.. SPDX-License-Identifier: CC-BY-4.0

Introduction
============

Xen is an open-source, baremetal hypervisor.  It runs as the most privileged
piece of software and shares the resources of the hardware between virtual
machines.

In Xen terminology, there are :term:`domains<domain>` (commonly abbreviated to
dom), which both Xen and its toolstack identify by numeric identifiers called
:term:`domids<domid>`.

When a typical Xen system boots, :term:`dom0` is automatically started as well.
Dom0 is a virtual machine which, by default, is granted full permissions.

.. note::

    A common misconception with Xen's architecture is that :term:`dom0` is
    somehow different to other guests.  The choice of id 0 is not an accident,
    and follows in UNIX heritage.

A typical setup might be:

.. image:: _static/xen-overview.drawio.svg

Dom0 takes the role of :term:`control domain`, responsible for creating and
managing other virtual machines, and the role of :term:`hardware domain`,
responsible for hardware and marshalling guest I/O.

Xen is deliberately minimal, and has no device drivers. It manages RAM,
schedules virtual CPUs on the available physical CPUs, and marshals interrupts.

.. note::

     This definition might be fuzzy.  Xen can talk to common serial UARTs, and
     knows how to drive various CPU internal devices such as IOMMUs, but has no
     knowledge of network cards, disks, etc.  All of that is the hardware
     domains' responsibility.

Xen also provides a hypercall interface to guests, including event channels
(virtual interrupts), grant tables (shared memory), on which a lot of higher
level functionality is built.