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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
     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 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
                     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 <[email protected]>
     and Doug Rabson <[email protected]>.  Further refinements were made by
     Justin Gibbs <[email protected]>, Adrian Chadd <[email protected]>, and
     Colin Percival <[email protected]>.  This manual page was written by
     Robert Watson <[email protected]>.

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

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

FreeBSD 11.1-RELEASE-p4         April 30, 2015         FreeBSD 11.1-RELEASE-p4
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