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<section id="microvm-virtual-platform-microvm">
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<h1>‘microvm’ virtual platform (<code class="docutils literal notranslate"><span class="pre">microvm</span></code>)<a class="headerlink" href="#microvm-virtual-platform-microvm" title="Link to this heading"></a></h1>
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<p><code class="docutils literal notranslate"><span class="pre">microvm</span></code> is a machine type inspired by <code class="docutils literal notranslate"><span class="pre">Firecracker</span></code> and
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constructed after its machine model.</p>
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<p>It’s a minimalist machine type without <code class="docutils literal notranslate"><span class="pre">PCI</span></code> nor <code class="docutils literal notranslate"><span class="pre">ACPI</span></code> support,
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designed for short-lived guests. microvm also establishes a baseline
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for benchmarking and optimizing both QEMU and guest operating systems,
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since it is optimized for both boot time and footprint.</p>
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<section id="supported-devices">
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<h2>Supported devices<a class="headerlink" href="#supported-devices" title="Link to this heading"></a></h2>
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<p>The microvm machine type supports the following devices:</p>
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<ul class="simple">
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<li><p>ISA bus</p></li>
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<li><p>i8259 PIC (optional)</p></li>
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<li><p>i8254 PIT (optional)</p></li>
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<li><p>MC146818 RTC (optional)</p></li>
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<li><p>One ISA serial port (optional)</p></li>
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<li><p>LAPIC</p></li>
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<li><p>IOAPIC (with kernel-irqchip=split by default)</p></li>
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<li><p>kvmclock (if using KVM)</p></li>
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<li><p>fw_cfg</p></li>
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<li><p>Up to eight virtio-mmio devices (configured by the user)</p></li>
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</ul>
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</section>
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<section id="limitations">
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<h2>Limitations<a class="headerlink" href="#limitations" title="Link to this heading"></a></h2>
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<p>Currently, microvm does <em>not</em> support the following features:</p>
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<ul class="simple">
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<li><p>PCI-only devices.</p></li>
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<li><p>Hotplug of any kind.</p></li>
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<li><p>Live migration across QEMU versions.</p></li>
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</ul>
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</section>
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<section id="using-the-microvm-machine-type">
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<h2>Using the microvm machine type<a class="headerlink" href="#using-the-microvm-machine-type" title="Link to this heading"></a></h2>
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<section id="machine-specific-options">
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<h3>Machine-specific options<a class="headerlink" href="#machine-specific-options" title="Link to this heading"></a></h3>
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<p>It supports the following machine-specific options:</p>
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<ul class="simple">
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<li><p>microvm.x-option-roms=bool (Set off to disable loading option ROMs)</p></li>
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<li><p>microvm.pit=OnOffAuto (Enable i8254 PIT)</p></li>
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<li><p>microvm.isa-serial=bool (Set off to disable the instantiation an ISA serial port)</p></li>
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<li><p>microvm.pic=OnOffAuto (Enable i8259 PIC)</p></li>
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<li><p>microvm.rtc=OnOffAuto (Enable MC146818 RTC)</p></li>
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<li><p>microvm.auto-kernel-cmdline=bool (Set off to disable adding virtio-mmio devices to the kernel cmdline)</p></li>
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</ul>
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</section>
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<section id="boot-options">
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<h3>Boot options<a class="headerlink" href="#boot-options" title="Link to this heading"></a></h3>
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<p>By default, microvm uses <code class="docutils literal notranslate"><span class="pre">qboot</span></code> as its BIOS, to obtain better boot
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times, but it’s also compatible with <code class="docutils literal notranslate"><span class="pre">SeaBIOS</span></code>.</p>
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<p>As no current FW is able to boot from a block device using
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<code class="docutils literal notranslate"><span class="pre">virtio-mmio</span></code> as its transport, a microvm-based VM needs to be run
