Documentation Guidelines

Project ACRN content is written using the reStructuredText markup language (.rst file extension) with Sphinx extensions, and processed using Sphinx to create a formatted stand-alone website. Developers can view this content either in its raw form as .rst markup files, or (with Sphinx installed) they can build the documentation using the Makefile (on Linux systems) to generate the HTML content. The HTML content can then be viewed using a web browser. This same .rst content is also fed into the Project ACRN documentation website.

You can read details about reStructuredText and about Sphinx extensions from their respective websites.

This document provides a quick reference for commonly used reST and Sphinx-defined directives and roles used to create the documentation you’re reading.


Document sections are identified through their heading titles, indicated with an underline below the title text. (While reST allows use of both and overline and matching underline to indicate a heading, we only use an underline indicator for headings.) For consistency in our documentation, we define the order of characters used to indicate the nested table of contents levels:

  • Use # for the Document title underline character

  • Use * for the First sub-section heading level

  • Use = for the Second sub-section heading level

  • Use - for the Third sub-section heading level

Additional heading level depth is discouraged.

The heading underline must be at least as long as the title it’s under.

Here’s an example of nested heading levels and the appropriate underlines to use:

Document Title heading

Section 1 heading

Section 2 heading

Section 2.1 heading

Section 2.1.1 heading

Section 2.2 heading

Section 3 heading

Content Highlighting

Some common reST inline markup samples:

  • one asterisk: *text* for emphasis (italics),

  • two asterisks: **text** for strong emphasis (boldface), and

  • two back quotes: ``text`` for inline code samples.

ReST rules for inline markup try to be forgiving to account for common cases of using these marks. For example, using an asterisk to indicate multiplication, such as 2 * (x + y) will not be interpreted as an unterminated italics section. For inline markup, the characters between the beginning and ending characters must not start or end with a space, so *this is italics* ( this is italics) while * this isn't* (* this isn’t*).

If asterisks or back quotes appear in running text and could be confused with inline markup delimiters, you can eliminate the confusion by adding a backslash (\) before it.


For bullet lists, place an asterisk (*) or hyphen (-) at the start of a paragraph and indent continuation lines with two spaces.

The first item in a list (or sublist) must have a blank line before it and should be indented at the same level as the preceding paragraph (and not indented itself).

For numbered lists start with a 1. or a) for example, and continue with autonumbering by using a # sign and a . or ) as used in the first list item. Indent continuation lines with spaces to align with the text of first list item:

* This is a bulleted list.
* It has two items, the second
  item and has more than one line of reST text.  Additional lines
  are indented to the first character of the
  text of the bullet list.

1. This is a new numbered list. If there wasn't a blank line before it,
   it would be a continuation of the previous list (or paragraph).
#. It has two items too.

a) This is a numbered list using alphabetic list headings
#) It has three items (and uses autonumbering for the rest of the list)
#) Here's the third item.  Use consistent punctuation on the list

#. This is an autonumbered list (default is to use numbers starting
   with 1).

   #. This is a second-level list under the first item (also
      autonumbered).  Notice the indenting.
   #. And a second item in the nested list.
#. And a second item back in the containing list.  No blank line
   needed, but it wouldn't hurt for readability.

Definition lists (with a term and its definition) are a convenient way to document a word or phrase with an explanation. For example, this reST content:

The Makefile has targets that include:

   Build the HTML output for the project

   Remove all generated output, restoring the folders to a
   clean state.

Would be rendered as:

The Makefile has targets that include:


Build the HTML output for the project


Remove all generated output, restoring the folders to a clean state.

Multi-Column Lists

If you have a long bullet list of items, where each item is short, you can indicate the list items should be rendered in multiple columns with a special .. rst-class:: rst-columns directive. The directive will apply to the next non-comment element (e.g., paragraph), or to content indented under the directive. For example, this unordered list:

.. rst-class:: rst-columns

* A list of
* short items
* that should be
* displayed
* horizontally
* so it doesn't
* use up so much
* space on
* the page

would be rendered as:

  • A list of

  • short items

  • that should be

  • displayed

  • horizontally

  • so it doesn’t

  • use up so much

  • space on

  • the page

A maximum of three columns will be displayed if you use rst-columns (or rst-columns3), and two columns for rst-columns2. The number of columns displayed can be reduced based on the available width of the display window, reducing to one column on narrow (phone) screens if necessary. We’ve deprecated use of the hlist directive because it misbehaves on smaller screens.


