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<section id="xen-hvm-guest-support">
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<h1>Xen HVM guest support<a class="headerlink" href="#xen-hvm-guest-support" 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>KVM has support for hosting Xen guests, intercepting Xen hypercalls and event
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channel (Xen PV interrupt) delivery. This allows guests which expect to be
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run under Xen to be hosted in QEMU under Linux/KVM instead.</p>
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<p>Using the split irqchip is mandatory for Xen support.</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>Xen mode is enabled by setting the <code class="docutils literal notranslate"><span class="pre">xen-version</span></code> property of the KVM
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accelerator, for example for Xen 4.17:</p>
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<pre class="literal-block">qemu-system-x86_64 --accel kvm,xen-version=0x40011,kernel-irqchip=split</pre>
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<p>Additionally, virtual APIC support can be advertised to the guest through the
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<code class="docutils literal notranslate"><span class="pre">xen-vapic</span></code> CPU flag:</p>
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<pre class="literal-block">qemu-system-x86_64 --accel kvm,xen-version=0x40011,kernel-irqchip=split --cpu host,+xen-vapic</pre>
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<p>When Xen support is enabled, QEMU changes hypervisor identification (CPUID
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0x40000000..0x4000000A) to Xen. The KVM identification and features are not
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advertised to a Xen guest. If Hyper-V is also enabled, the Xen identification
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moves to leaves 0x40000100..0x4000010A.</p>
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</section>
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<section id="properties">
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<h2>Properties<a class="headerlink" href="#properties" title="Link to this heading"></a></h2>
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<p>The following properties exist on the KVM accelerator object:</p>
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<dl class="simple">
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<dt><code class="docutils literal notranslate"><span class="pre">xen-version</span></code></dt><dd><p>This property contains the Xen version in <code class="docutils literal notranslate"><span class="pre">XENVER_version</span></code> form, with the
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major version in the top 16 bits and the minor version in the low 16 bits.
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Setting this property enables the Xen guest support. If Xen version 4.5 or
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greater is specified, the HVM leaf in Xen CPUID is populated. Xen version
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4.6 enables the vCPU ID in CPUID, and version 4.17 advertises vCPU upcall
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vector support to the guest.</p>
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</dd>
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<dt><code class="docutils literal notranslate"><span class="pre">xen-evtchn-max-pirq</span></code></dt><dd><p>Xen PIRQs represent an emulated physical interrupt, either GSI or MSI, which
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can be routed to an event channel instead of to the emulated I/O or local
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APIC. By default, QEMU permits only 256 PIRQs because this allows maximum
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compatibility with 32-bit MSI where the higher bits of the PIRQ# would need
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to be in the upper 64 bits of the MSI message. For guests with large numbers
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of PCI devices (and none which are limited to 32-bit addressing) it may be
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desirable to increase this value.</p>
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</dd>
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<dt><code class="docutils literal notranslate"><span class="pre">xen-gnttab-max-frames</span></code></dt><dd><p>Xen grant tables are the means by which a Xen guest grants access to its
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memory for PV back ends (disk, network, etc.). Since QEMU only supports v1
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grant tables which are 8 bytes in size, each page (each frame) of the grant
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table can reference 512 pages of guest memory. The default number of frames
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is 64, allowing for 32768 pages of guest memory to be accessed by PV backends
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through simultaneous grants. For guests with large numbers of PV devices and
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high throughput, it may be desirable to increase this value.</p>
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</dd>
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</dl>
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</section>
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<section id="xen-paravirtual-devices">
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<h2>Xen paravirtual devices<a class="headerlink" href="#xen-paravirtual-devices" title="Link to this heading"></a></h2>
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<p>The Xen PCI platform device is enabled automatically for a Xen guest. This
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allows a guest to unplug all emulated devices, in order to use paravirtual
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block and network drivers instead.</p>
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<p>Those paravirtual Xen block, network (and console) devices can be created
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through the command line, and/or hot-plugged.</p>
