NAME

       xen — Xen Hypervisor Guest (DomU) Support

SYNOPSIS

       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

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.

       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 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 hypervisor.

   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

SEE ALSO

       pae(4)

HISTORY

       Support for xen first appeared in FreeBSD 8.1.

AUTHORS

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

BUGS

       FreeBSD is only able to run as a Xen guest (DomU) and not as a Xen host (Dom0).

       A fully para-virtualized (PV) kernel is only supported on i386, and not amd64.

       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-exercised.

Debian                                          December 17, 2010                                         XEN(4)