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:
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
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.
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.
These para-virtualized drivers are supported:
- Allow physical memory pages to be returned to the hypervisor as a result
of manual tuning or automatic policy.
- Exports local block devices or files to other Xen domains where they can
then be imported via
- Import block devices from other Xen domains as local block devices, to be
used for file systems, swap, etc.
- Export the low-level system console via the Xen console service.
- Process management operations from Domain 0, including power off, reboot,
suspend, crash, and halt requests.
- Expose Xen events via the
/dev/xen/evtchn special device.
- Export local network interfaces to other Xen domains where they can be
- Import network interfaces from other Xen domains as local network
interfaces, which may be used for IPv4, IPv6, etc.
- Allow physical PCI devices to be passed through into a PV domain.
- 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
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
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:
first appeared in
support for Xen was first added by
and Doug Rabson
Further refinements were made by Justin
and Colin Percival
This manual page was written by Robert
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