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XEN(4)		       FreeBSD Kernel Interfaces Manual			XEN(4)

     xen -- Xen	Hypervisor Guest (DomU)	Support

     To	compile	hardware-assisted virtualization (HVM) Xen guest support with
     para-virtualized drivers into an amd64 or i386 kernel, place the follow-
     ing 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 hardware-assisted	virtualization (HVM) on	both i386 and
     amd64 kernels.

     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
     These para-virtualized 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

     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-

FreeBSD	13.0			April 30, 2015			  FreeBSD 13.0


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