There are a few ways to create tables, each with their limitations or quirks. Grid tables offer the most capability for defining merged rows and columns, but are hard to maintain:

| Header row, column 1   | Header 2   | Header 3 | Header 4 |
| (header rows optional) |            |          |          |
| body row 1, column 1   | column 2   | column 3 | column 4 |
| body row 2             | ...        | ...      | you can  |
+------------------------+------------+----------+ easily   +
| body row 3 with a two column span   | ...      | span     |
+------------------------+------------+----------+ rows     +
| body row 4             | ...        | ...      | too      |

This example would render as:

Header row, column 1 (header rows optional)

Header 2

Header 3

Header 4

body row 1, column 1

column 2

column 3

column 4

body row 2

you can easily span rows too

body row 3 with a two column span

body row 4

List tables are much easier to maintain, but don’t support row or column spans:

.. list-table:: Table title
   :widths: 15 20 40
   :header-rows: 1

   * - Heading 1
     - Heading 2
     - Heading 3
   * - body row 1, column 1
     - body row 1, column 2
     - body row 1, column 3
   * - body row 2, column 1
     - body row 2, column 2
     - body row 2, column 3

This example would render as:

Table 10 Table title

Heading 1

Heading 2

Heading 3

body row 1, column 1

body row 1, column 2

body row 1, column 3

body row 2, column 1

body row 2, column 2

body row 2, column 3

The :widths: parameter lets you define relative column widths. The default is equal column widths. If you have a three-column table and you want the first column to be half as wide as the other two equal-width columns, you can specify :widths: 1 2 2. If you’d like the browser to set the column widths automatically based on the column contents, you can use :widths: auto.

File Names and Commands

Sphinx extends reST by supporting additional inline markup elements (called “roles”) used to tag text with special meanings and allow style output formatting. (You can refer to the Sphinx Inline Markup documentation for the full list).

For example, there are roles for marking filenames (:file:`name`) and command names such as make (:command:`make`). You can also use the ``inline code`` markup (double backticks) to indicate a filename.

Don’t use items within a single backtick, for example `word`.

For references to files that are in the ACRN Hypervisor GitHub tree, a special role can be used that creates a hyperlink to that file. For example a GitHub link to the reST file used to create this document can be generated using :acrn_file:`doc/developer-guides/doc_guidelines.rst` that will show up as doc/developer-guides/doc_guidelines.rst, a link to the “blob” file in the GitHub repo as displayed by GitHub. There’s also an :acrn_raw:`doc/developer-guides/doc_guidelines.rst` role that will link to the “raw” uninterpreted file, doc/developer-guides/doc_guidelines.rst file. (Click on these links to see the difference.)

Internal Cross-Reference Linking

Traditional ReST links are only supported within the current file using the notation:

refer to the `internal-linking`_ page

which renders as,

refer to the internal-linking page

Note the use of a trailing underscore to indicate an outbound link. In this example, the label was added immediately before a heading, so the text that’s displayed is the heading text itself.

With Sphinx however, we can create link-references to any tagged text within the project documentation.

Target locations within documents are defined with a label directive:

.. _my label name:

Note the leading underscore indicating an inbound link. The content immediately following this label is the target for a :ref:`my label name` reference from anywhere within the documentation set. The label must be added immediately before a heading so there’s a natural phrase to show when referencing this label (e.g., the heading text).

This is the same directive used to define a label that’s a reference to a URL:

.. _Hypervisor Wikipedia Page:

To enable easy cross-page linking within the site, each file should have a reference label before its title so it can be referenced from another file. These reference labels must be unique across the whole site, so generic names such as “samples” should be avoided. For example the top of this document’s .rst file is:

.. _doc_guidelines:

Documentation Guidelines

Other .rst documents can link to this document using the :ref:`doc_guidelines` tag and it will show up as Documentation Guidelines. This type of internal cross reference works across multiple files, and the link text is obtained from the document source so if the title changes, the link text will update as well.

There may be times when you’d like to change the link text that’s shown in the generated document. In this case, you can specify alternate text using :ref:`alternate text <doc_guidelines>` (renders as alternate text).

Non-ASCII Characters

You can insert non-ASCII characters such as a Trademark symbol (™), by using the notation |trade|. (It’s also allowed to use the UTF-8 characters directly.) Available replacement names are defined in an include file used during the Sphinx processing of the reST files. The names of these replacement characters are the same as used in HTML entities used to insert characters in HTML, e.g., &trade; and are defined in the file sphinx_build/substitutions.txt as listed here:

.. |br|     raw:: html        .. force a line break in HTML output (blank lines needed here)

   <br style="clear:both" />

.. These are replacement strings for non-ASCII characters used within the project
   using the same name as the html entity names (e.g., &copy;) for that character

.. |copy|   unicode:: U+000A9 .. COPYRIGHT SIGN
.. |trade|  unicode:: U+02122 .. TRADEMARK SIGN
.. |reg|    unicode:: U+000AE .. REGISTERED TRADEMARK SIGN
.. |deg|    unicode:: U+000B0 .. DEGREE SIGN
.. |plusminus|  unicode:: U+000B1 .. PLUS-MINUS SIGN
.. |micro|  unicode:: U+000B5 .. MICRO SIGN
.. |check|  unicode:: U+02714 .. HEAVY CHECK MARK
.. |oplus|  unicode:: U+02295 .. CIRCLED PLUS SIGN
.. |rarr|   unicode:: U+02192 .. RIGHTWARDS ARROW

We’ve kept the substitutions list small but others can be added as needed by submitting a change to the substitutions.txt file.

