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  <div class="section" id="paravirtualized-kvm-features">
<h1>Paravirtualized KVM features<a class="headerlink" href="#paravirtualized-kvm-features" title="Permalink to this headline">¶</a></h1>
<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
<p>In some cases when implementing hardware interfaces in software is slow, <code class="docutils literal notranslate"><span class="pre">KVM</span></code>
implements its own paravirtualized interfaces.</p>
</div>
<div class="section" id="setup">
<h2>Setup<a class="headerlink" href="#setup" title="Permalink to this headline">¶</a></h2>
<p>Paravirtualized <code class="docutils literal notranslate"><span class="pre">KVM</span></code> features are represented as CPU flags. The following
features are enabled by default for any CPU model when <code class="docutils literal notranslate"><span class="pre">KVM</span></code> acceleration is
enabled:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">kvmclock</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">kvm-nopiodelay</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">kvm-asyncpf</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">kvm-steal-time</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">kvm-pv-eoi</span></code></li>
<li><code class="docutils literal notranslate"><span class="pre">kvmclock-stable-bit</span></code></li>
</ul>
<p><code class="docutils literal notranslate"><span class="pre">kvm-msi-ext-dest-id</span></code> feature is enabled by default in x2apic mode with split
irqchip (e.g. “-machine …,kernel-irqchip=split -cpu …,x2apic”).</p>
<p>Note: when CPU model <code class="docutils literal notranslate"><span class="pre">host</span></code> is used, QEMU passes through all supported
paravirtualized <code class="docutils literal notranslate"><span class="pre">KVM</span></code> features to the guest.</p>
</div>
<div class="section" id="existing-features">
<h2>Existing features<a class="headerlink" href="#existing-features" title="Permalink to this headline">¶</a></h2>
<dl class="docutils">
<dt><code class="docutils literal notranslate"><span class="pre">kvmclock</span></code></dt>
<dd>Expose a <code class="docutils literal notranslate"><span class="pre">KVM</span></code> specific paravirtualized clocksource to the guest. Supported
since Linux v2.6.26.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-nopiodelay</span></code></dt>
<dd>The guest doesn’t need to perform delays on PIO operations. Supported since
Linux v2.6.26.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-mmu</span></code></dt>
<dd>This feature is deprecated.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-asyncpf</span></code></dt>
<dd>Enable asynchronous page fault mechanism. Supported since Linux v2.6.38.
Note: since Linux v5.10 the feature is deprecated and not enabled by <code class="docutils literal notranslate"><span class="pre">KVM</span></code>.
Use <code class="docutils literal notranslate"><span class="pre">kvm-asyncpf-int</span></code> instead.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-steal-time</span></code></dt>
<dd>Enable stolen (when guest vCPU is not running) time accounting. Supported
since Linux v3.1.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-pv-eoi</span></code></dt>
<dd>Enable paravirtualized end-of-interrupt signaling. Supported since Linux
v3.10.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-pv-unhalt</span></code></dt>
<dd>Enable paravirtualized spinlocks support. Supported since Linux v3.12.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-pv-tlb-flush</span></code></dt>
<dd>Enable paravirtualized TLB flush mechanism. Supported since Linux v4.16.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-pv-ipi</span></code></dt>
<dd>Enable paravirtualized IPI mechanism. Supported since Linux v4.19.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-poll-control</span></code></dt>
<dd>Enable host-side polling on HLT control from the guest. Supported since Linux
v5.10.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-pv-sched-yield</span></code></dt>
<dd>Enable paravirtualized sched yield feature. Supported since Linux v5.10.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-asyncpf-int</span></code></dt>
<dd>Enable interrupt based asynchronous page fault mechanism. Supported since Linux
v5.10.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvm-msi-ext-dest-id</span></code></dt>
<dd>Support ‘Extended Destination ID’ for external interrupts. The feature allows
to use up to 32768 CPUs without IRQ remapping (but other limits may apply making
the number of supported vCPUs for a given configuration lower). Supported since
Linux v5.10.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">kvmclock-stable-bit</span></code></dt>
<dd>Tell the guest that guest visible TSC value can be fully trusted for kvmclock
computations and no warps are expected. Supported since Linux v2.6.35.</dd>
</dl>
</div>
<div class="section" id="supplementary-features">
<h2>Supplementary features<a class="headerlink" href="#supplementary-features" title="Permalink to this headline">¶</a></h2>
<dl class="docutils">
<dt><code class="docutils literal notranslate"><span class="pre">kvm-pv-enforce-cpuid</span></code></dt>
<dd>Limit the supported paravirtualized feature set to the exposed features only.
Note, by default, <code class="docutils literal notranslate"><span class="pre">KVM</span></code> allows the guest to use all currently supported
paravirtualized features even when they were not announced in guest visible
CPUIDs. Supported since Linux v5.10.</dd>
</dl>
</div>
<div class="section" id="useful-links">
<h2>Useful links<a class="headerlink" href="#useful-links" title="Permalink to this headline">¶</a></h2>
<p>Please refer to Documentation/virt/kvm in Linux for additional details.</p>
</div>
</div>

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