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+Xen HVM guest support
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+=====================
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+
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+
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+Description
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+-----------
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+
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+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.
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+
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+Setup
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+-----
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+
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+Xen mode is enabled by setting the ``xen-version`` property of the KVM
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+accelerator, for example for Xen 4.10:
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+
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+.. parsed-literal::
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+
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+ |qemu_system| --accel kvm,xen-version=0x4000a
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+
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+Additionally, virtual APIC support can be advertised to the guest through the
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+``xen-vapic`` CPU flag:
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+
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+.. parsed-literal::
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+
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+ |qemu_system| --accel kvm,xen-version=0x4000a --cpu host,+xen_vapic
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+
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+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.
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+
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+The Xen platform device is enabled automatically for a Xen guest. This allows
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+a guest to unplug all emulated devices, in order to use Xen PV block and network
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+drivers instead. Note that until the Xen PV device back ends are enabled to work
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+with Xen mode in QEMU, that is unlikely to cause significant joy. Linux guests
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+can be dissuaded from this by adding 'xen_emul_unplug=never' on their command
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+line, and it can also be noted that AHCI disk controllers are exempt from being
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+unplugged, as are passthrough VFIO PCI devices.
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+
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+Properties
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+----------
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+
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+The following properties exist on the KVM accelerator object:
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+
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+``xen-version``
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+ This property contains the Xen version in ``XENVER_version`` 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.
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+
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+``xen-evtchn-max-pirq``
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+ 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.
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+
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+``xen-gnttab-max-frames``
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+ 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.
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+
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+OS requirements
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+---------------
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+
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+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
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+accelerated Xen PV timers and inter-processor interrupts (IPIs).
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David Woodhouse