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