The Sphinx QAPI Domain — QEMU documentation

# The Sphinx QAPI Domain

An extension to the rST syntax in Sphinx is provided by the QAPI Domain, located in `docs/sphinx/qapi_domain.py`. This extension is analogous to the Python Domain included with Sphinx, but provides special directives and roles speciically for annotating and documenting QAPI definitions specifically.

A Domain provides a set of special rST directives and cross-referencing roles to Sphinx for understanding rST markup written to document a specific language. By itself, this QAPI extension is only sufficient to parse rST markup written by hand; the autodoc functionality is provided elsewhere, in `docs/sphinx/qapidoc.py`, by the “Transmogrifier”.

It is not expected that any developer nor documentation writer would never need to write nor read these special rST forms. However, in the event that something needs to be debugged, knowing the syntax of the domain is quite handy. This reference may also be useful as a guide for understanding the QAPI Domain extension code itself. Although most of these forms will not be needed for documentation writing purposes, understanding the cross-referencing syntax will be helpful when writing rST documentation elsewhere, or for enriching the body of QAPIDoc blocks themselves.

## Concepts

The QAPI Domain itself provides no mechanisms for reading the QAPI Schema or generating documentation from code that exists. It is merely the rST syntax used to describe things. For instance, the Sphinx Python domain adds syntax like `:py:func:` for describing Python functions in documentation, but it’s the autodoc module that is responsible for reading Python code and generating such syntax. QAPI is analogous here: qapidoc.py is responsible for reading the QAPI Schema and generating rST syntax, and qapi_domain.py is responsible for translating that special syntax and providing APIs for Sphinx internals.

In other words:

qapi_domain.py adds syntax like `.. qapi:command::` to Sphinx, and qapidoc.py transforms the documentation in `qapi/*.json` into rST using directives defined by the domain.

Or even shorter:

`:py:` is to `:qapi:` as autodoc is to qapidoc.

## Info Field Lists

Field lists are a standard syntax in reStructuredText. Sphinx extends that syntax to give certain field list entries special meaning and parsing to, for example, add cross-references. The QAPI Domain takes advantage of this field list extension to document things like Arguments, Members, Values, and so on.

The special parsing and handling of info field lists in Sphinx is provided by three main classes; Field, GroupedField, and TypedField. The behavior and formatting for each configured field list entry in the domain changes depending on which class is used.

Field:

  - Creates an ungrouped field: i.e., each entry will create its own section and they will not be combined.

  - May optionally support an argument.

  - May apply cross-reference roles to either the argument or the content body, both, or neither.

This is used primarily for entries which are not expected to be repeated, i.e., items that may only show up at most once. The QAPI domain uses this class for “Errors” section.

GroupedField:

  - Creates a grouped field: i.e. multiple adjacent entries will be merged into one section, and the content will form a bulleted list.

  - Must take an argument.

  - May optionally apply a cross-reference role to the argument, but not the body.

  - Can be configured to remove the bulleted list if there is only a single entry.

  - All items will be generated with the form: “argument – body”

This is used for entries which are expected to be repeated, but aren’t expected to have two arguments, i.e. types without names, or names without types. The QAPI domain uses this class for features, returns, and enum values.

TypedField:

  - Creates a grouped, typed field. Multiple adjacent entres will be merged into one section, and the content will form a bulleted list.

  - Must take at least one argument, but supports up to two - nominally, a name and a type.

  - May optionally apply a cross-reference role to the type or the name argument, but not the body.

  - Can be configured to remove the bulleted list if there is only a single entry.

  - All items will be generated with the form “name (type) – body”

This is used for entries that are expected to be repeated and will have a name, a type, and a description. The QAPI domain uses this class for arguments, alternatives, and members. Wherever type names are referenced below, They must be a valid, documented type that will be cross-referenced in the HTML output; or one of the built-in JSON types (string, number, int, boolean, null, value, q_empty).

