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Andrew Cooper
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Creative Commons is a more common license than GPL for documentation purposes. Switch to using CC-BY-4.0 to explicitly permit re-purposing and remixing of the content. Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com> Reviewed-by: Lars Kurth <lars.kurth@citrix.com> Release-acked-by: Juergen Gross <jgross@suse.com>
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41 lines
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.. SPDX-License-Identifier: CC-BY-4.0
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Introduction
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============
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Xen is an open source, bare metal hypervisor. It runs as the most privileged
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piece of software, and shares the resources of the hardware between virtual
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machines.
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In Xen terminology, there are :term:`domains<domain>`, commonly abbreviated to
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dom, which are identified by their numeric :term:`domid`.
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When Xen boots, dom0 is automatically started as well. Dom0 is a virtual
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machine which, by default, is granted full permissions [1]_. A typical setup
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might be:
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.. image:: xen-overview.drawio.svg
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Dom0 takes the role of :term:`control domain`, responsible for creating and
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managing other virtual machines, and the role of :term:`hardware domain`,
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responsible for hardware and marshalling guest I/O.
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Xen is deliberately minimal, and has no device drivers [2]_. Xen manages RAM,
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schedules virtual CPUs on the available physical CPUs, and marshals
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interrupts.
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Xen also provides a hypercall interface to guests, including event channels
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(virtual interrupts), grant tables (shared memory), on which a lot of higher
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level functionality is built.
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.. rubric:: Footnotes
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.. [1] A common misconception with Xen's architecture is that dom0 is somehow
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different to other guests. The choice of id 0 is not an accident, and
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follows in UNIX heritage.
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.. [2] This definition might be fuzzy. Xen can talk to common serial UARTs,
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and knows how to drive various CPU internal devices such as IOMMUs, but
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has no knowledge of network cards, disks, etc. All of that is the
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hardware domains responsibility.
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