XEN(4) FreeBSD Kernel Interfaces Manual XEN(4)
NAME
xen - Xen Hypervisor Support
SYNOPSIS
FreeBSD supports running both as a Xen guest and host on amd64 hardware.
Guest support is limited to HVM and PVH modes, while host support is
limited to PVH mode only.
Xen support is built by default in the i386 and amd64 GENERIC kernels;
note however that host mode is only available on amd64.
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
and PVH). 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. PVH configurations rely on para-virtualized
drivers exclusively for IO.
FreeBSD 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 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
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.
gntdev Allow access to the grant table interface via the
/dev/xen/gntdev 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.
privcmd Allow issuing hypercalls via the /dev/xen/privcmd special
device.
timer Implementation of a one-shot high resolution per-CPU
timer using the hypercall interface.
acpi cpu When running as a host forwards power management related
information from ACPI to the hypervisor for better
performance management.
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.
xenstore Information storage space shared between domains.
HISTORY
Support for xen first appeared in FreeBSD 8.1. Support for host mode was
added in 11.0 .
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>,
Colin Percival <cperciva@FreeBSD.org>, and Roger Pau Monne
<royger@FreeBSD.org>. This manual page was written by Robert Watson
<rwatson@FreeBSD.org>, and Roger Pau Monne <royger@FreeBSD.org>.
FreeBSD 15.1-STABLE-HBSD January 8, 2024 XEN(4)