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<p>To provide a Xen console device, define a character device and then a device
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of type <code class="docutils literal notranslate"><span class="pre">xen-console</span></code> to connect to it. For the Xen console equivalent of
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the handy <code class="docutils literal notranslate"><span class="pre">-serial</span> <span class="pre">mon:stdio</span></code> option, for example:</p>
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<pre class="literal-block">-chardev stdio,mux=on,id=char0,signal=off -mon char0 \
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-device xen-console,chardev=char0</pre>
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<p>The Xen network device is <code class="docutils literal notranslate"><span class="pre">xen-net-device</span></code>, which becomes the default NIC
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model for emulated Xen guests, meaning that just the default NIC provided
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by QEMU should automatically work and present a Xen network device to the
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guest.</p>
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<p>Disks can be configured with ‘<code class="docutils literal notranslate"><span class="pre">-drive</span> <span class="pre">file=${GUEST_IMAGE},if=xen</span></code>’ and will
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appear to the guest as <code class="docutils literal notranslate"><span class="pre">xvda</span></code> onwards.</p>
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<p>Under Xen, the boot disk is typically available both via IDE emulation, and
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as a PV block device. Guest bootloaders typically use IDE to load the guest
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kernel, which then unplugs the IDE and continues with the Xen PV block device.</p>
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<p>This configuration can be achieved as follows:</p>
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<pre class="literal-block">qemu-system-x86_64 --accel kvm,xen-version=0x40011,kernel-irqchip=split \
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-drive file=${GUEST_IMAGE},if=xen \
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-drive file=${GUEST_IMAGE},file.locking=off,if=ide</pre>
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<p>VirtIO devices can also be used; Linux guests may need to be dissuaded from
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umplugging them by adding ‘<code class="docutils literal notranslate"><span class="pre">xen_emul_unplug=never</span></code>’ on their command line.</p>
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</section>
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<section id="booting-xen-pv-guests">
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<h2>Booting Xen PV guests<a class="headerlink" href="#booting-xen-pv-guests" title="Link to this heading"></a></h2>
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<p>Booting PV guest kernels is possible by using the Xen PV shim (a version of Xen
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itself, designed to run inside a Xen HVM guest and provide memory management
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services for one guest alone).</p>
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<p>The Xen binary is provided as the <code class="docutils literal notranslate"><span class="pre">-kernel</span></code> and the guest kernel itself (or
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PV Grub image) as the <code class="docutils literal notranslate"><span class="pre">-initrd</span></code> image, which actually just means the first
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multiboot “module”. For example:</p>
|
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<pre class="literal-block">qemu-system-x86_64 --accel kvm,xen-version=0x40011,kernel-irqchip=split \
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-chardev stdio,id=char0 -device xen-console,chardev=char0 \
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-display none -m 1G -kernel xen -initrd bzImage \
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-append "pv-shim console=xen,pv -- console=hvc0 root=/dev/xvda1" \
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-drive file=${GUEST_IMAGE},if=xen</pre>
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<p>The Xen image must be built with the <code class="docutils literal notranslate"><span class="pre">CONFIG_XEN_GUEST</span></code> and <code class="docutils literal notranslate"><span class="pre">CONFIG_PV_SHIM</span></code>
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options, and as of Xen 4.17, Xen’s PV shim mode does not support using a serial
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port; it must have a Xen console or it will panic.</p>
|
||
<p>The example above provides the guest kernel command line after a separator
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(” <code class="docutils literal notranslate"><span class="pre">--</span></code> “) on the Xen command line, and does not provide the guest kernel
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||
with an actual initramfs, which would need to listed as a second multiboot
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||
module. For more complicated alternatives, see the command line
|
||
documentation for the <code class="docutils literal notranslate"><span class="pre">-initrd</span></code> option.</p>
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</section>
|
||
<section id="host-os-requirements">
|
||
<h2>Host OS requirements<a class="headerlink" href="#host-os-requirements" title="Link to this heading"></a></h2>
|
||
<p>The minimal Xen support in the KVM accelerator requires the host to be running
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Linux v5.12 or newer. Later versions add optimisations: Linux v5.17 added
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||
acceleration of interrupt delivery via the Xen PIRQ mechanism, and Linux v5.19
|
||
accelerated Xen PV timers and inter-processor interrupts (IPIs).</p>
|
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</section>
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