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     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
     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 semantics.

     FreeBSD supports a fully para-virtualized (PV) kernel on the i386
     architecture 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
                     hypervisor as a result of manual tuning or automatic

           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
                     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

     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;
     instability has been reported during VM migration of PV kernels.

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