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     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 following lines in your kernel configuration

           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 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.

     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

     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,

   Xen DomU device drivers
     These para-virtualized drivers are supported:

           balloon   Allow physical memory pages to be returned to the hypervisor 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 service.

           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, etc.

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

           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

   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_MUTEXES
           options NO_ADAPTIVE_RWLOCKS
           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 (Dom0).

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

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