"cpu-2"</span><span class="p">,</span>
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<span class="s2">"core-id"</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span>
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<span class="s2">"thread-id"</span><span class="p">:</span> <span class="mi">0</span>
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<span class="p">}</span>
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<span class="p">}</span>
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<span class="p">{</span>
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<span class="s2">"return"</span><span class="p">:</span> <span class="p">{}</span>
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<span class="p">}</span>
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<span class="p">(</span><span class="n">QEMU</span><span class="p">)</span>
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</pre></div>
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</div>
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</li>
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<li><p>Optionally, run QMP <code class="docutils literal notranslate"><span class="pre">query-cpus-fast</span></code> for some details about the
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vCPUs:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">query</span><span class="o">-</span><span class="n">cpus</span><span class="o">-</span><span class="n">fast</span>
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<span class="p">{</span>
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<span class="s2">"execute"</span><span class="p">:</span> <span class="s2">"query-cpus-fast"</span><span class="p">,</span>
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<span class="s2">"arguments"</span><span class="p">:</span> <span class="p">{}</span>
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||
<span class="p">}</span>
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<span class="p">{</span>
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<span class="s2">"return"</span><span class="p">:</span> <span class="p">[</span>
|
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<span class="p">{</span>
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<span class="s2">"qom-path"</span><span class="p">:</span> <span class="s2">"/machine/unattached/device[0]"</span><span class="p">,</span>
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<span class="s2">"target"</span><span class="p">:</span> <span class="s2">"x86_64"</span><span class="p">,</span>
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<span class="s2">"thread-id"</span><span class="p">:</span> <span class="mi">11534</span><span class="p">,</span>
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||
<span class="s2">"cpu-index"</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span>
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<span class="s2">"props"</span><span class="p">:</span> <span class="p">{</span>
|
||
<span class="s2">"socket-id"</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span>
|
||
<span class="s2">"core-id"</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span>
|
||
<span class="s2">"thread-id"</span><span class="p">:</span> <span class="mi">0</span>
|
||
<span class="p">},</span>
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||
<span class="s2">"arch"</span><span class="p">:</span> <span class="s2">"x86"</span>
|
||
<span class="p">},</span>
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||
<span class="p">{</span>
|
||
<span class="s2">"qom-path"</span><span class="p">:</span> <span class="s2">"/machine/peripheral/cpu-2"</span><span class="p">,</span>
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||
<span class="s2">"target"</span><span class="p">:</span> <span class="s2">"x86_64"</span><span class="p">,</span>
|
||
<span class="s2">"thread-id"</span><span class="p">:</span> <span class="mi">12106</span><span class="p">,</span>
|
||
<span class="s2">"cpu-index"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
|
||
<span class="s2">"props"</span><span class="p">:</span> <span class="p">{</span>
|
||
<span class="s2">"socket-id"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
|
||
<span class="s2">"core-id"</span><span class="p">:</span> <span class="mi">0</span><span class="p">,</span>
|
||
<span class="s2">"thread-id"</span><span class="p">:</span> <span class="mi">0</span>
|
||
<span class="p">},</span>
|
||
<span class="s2">"arch"</span><span class="p">:</span> <span class="s2">"x86"</span>
|
||
<span class="p">}</span>
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||
<span class="p">]</span>
|
||
<span class="p">}</span>
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||
<span class="p">(</span><span class="n">QEMU</span><span class="p">)</span>
|
||
</pre></div>
|
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</div>
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</li>
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</ol>
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</section>
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<section id="vcpu-hot-unplug">
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<h2>vCPU hot-unplug<a class="headerlink" href="#vcpu-hot-unplug" title="Link to this heading"></a></h2>
|
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<p>From the ‘qmp-shell’, invoke the QMP <code class="docutils literal notranslate"><span class="pre">device_del</span></code> command:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">device_del</span> <span class="nb">id</span><span class="o">=</span><span class="n">cpu</span><span class="o">-</span><span class="mi">2</span>
|
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<span class="p">{</span>
|
||
<span class="s2">"execute"</span><span class="p">:</span> <span class="s2">"device_del"</span><span class="p">,</span>
|
||
<span class="s2">"arguments"</span><span class="p">:</span> <span class="p">{</span>
|
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<span class="s2">"id"</span><span class="p">:</span> <span class="s2">"cpu-2"</span>
|
||
<span class="p">}</span>
|
||
<span class="p">}</span>
|
||
<span class="p">{</span>
|
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<span class="s2">"return"</span><span class="p">:</span> <span class="p">{}</span>
|
||
<span class="p">}</span>
|
||
<span class="p">(</span><span class="n">QEMU</span><span class="p">)</span>
|
||
</pre></div>
|
||
</div>
|
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<div class="admonition note">
|
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<p class="admonition-title">Note</p>
|
||
<p>vCPU hot-unplug requires guest cooperation; so the <code class="docutils literal notranslate"><span class="pre">device_del</span></code>
|
||
command above does not guarantee vCPU removal – it’s a “request to
|
||
unplug”. At this point, the guest will get a System Control
|
||
Interrupt (SCI) and calls the ACPI handler for the affected vCPU
|
||
device. Then the guest kernel will bring the vCPU offline and tell
|
||
QEMU to unplug it.</p>
|
||
</div>
|
||
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|
||
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||
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