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<li class="toctree-l2 current"><a class="current reference internal" href="#">Virtual CPU hotplug</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#vcpu-hotplug">vCPU hotplug</a></li>
<li class="toctree-l3"><a class="reference internal" href="#vcpu-hot-unplug">vCPU hot-unplug</a></li>
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 <div class="section" id="virtual-cpu-hotplug">
<h1>Virtual CPU hotplug<a class="headerlink" href="#virtual-cpu-hotplug" title="Permalink to this headline">¶</a></h1>
A complete example of vCPU hotplug (and hot-unplug) using QMP
<code class="docutils literal notranslate">device_add</code> and <code class="docutils literal notranslate">device_del</code>.
<div class="section" id="vcpu-hotplug">
<h2>vCPU hotplug<a class="headerlink" href="#vcpu-hotplug" title="Permalink to this headline">¶</a></h2>
<ol class="arabic">
<li>Launch QEMU as follows (note that the “maxcpus” is mandatory to
allow vCPU hotplug):
<div class="highlight-default notranslate"><div class="highlight"><pre>$ qemu-system-x86_64 -display none -no-user-config -m 2048 \
 -nodefaults -monitor stdio -machine pc,accel=kvm,usb=off \
 -smp 1,maxcpus=2 -cpu IvyBridge-IBRS \
 -qmp unix:/tmp/qmp-sock,server=on,wait=off
</pre></div>
</div>
</li>
<li>Run ‘qmp-shell’ (located in the source tree, under: “scripts/qmp/)
to connect to the just-launched QEMU:
<div class="highlight-default notranslate"><div class="highlight"><pre>$&gt; ./qmp-shell -p -v /tmp/qmp-sock
[...]
(QEMU)
</pre></div>
</div>
</li>
<li>Find out which CPU types could be plugged, and into which sockets:
<div class="highlight-default notranslate"><div class="highlight"><pre>(QEMU) query-hotpluggable-cpus
{
 &quot;execute&quot;: &quot;query-hotpluggable-cpus&quot;,
 &quot;arguments&quot;: {}
}
{
 &quot;return&quot;: [
 {
 &quot;type&quot;: &quot;IvyBridge-IBRS-x86_64-cpu&quot;,
 &quot;vcpus-count&quot;: 1,
 &quot;props&quot;: {
 &quot;socket-id&quot;: 1,
 &quot;core-id&quot;: 0,
 &quot;thread-id&quot;: 0
 }
 },
 {
 &quot;qom-path&quot;: &quot;/machine/unattached/device[0]&quot;,
 &quot;type&quot;: &quot;IvyBridge-IBRS-x86_64-cpu&quot;,
 &quot;vcpus-count&quot;: 1,
 &quot;props&quot;: {
 &quot;socket-id&quot;: 0,
 &quot;core-id&quot;: 0,
 &quot;thread-id&quot;: 0
 }
 }
 ]
}
(QEMU)
</pre></div>
</div>
</li>
<li>The <code class="docutils literal notranslate">query-hotpluggable-cpus</code> command returns an object for CPUs
that are present (containing a “qom-path” member) or which may be
hot-plugged (no “qom-path” member). From its output in step (3), we
can see that <code class="docutils literal notranslate">IvyBridge-IBRS-x86_64-cpu</code> is present in socket 0,
while hot-plugging a CPU into socket 1 requires passing the listed
properties to QMP <code class="docutils literal notranslate">device_add</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>(QEMU) device_add id=cpu-2 driver=IvyBridge-IBRS-x86_64-cpu socket-id=1 core-id=0 thread-id=0
{
 &quot;execute&quot;: &quot;device_add&quot;,
 &quot;arguments&quot;: {
 &quot;socket-id&quot;: 1,
 &quot;driver&quot;: &quot;IvyBridge-IBRS-x86_64-cpu&quot;,
 &quot;id&quot;: &quot;cpu-2&quot;,
 &quot;core-id&quot;: 0,
 &quot;thread-id&quot;: 0
 }
}
{
 &quot;return&quot;: {}
}
(QEMU)
</pre></div>
</div>
</li>
<li>Optionally, run QMP <code class="docutils literal notranslate">query-cpus-fast</code> for some details about the
vCPUs:
<div class="highlight-default notranslate"><div class="highlight"><pre>(QEMU) query-cpus-fast
{
 &quot;execute&quot;: &quot;query-cpus-fast&quot;,
 &quot;arguments&quot;: {}
}
{
 &quot;return&quot;: [
 {
 &quot;qom-path&quot;: &quot;/machine/unattached/device[0]&quot;,
 &quot;target&quot;: &quot;x86_64&quot;,
 &quot;thread-id&quot;: 11534,
 &quot;cpu-index&quot;: 0,
 &quot;props&quot;: {
 &quot;socket-id&quot;: 0,
 &quot;core-id&quot;: 0,
 &quot;thread-id&quot;: 0
 },
 &quot;arch&quot;: &quot;x86&quot;
 },
 {
 &quot;qom-path&quot;: &quot;/machine/peripheral/cpu-2&quot;,
 &quot;target&quot;: &quot;x86_64&quot;,
 &quot;thread-id&quot;: 12106,
 &quot;cpu-index&quot;: 1,
 &quot;props&quot;: {
 &quot;socket-id&quot;: 1,
 &quot;core-id&quot;: 0,
 &quot;thread-id&quot;: 0
 },
 &quot;arch&quot;: &quot;x86&quot;
 }
 ]
}
(QEMU)
</pre></div>
</div>
</li>
</ol>
</div>
<div class="section" id="vcpu-hot-unplug">
<h2>vCPU hot-unplug<a class="headerlink" href="#vcpu-hot-unplug" title="Permalink to this headline">¶</a></h2>
From the ‘qmp-shell’, invoke the QMP <code class="docutils literal notranslate">device_del</code> command:
<div class="highlight-default notranslate"><div class="highlight"><pre>(QEMU) device_del id=cpu-2
{
 &quot;execute&quot;: &quot;device_del&quot;,
 &quot;arguments&quot;: {
 &quot;id&quot;: &quot;cpu-2&quot;
 }
}
{
 &quot;return&quot;: {}
}
(QEMU)
</pre></div>
</div>
<div class="admonition note">
Note
vCPU hot-unplug requires guest cooperation; so the <code class="docutils literal notranslate">device_del</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.
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