### `:feat:`

Document a feature attached to a QAPI definition.

availability:

  This field list is available in the body of Command, Event, Enum, Object and Alternate directives.

syntax:

  `:feat name: Lorem ipsum, dolor sit amet...`

type:

  sphinx.util.docfields.GroupedField

Example:

    .. qapi:object:: BlockdevOptionsVirtioBlkVhostVdpa
       :since: 7.2
       :ifcond: CONFIG_BLKIO

       Driver specific block device options for the virtio-blk-vhost-vdpa
       backend.

    :memb string path: path to the vhost-vdpa character device.
    :feat fdset: Member ``path`` supports the special "/dev/fdset/N" path
        (since 8.1)

### `:arg:`

Document an argument to a QAPI command.

availability:

  This field list is only available in the body of the Command directive.

syntax:

  `:arg type name: description`

type:

  sphinx.util.docfields.TypedField

Example:

    .. qapi:command:: job-pause
       :since: 3.0

       Pause an active job.

       This command returns immediately after marking the active job for
       pausing.  Pausing an already paused job is an error.

       The job will pause as soon as possible, which means transitioning
       into the PAUSED state if it was RUNNING, or into STANDBY if it was
       READY.  The corresponding JOB_STATUS_CHANGE event will be emitted.

       Cancelling a paused job automatically resumes it.

       :arg string id: The job identifier.

### `:error:`

Document the error condition(s) of a QAPI command.

availability:

  This field list is only available in the body of the Command directive.

syntax:

  `:error: Lorem ipsum dolor sit amet ...`

type:

  sphinx.util.docfields.Field

The format of the :errors: field list description is free-form rST. The alternative spelling “:errors:” is also permitted, but strictly analogous.

Example:

    .. qapi:command:: block-job-set-speed
       :since: 1.1

       Set maximum speed for a background block operation.

       This command can only be issued when there is an active block job.

       Throttling can be disabled by setting the speed to 0.

       :arg string device: The job identifier.  This used to be a device
           name (hence the name of the parameter), but since QEMU 2.7 it
           can have other values.
       :arg int speed: the maximum speed, in bytes per second, or 0 for
           unlimited.  Defaults to 0.
       :error:
           - If no background operation is active on this device,
             DeviceNotActive

### `:return:`

Document the return type(s) and value(s) of a QAPI command.

availability:

  This field list is only available in the body of the Command directive.

syntax:

  `:return type: Lorem ipsum dolor sit amet ...`

type:

  sphinx.util.docfields.GroupedField

Example:

    .. qapi:command:: query-replay
       :since: 5.2

       Retrieve the record/replay information.  It includes current
       instruction count which may be used for ``replay-break`` and
       ``replay-seek`` commands.

       :return ReplayInfo: record/replay information.

       .. qmp-example::

           -> { "execute": "query-replay" }
           <- { "return": {
                  "mode": "play", "filename": "log.rr", "icount": 220414 }
              }

### `:value:`

Document a possible value for a QAPI enum.

availability:

  This field list is only available in the body of the Enum directive.

syntax:

  `:value name: Lorem ipsum, dolor sit amet ...`

type:

  sphinx.util.docfields.GroupedField

Example:

    .. qapi:enum:: QapiErrorClass
       :since: 1.2

       QEMU error classes

       :value GenericError: this is used for errors that don't require a specific
           error class.  This should be the default case for most errors
       :value CommandNotFound: the requested command has not been found
       :value DeviceNotActive: a device has failed to be become active
       :value DeviceNotFound: the requested device has not been found
       :value KVMMissingCap: the requested operation can't be fulfilled because a
           required KVM capability is missing

### `:alt:`

Document a possible branch for a QAPI alternate.

availability:

  This field list is only available in the body of the Alternate directive.

syntax:

  `:alt type name: Lorem ipsum, dolor sit amet ...`

type:

  sphinx.util.docfields.TypedField

As a limitation of Sphinx, we must document the “name” of the branch in addition to the type, even though this information is not visible on the wire in the QMP protocol format. This limitation may be lifted at a future date.

