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.Dd April 30, 2015
.Dt XEN 4
.Os
.Sh NAME
.Nm xen
.Nd Xen Hypervisor Guest (DomU) Support
.Sh SYNOPSIS
To compile hardware-assisted virtualization (HVM) Xen guest support with
para-virtualized drivers into an amd64 or i386 kernel,
place the following lines in your kernel configuration file:
.Bd -ragged -offset indent
.Cd "options XENHVM"
.Cd "device xenpci"
.Ed
.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).
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.
.Pp
.Fx
supports hardware-assisted virtualization (HVM) on both i386 and amd64
kernels.
.Pp
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 DomU 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 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 pcifront
Allow physical PCI devices to be passed through into a PV domain.
.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.
.El
.Ss Performance considerations
In general, PV drivers will perform better than emulated hardware, and are
the recommended configuration for HVM installations.
.Pp
Using a hypervisor introduces a second layer of scheduling that may limit the
effectiveness of certain
.Fx
scheduling optimisations.
Among these is adaptive locking, which is no longer able to determine whether
a thread holding a lock is in execution.
It is recommended that adaptive locking be disabled when using Xen:
.Bd -unfilled -offset indent
.Cd "options NO_ADAPTIVE_MUTEXES"
.Cd "options NO_ADAPTIVE_RWLOCKS"
.Cd "options NO_ADAPTIVE_SX"
.Ed
.Sh HISTORY
Support for
.Nm
first appeared in
.Fx 8.1 .
.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 ,
and
.An Colin Percival Aq Mt cperciva@FreeBSD.org .
This manual page was written by
.An Robert Watson Aq Mt rwatson@FreeBSD.org .
.Sh BUGS
.Fx
is only able to run as a Xen guest (DomU) and not as a Xen host (Dom0).
.Pp
As of this release, Xen PV DomU support is not heavily tested; instability
has been reported during VM migration of PV kernels.
.Pp
Certain PV driver features, such as the balloon driver, are under-exercised.