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.
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.