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using a host-side kernel and, optionally, an initrd image.</p>
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</section>
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<section id="running-a-microvm-based-vm">
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<h3>Running a microvm-based VM<a class="headerlink" href="#running-a-microvm-based-vm" title="Link to this heading"></a></h3>
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<p>By default, microvm aims for maximum compatibility, enabling both
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legacy and non-legacy devices. In this example, a VM is created
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without passing any additional machine-specific option, using the
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legacy <code class="docutils literal notranslate"><span class="pre">ISA</span> <span class="pre">serial</span></code> device as console:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-system-x86_64 -M microvm \
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-enable-kvm -cpu host -m 512m -smp 2 \
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-kernel vmlinux -append "earlyprintk=ttyS0 console=ttyS0 root=/dev/vda" \
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-nodefaults -no-user-config -nographic \
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-serial stdio \
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-drive id=test,file=test.img,format=raw,if=none \
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-device virtio-blk-device,drive=test \
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-netdev tap,id=tap0,script=no,downscript=no \
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-device virtio-net-device,netdev=tap0
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</pre></div>
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</div>
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<p>While the example above works, you might be interested in reducing the
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footprint further by disabling some legacy devices. If you’re using
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<code class="docutils literal notranslate"><span class="pre">KVM</span></code>, you can disable the <code class="docutils literal notranslate"><span class="pre">RTC</span></code>, making the Guest rely on
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<code class="docutils literal notranslate"><span class="pre">kvmclock</span></code> exclusively. Additionally, if your host’s CPUs have the
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<code class="docutils literal notranslate"><span class="pre">TSC_DEADLINE</span></code> feature, you can also disable both the i8259 PIC and
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the i8254 PIT (make sure you’re also emulating a CPU with such feature
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in the guest).</p>
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<p>This is an example of a VM with all optional legacy features
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disabled:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-system-x86_64 \
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-M microvm,x-option-roms=off,pit=off,pic=off,isa-serial=off,rtc=off \
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-enable-kvm -cpu host -m 512m -smp 2 \
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-kernel vmlinux -append "console=hvc0 root=/dev/vda" \
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-nodefaults -no-user-config -nographic \
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||
-chardev stdio,id=virtiocon0 \
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||
-device virtio-serial-device \
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-device virtconsole,chardev=virtiocon0 \
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||
-drive id=test,file=test.img,format=raw,if=none \
|
||
-device virtio-blk-device,drive=test \
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||
-netdev tap,id=tap0,script=no,downscript=no \
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-device virtio-net-device,netdev=tap0
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="triggering-a-guest-initiated-shut-down">
|
||
<h3>Triggering a guest-initiated shut down<a class="headerlink" href="#triggering-a-guest-initiated-shut-down" title="Link to this heading"></a></h3>
|
||
<p>As the microvm machine type includes just a small set of system
|
||
devices, some x86 mechanisms for rebooting or shutting down the
|
||
system, like sending a key sequence to the keyboard or writing to an
|
||
ACPI register, doesn’t have any effect in the VM.</p>
|
||
<p>The recommended way to trigger a guest-initiated shut down is by
|
||
generating a <code class="docutils literal notranslate"><span class="pre">triple-fault</span></code>, which will cause the VM to initiate a
|
||
reboot. Additionally, if the <code class="docutils literal notranslate"><span class="pre">-no-reboot</span></code> argument is present in the
|
||
command line, QEMU will detect this event and terminate its own
|
||
execution gracefully.</p>
|
||
<p>Linux does support this mechanism, but by default will only be used
|
||
after other options have been tried and failed, causing the reboot to
|
||
be delayed by a small number of seconds. It’s possible to instruct it
|
||
to try the triple-fault mechanism first, by adding <code class="docutils literal notranslate"><span class="pre">reboot=t</span></code> to the
|
||
kernel’s command line.</p>
|
||
</section>
|
||
</section>
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</section>
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<p>© Copyright 2025, The QEMU Project Developers.</p>
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<p>This documentation is for QEMU version 8.2.2.</p>
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<p><a href="../../about/license.html">QEMU and this manual are released under the
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