Example:

    .. qapi:alternate:: StrOrNull
       :since: 2.10

       This is a string value or the explicit lack of a string (null
       pointer in C).  Intended for cases when 'optional absent' already
       has a different meaning.

        :alt string s: the string value
        :alt null n: no string value

### `:memb:`

Document a member of an Event or Object.

availability:

  This field list is available in the body of Event or Object directives.

syntax:

  `:memb type name: Lorem ipsum, dolor sit amet ...`

type:

  sphinx.util.docfields.TypedField

This is fundamentally the same as `:arg:` and `:alt:`, but uses the “Members” phrasing for Events and Objects (Structs and Unions).

Example:

    .. qapi:event:: JOB_STATUS_CHANGE
       :since: 3.0

       Emitted when a job transitions to a different status.

       :memb string id: The job identifier
       :memb JobStatus status: The new job status

### Arbitrary field lists

Other field list names, while valid rST syntax, are prohibited inside of QAPI directives to help prevent accidental misspellings of info field list names. If you want to add a new arbitrary “non-value-added” field list to QAPI documentation, you must add the field name to the allow list in `docs/conf.py`

For example:

    qapi_allowed_fields = {
        "see also",
    }

Will allow you to add arbitrary field lists in QAPI directives:

    .. qapi:command:: x-fake-command

       :see also: Lorem ipsum, dolor sit amet ...

## Cross-references

Cross-reference roles in the QAPI domain are modeled closely after the Python cross-referencing syntax.

QAPI definitions can be referenced using the standard any role cross-reference syntax, such as with ``query-blockstats``. In the event that disambiguation is needed, cross-references can also be written using a number of explicit cross-reference roles:

- `:qapi:mod:`block-core`` – Reference a QAPI module. The link will take you to the beginning of that section in the documentation.

- `:qapi:cmd:`query-block`` – Reference a QAPI command.

- `:qapi:event:`JOB_STATUS_CHANGE`` – Reference a QAPI event.

- `:qapi:enum:`QapiErrorClass`` – Reference a QAPI enum.

- `:qapi:obj:`BlockdevOptionsVirtioBlkVhostVdpa` – Reference a QAPI object (struct or union)

- `:qapi:alt:`StrOrNull`` – Reference a QAPI alternate.

- `:qapi:type:`BlockDirtyInfo`` – Reference any QAPI type; this excludes modules, commands, and events.

- `:qapi:any:`block-job-set-speed`` – Reference absolutely any QAPI entity.

Type arguments in info field lists are converted into references as if you had used the `:qapi:type:` role. All of the special syntax below applies to both info field lists and standalone explicit cross-references.

### Type decorations

Type names in references can be surrounded by brackets, like `[typename]`, to indicate an array of that type. The cross-reference will apply only to the type name between the brackets. For example; `:qapi:type:`[Qcow2BitmapInfoFlags]`` renders to: `[``QMP:Qcow2BitmapInfoFlags``]`

To indicate an optional argument/member in a field list, the type name can be suffixed with `?`. The cross-reference will be transformed to “type, Optional” with the link applying only to the type name. For example; `:qapi:type:`BitmapSyncMode?`` renders to: `QMP:BitmapSyncMode`, optional

### Namespaces

Mimicking the Python domain target specification syntax, QAPI allows you to specify the fully qualified path for a data type.

- A namespace can be explicitly provided; e.g. `:qapi:type:`QMP:BitmapSyncMode`

- A module can be explicitly provided; `:qapi:type:`QMP:block-core.BitmapSyncMode`` will render to: `QMP:block-core.BitmapSyncMode`

- If you don’t want to display the “fully qualified” name, it can be prefixed with a tilde; `:qapi:type:`~QMP:block-core.BitmapSyncMode`` will render to: `BitmapSyncMode`

### Target resolution

Any cross-reference to a QAPI type, whether using the ``any`` style of reference or the more explicit ``:qapi:any:`target`` syntax, allows for the presence or absence of either the namespace or module information.

