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XEN(4)			 BSD Kernel Interfaces Manual			XEN(4)

     xen -- Xen	Hypervisor Guest (DomU)	Support

     To	compile	para-virtualized (PV) Xen guest	support	into an	i386 kernel,
     place the following lines in your kernel configuration file:

	   options PAE
	   options XEN
	   nooptions NATIVE

     To	compile	hardware-assisted virtualization (HVM) Xen guest support with
     para-virtualized drivers into an amd64 kernel, place the following	lines
     in	your kernel configuration file:

	   options XENHVM
	   device xenpci

     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 vir-
     tualizable.  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.

     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 pe-
     ripherals,	or para-virtualized drivers, which are aware of	virtualiza-
     tion, and hence able to optimize certain behaviors	to improve performance
     or	semantics.

     FreeBSD supports a	fully para-virtualized (PV) kernel on the i386 archi-
     tecture using options XEN and nooptions NATIVE; currently,	this requires
     use of a PAE kernel, enabled via options PAE.

     FreeBSD 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 options XENHVM and
     device xenpci.

     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.

   Xen DomU device drivers
     Xen para-virtualized drivers are automatically added to the kernel	if a
     PV	kernel is compiled using options XEN; for HVM environments, options
     XENHVM and	device xenpci are required.  The follow	drivers	are supported:

	   balloon   Allow physical memory pages to be returned	to the hyper-
		     visor as a	result of manual tuning	or automatic policy.

	   blkback   Exports local block devices or files to other Xen domains
		     where they	can then be imported via blkfront.

	   blkfront  Import block devices from other Xen domains as local
		     block devices, to be used for file	systems, swap, etc.

	   console   Export the	low-level system console via the Xen console

	   control   Process management	operations from	Domain 0, including
		     power off,	reboot,	suspend, crash,	and halt requests.

	   evtchn    Expose Xen	events via the /dev/xen/evtchn special device.

	   netback   Export local network interfaces to	other Xen domains
		     where they	can be imported	via netfront.

	   netfront  Import network interfaces from other Xen domains as local
		     network interfaces, which may be used for IPv4, IPv6,

	   pcifront  Allow physical PCI	devices	to be passed through into a PV

	   xenpci    Represents	the Xen	PCI device, an emulated	PCI device
		     that is exposed to	HVM domains.  This device allows de-
		     tection of	the Xen	hypervisor, and	provides interrupt and
		     shared memory services required to	interact with the hy-

   Performance considerations
     In	general, PV drivers will perform better	than emulated hardware,	and
     are the recommended configuration for HVM installations.

     Using a hypervisor	introduces a second layer of scheduling	that may limit
     the effectiveness of certain FreeBSD 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:

	   options NO_ADAPTIVE_SX


     Support for xen first appeared in FreeBSD 8.1.

     FreeBSD support for Xen was first added by	Kip Macy <>
     and Doug Rabson <>.	 Further refinements were made by
     Justin Gibbs <>, Adrian Chadd <>, and
     Colin Percival <>.  This manual page was written by
     Robert Watson <>.

     FreeBSD is	only able to run as a Xen guest	(DomU) and not as a Xen	host

     A fully para-virtualized (PV) kernel is only supported on i386, and not

     Para-virtualized drivers under hardware-assisted virtualization (HVM)
     kernel are	only supported on amd64, not i386.

     As	of this	release, Xen PV	DomU support is	not heavily tested; instabil-
     ity has been reported during VM migration of PV kernels.

     Certain PV	driver features, such as the balloon driver, are under-exer-

BSD			       December	17, 2010			   BSD


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