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

Glossary
========

.. Terms should appear in alphabetical order

.. glossary::

   control domain
     A :term:`domain`, commonly dom0, with the permission and responsibility
     to create and manage other domains on the system.

   domain
   domains
     A domain is Xen's unit of resource ownership, and generally has at the
     minimum some RAM and virtual CPUs.

     The terms :term:`domain` and :term:`guest` are commonly used
     interchangeably, but they mean subtly different things.

     A guest is a single, end user, virtual machine.

     In some cases, e.g. during live migration, one guest will be comprised of
     two domains for a period of time, while it is in transit.

   domid
     The numeric identifier of a running :term:`domain`.  It is unique to a
     single instance of Xen, used as the identifier in various APIs, and is
     typically allocated sequentially from 0.

   CET
     Control-flow Enforcement Technology is a facility in x86 CPUs for
     defending against memory safety vulnerabilities.  It is formed of two
     independent features:

     * CET-SS, Shadow Stacks, are designed to protect against Return Oriented
       Programming (ROP) attacks.

     * CET-IBT, Indirect Branch Tracking, is designed to protect against Call
       or Jump Oriented Programming (COP/JOP) attacks.

     Intel support CET-SS and CET-IBT from the Tiger Lake (Client, 2020) and
     Sapphire Rapids (Server, 2023) CPUs.  AMD support only CET-SS, starting
     with Zen3 (Both client and server, 2020) CPUs.

   event channel
     A paravirtual facility for guests to send and receive interrupts.

   guest
     The term 'guest' has two different meanings, depending on context, and
     should not be confused with :term:`domain`.

     When discussing a Xen system as a whole, a 'guest' refer to a virtual
     machine which is the "useful output" of running the system in the first
     place (e.g. an end-user VM).  Virtual machines providing system services,
     (e.g. the control and/or hardware domains), are not considered guests in
     this context.

     In the code, "guest context" and "guest state" is considered in terms of
     the CPU architecture, and contrasted against hypervisor context/state.
     In this case, it refers to all code running lower privilege level than
     the hypervisor.  As such, it covers all domains, including ones
     providing system services.

   hardware domain
     A :term:`domain`, commonly dom0, which shares responsibility with Xen
     about the system as a whole.

     By default, it gets all devices, including all disks and network cards, and
     is responsible for multiplexing guest I/O.

   hypercall
   hypercalls
     A mechanism for a :term:`guest` to request services from the hypervisor.
     Hypercalls are analogous to system calls in a traditional operating system.