1.. SPDX-License-Identifier: CC-BY-4.0
2
3Introduction
4============
5
6Xen is an open source, bare metal hypervisor.  It runs as the most privileged
7piece of software, and shares the resources of the hardware between virtual
8machines.
9
10In Xen terminology, there are :term:`domains<domain>`, commonly abbreviated to
11dom, which are identified by their numeric :term:`domid`.
12
13When Xen boots, dom0 is automatically started as well.  Dom0 is a virtual
14machine which, by default, is granted full permissions [1]_.  A typical setup
15might be:
16
17.. image:: xen-overview.drawio.svg
18
19Dom0 takes the role of :term:`control domain`, responsible for creating and
20managing other virtual machines, and the role of :term:`hardware domain`,
21responsible for hardware and marshalling guest I/O.
22
23Xen is deliberately minimal, and has no device drivers [2]_.  Xen manages RAM,
24schedules virtual CPUs on the available physical CPUs, and marshals
25interrupts.
26
27Xen also provides a hypercall interface to guests, including event channels
28(virtual interrupts), grant tables (shared memory), on which a lot of higher
29level functionality is built.
30
31.. rubric:: Footnotes
32
33.. [1] A common misconception with Xen's architecture is that dom0 is somehow
34       different to other guests.  The choice of id 0 is not an accident, and
35       follows in UNIX heritage.
36
37.. [2] This definition might be fuzzy.  Xen can talk to common serial UARTs,
38       and knows how to drive various CPU internal devices such as IOMMUs, but
39       has no knowledge of network cards, disks, etc.  All of that is the
40       hardware domains responsibility.
41