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+.\" Copyright (c) 2010 Robert N. M. Watson
+.\" All rights reserved.
+.\"
+.\" This software was developed by SRI International and the University of
+.\" Cambridge Computer Laboratory under DARPA/AFRL contract FA8750-10-C-0237
+.\" ("CTSRD"), as part of the DARPA CRASH research program.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.Dd January 8, 2024
+.Dt XEN 4
+.Os
+.Sh NAME
+.Nm xen
+.Nd Xen Hypervisor Support
+.Sh SYNOPSIS
+FreeBSD supports running both as a Xen guest and host on amd64 hardware.
+Guest support is limited to HVM and PVH modes, while host support is limited to
+PVH mode only.
+.Pp
+Xen support is built by default in the i386 and amd64 GENERIC kernels; note
+however that host mode is only available on amd64.
+.Sh DESCRIPTION
+The Xen Hypervisor allows multiple virtual machines to be run on a single
+computer system.
+When first released, Xen required that i386 kernels be compiled
+"para-virtualized" as the x86 instruction set was not fully virtualizable.
+Primarily, para-virtualization modifies the virtual memory system to use
+hypervisor calls (hypercalls) rather than direct hardware instructions to
+modify the TLB, although para-virtualized device drivers were also required
+to access resources such as virtual network interfaces and disk devices.
+.Pp
+With later instruction set extensions from AMD and Intel to support fully
+virtualizable instructions, unmodified virtual memory systems can also be
+supported; this is referred to as hardware-assisted virtualization (HVM and PVH).
+HVM configurations may either rely on transparently emulated hardware
+peripherals, or para-virtualized drivers, which are aware of virtualization,
+and hence able to optimize certain behaviors to improve performance or
+semantics.
+PVH configurations rely on para-virtualized drivers exclusively for IO.
+.Pp
+.Fx
+Para-virtualized device drivers are required in order to support certain
+functionality, such as processing management requests, returning idle
+physical memory pages to the hypervisor, etc.
+.Ss Xen device drivers
+These para-virtualized drivers are supported:
+.Bl -hang -offset indent -width blkfront
+.It Nm balloon
+Allow physical memory pages to be returned to the hypervisor as a result of
+manual tuning or automatic policy.
+.It Nm blkback
+Exports local block devices or files to other Xen domains where they can
+then be imported via
+.Nm blkfront .
+.It Nm blkfront
+Import block devices from other Xen domains as local block devices, to be
+used for file systems, swap, etc.
+.It Nm console
+Export the low-level system console via the Xen console service.
+.It Nm control
+Process management operations from Domain 0, including power off, reboot,
+suspend, crash, and halt requests.
+.It Nm evtchn
+Expose Xen events via the
+.Pa /dev/xen/evtchn
+special device.
+.It Nm gntdev
+Allow access to the grant table interface via the
+.Pa /dev/xen/gntdev
+special device.
+.It Nm netback
+Export local network interfaces to other Xen domains where they can be
+imported via
+.Nm netfront .
+.It Nm netfront
+Import network interfaces from other Xen domains as local network interfaces,
+which may be used for IPv4, IPv6, etc.
+.It Nm privcmd
+Allow issuing hypercalls via the
+.Pa /dev/xen/privcmd
+special device.
+.It Nm timer
+Implementation of a one-shot high resolution per-CPU timer using the hypercall
+interface.
+.It Nm acpi cpu
+When running as a host forwards power management related information from ACPI
+to the hypervisor for better performance management.
+.It Nm xenpci
+Represents the Xen PCI device, an emulated PCI device that is exposed to
+HVM domains.
+This device allows detection of the Xen hypervisor, and provides interrupt
+and shared memory services required to interact with the hypervisor.
+.It Nm xenstore
+Information storage space shared between domains.
+.El
+.Sh HISTORY
+Support for
+.Nm
+first appeared in
+.Fx 8.1 .
+Support for host mode was added in 11.0 .
+.Sh AUTHORS
+.An -nosplit
+.Fx
+support for Xen was first added by
+.An Kip Macy Aq Mt kmacy@FreeBSD.org
+and
+.An Doug Rabson Aq Mt dfr@FreeBSD.org .
+Further refinements were made by
+.An Justin Gibbs Aq Mt gibbs@FreeBSD.org ,
+.An Adrian Chadd Aq Mt adrian@FreeBSD.org ,
+.An Colin Percival Aq Mt cperciva@FreeBSD.org ,
+and
+.An Roger Pau Monné Aq Mt royger@FreeBSD.org .
+This manual page was written by
+.An Robert Watson Aq Mt rwatson@FreeBSD.org ,
+and
+.An Roger Pau Monné Aq Mt royger@FreeBSD.org .