Manual Pages  — XEN

xen – Xen Hypervisor Support

SYNOPSIS DESCRIPTION Xen device drivers HISTORY AUTHORS

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.

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.

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	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.

Support for xen first appeared in FreeBSD 8.1 . Support for host mode was added in 11.0 .

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