xref: /freebsd/share/man/man4/xen.4 (revision 3b56015b224e155a5832fb519a43c7dc01103df2)

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.Dd December 13, 2010
.Dt XEN 4
.Os
.Sh NAME
.Nm xen
.Nd Xen Hypervisor Guest (DomU) Support
.Sh SYNOPSIS
To compile para-virtualized (PV) Xen guest support into an i386 kernel, place
the following lines in your kernel configuration file:
.Bd -ragged -offset indent
.Cd "options PAE"
.Cd "options XEN"
.Ed
.Pp
To compile hardware-assisted virtualization (HVM) Xen guest support into an
amd64 kernel, place the following line in your kernel configuration file:
.Bd -ragged -offset indent
.Cd "options XENHVM"
.Ed
.Pp
To compile support for Xenbux and Xen PV drivers into an amd64 or i386
kernel, place the following line in your kernel configuration file:
.Bd -ragged -offset indent
.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 a fully para-virtualized (PV) kernel on the i386 architecture using
.Cd "options XEN" ;
currently, this requires use of a PAE kernel, enabled via
.Cd "options PAE" .
.Pp
.Fx
supports hardware-assisted virtualization (HVM) on both the i386 and amd64
kernels; however, PV device drivers with an HVM kernel are only supported on
the amd64 architecture, and require
.Cd "options XENHVM" .
.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 hypevisor, etc.
.Ss Para-virtualized drivers
Adding
.Cd "options xenpci"
to the kernel configuration enables the Xen para-virtualized drivers:
.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 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 interfacest 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
No description.
.It Nm xenpci
No description.
.El
.Ss Performance
In general, PV drivers will perform better than emulated hardware, and are
the recommended configuration.
.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 -ragged -offset indent
.Cd "options NO_ADAPTIVE_MUTEXES"
.Cd "options NO_ADAPTIVE_RWLOCKS"
.Cd "options NO_ADAPTIVE_SX"
.Ed
.Sh SEE ALSO
.Xr pae 4
.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 kmacy@FreeBSD.org
and
.An Doug Rabson Aq dfr@FreeBSD.org .
Further refinements were made by
.An Justin Gibbs Aq gibbs@FreeBSD.org ,
.An Adrian Chadd Aq adrian@FreeBSD.org ,
and
.An Colin Percival Aq cperciva@FreeBSD.org .
This manual page was written by
.An Robert Watson Aq rwatson@FreeBSD.org .
.Sh BUGS
.Fx
is only able to run as a Xen guest (DomU) and not as a Xen host (Dom0).
.Pp
A fully para-virtualized (PV) kernel is only supported on i386, and not
amd64.
.Pp
Para-virtualized drivers under hardware-assisted virtualization (HVM) kernel
are only supported on amd64, not i386.
.P
As of this release, Xen DomU support is not heavily tested; instability has
been reported during VM migration of PV kernels, and certain PV driver
features, such as the balloon driver, are under-exercised.