Code and Command Examples

Use the reST code-block directive to create a highlighted block of fixed-width text, typically used for showing formatted code or console commands and output. Smart syntax highlighting is also supported (using the Pygments package). You can also directly specify the highlighting language. For example:

.. code-block:: c

   struct _k_object {
      char *name;
      u8_t perms[CONFIG_MAX_THREAD_BYTES];
      u8_t type;
      u8_t flags;
      u32_t data;
   } __packed;

Note the blank line between the code-block directive and the first line of the code-block body, and the body content is indented three spaces (to the first non-blank space of the directive name).

This would be rendered as:

struct _k_object {
   char *name;
   u8_t type;
   u8_t flags;
   u32_t data;
} __packed;

You can specify other languages for the code-block directive, including c, python, and rst, and also console, bash, or shell. If you want no syntax highlighting, use the language none, for example:

.. code-block:: none

   This would be a block of text styled with a background
   and box, but with no syntax highlighting.

Would display as:

This would be a block of text styled with a background
and box, but with no syntax highlighting.

There’s a shorthand for writing code blocks too: end the introductory paragraph with a double colon (::) and indent the code block content by three spaces. On output, only one colon will be shown. The highlighting package makes a best guess at the type of content in the block and highlighting purposes. This can lead to some odd highlighting in the generated output.


Images are included in documentation by using an image directive:

.. image:: ../../images/doc-gen-flow.png
   :align: center
   :alt: alt text for the image

or if you’d like to add an image caption, use:

.. figure:: ../../images/doc-gen-flow.png
   :alt: image description

   Caption for the figure

The file name specified is relative to the document source file, and we recommend putting images into an images folder where the document source is found. The usual image formats handled by a web browser are supported: JPEG, PNG, GIF, and SVG. Keep the image size only as large as needed, generally at least 500 px wide but no more than 1000 px, and no more than 250 KB unless a particularly large image is needed for clarity.

Tabs, Spaces, and Indenting

Indenting is significant in reST file content, and using spaces is preferred. Extra indenting can (unintentionally) change the way content is rendered too. For lists and directives, indent the content text to the first non-blank space in the preceding line. For example:

* List item that spans multiple lines of text
  showing where to indent the continuation line.

1. And for numbered list items, the continuation
   line should align with the text of the line above.

.. code-block::

   The text within a directive block should align with the
   first character of the directive name.

Keep the line length for documentation less than 80 characters to make it easier for reviewing in GitHub. Long lines because of URL references are an allowed exception.


You can include a picture (.jpg, .png, .svg for example) by using the .. image directive:

.. image:: ../images/ACRNlogo.png
   :align: center

This results in the image being placed in the document:


Alternatively, use the .. figure directive to include a picture with a caption and automatic figure numbering for your image, (so you can say see Figure 312, by using the notation :numref:`acrn-logo-figure` and specifying the name of figure):

.. figure:: ../images/ACRNlogo.png
   :align: center
   :name: acrn-logo-figure

   Caption for the figure

Figure 312 Caption for the figure

We’ve also included the graphviz Sphinx extension to let you use a text description language to render drawings. See Drawings Using Graphviz for more information.

Alternative Tabbed Content

Instead of creating multiple documents with common material except for some specific sections, you can write one document and provide alternative content to the reader via a tabbed interface. When the reader clicks on a tab, the content for that tab is displayed, for example:

.. tabs::

   .. tab:: Apples

      Apples are green, or sometimes red.

   .. tab:: Pears

      Pears are green.

   .. tab:: Oranges

      Oranges are orange.

will display as:

Apples are green, or sometimes red.

Pears are green.

Oranges are orange.

Tabs can also be grouped, so that changing the current tab in one area changes all tabs with the same name throughout the page. For example:

Linux Line 1

macOS Line 1

Windows Line 1

Linux Line 2

macOS Line 2

Windows Line 2

In this latter case, we’re using a .. group-tab:: directive instead of a .. tab:: directive. Under the hood, we’re using the sphinx-tabs extension that’s included in the ACRN (requirements.txt) setup. Within a tab, you can have most any content other than a heading (code-blocks, ordered and unordered lists, pictures, paragraphs, and such). You can read more about sphinx-tabs from the link above.

Instruction Steps

A numbered instruction steps style makes it easy to create tutorial guides with clearly identified steps. Add the .. rst-class:: numbered-step directive immediately before a second-level heading (by project convention, a heading underlined with asterisks ******, and it will be displayed as a numbered step, sequentially numbered within the document. (Second-level headings without this rst-class directive will not be numbered.) For example:

.. rst-class:: numbered-step

Put your right hand in

First Instruction Step

This is the first instruction step material. You can do the usual paragraphs and pictures as you’d use in normal document writing. Write the heading to be a summary of what the step is (the step numbering is automated so you can move steps around easily if needed).

Second Instruction Step

This is the second instruction step.


As implemented, only one set of numbered steps is intended per document and the steps must be level 2 headings.

Documentation Generation

For instructions on building the documentation, see ACRN Documentation Generation.