When absent, their value will be inferred from context by the presence of any `qapi:namespace` or `qapi:module` directives preceding the cross-reference.

If no results are found when using the inferred values, other namespaces/modules will be searched as a last resort; but any explicitly provided values must always match in order to succeed.

This allows for efficient cross-referencing with a minimum of syntax in the large majority of cases, but additional context or namespace markup may be required outside of the QAPI reference documents when linking to items that share a name across multiple documented QAPI schema.

### Custom link text

The name of a cross-reference link can be explicitly overridden like most stock Sphinx references using the `custom text <target>` syntax.

For example, `:qapi:cmd:`Merge dirty bitmaps <block-dirty-bitmap-merge>`` will render as: `Merge dirty bitmaps`

## Directives

The QAPI domain adds a number of custom directives for documenting various QAPI/QMP entities. The syntax is plain rST, and follows this general format:

    .. qapi:directive:: argument
       :option:
       :another-option: with an argument

       Content body, arbitrary rST is allowed here.

### Sphinx standard options

All QAPI directives inherit a number of standard options from Sphinx’s ObjectDescription class.

The dashed spellings of the below options were added in Sphinx 7.2, the undashed spellings are currently retained as aliases, but will be removed in a future version.

- `:no-index:` and `:noindex:` – Do not add this item into the Index, and do not make it available for cross-referencing.

- `no-index-entry:` and `:noindexentry:` – Do not add this item into the Index, but allow it to be cross-referenced.

- `no-contents-entry` and `:nocontentsentry:` – Exclude this item from the Table of Contents.

- `no-typesetting` – Create TOC, Index and cross-referencing entities, but don’t actually display the content.

### QAPI standard options

All QAPI directives – except for namespace and module – support these common options.

- `:namespace: name` – This option allows you to override the namespace association of a given definition.

- `:module: modname` – Borrowed from the Python domain, this option allows you to override the module association of a given definition.

- `:since: x.y` – Allows the documenting of “Since” information, which is displayed in the signature bar.

- `:ifcond: CONDITION` – Allows the documenting of conditional availability information, which is displayed in an eyecatch just below the signature bar.

- `:deprecated:` – Adds an eyecatch just below the signature bar that advertises that this definition is deprecated and should be avoided.

- `:unstable:` – Adds an eyecatch just below the signature bar that advertises that this definition is unstable and should not be used in production code.

### qapi:namespace

The `qapi:namespace` directive marks the start of a QAPI namespace. It does not take a content body, nor any options. All subsequent QAPI directives are associated with the most recent namespace. This affects the definition’s “fully qualified name”, allowing two different namespaces to create an otherwise identically named definition.

This directive also influences how reference resolution works for any references that do not explicitly specify a namespace, so this directive can be used to nudge references into preferring targets from within that namespace.

Example:

    .. qapi:namespace:: QMP

This directive has no visible effect.

### qapi:module

The `qapi:module` directive marks the start of a QAPI module. It may have a content body, but it can be omitted. All subsequent QAPI directives are associated with the most recent module; this effects their “fully qualified” name, but has no other effect.

Example:

    .. qapi:module:: block-core

       Welcome to the block-core module!

Will be rendered as:

Welcome to the block-core module!

### qapi:command

This directive documents a QMP command. It may use any of the standard Sphinx or QAPI options, and the documentation body may contain `:arg:`, `:feat:`, `:error:`, or `:return:` info field list entries.

Example:

    .. qapi:command:: x-fake-command
       :since: 42.0
       :unstable:

       This command is fake, so it can't hurt you!

       :arg int foo: Your favorite number.
       :arg string? bar: Your favorite season.
       :return [string]: A lovely computer-written poem for you.

Will be rendered as:

> Command x-fake-command (Since: 42.0)
>   This command is unstable/experimental.

>   This command is fake, so it can’t hurt you!

>   Arguments:

>     - foo (`int`) – Your favorite number.

>     - bar (`string`, optional) – Your favorite season.

>   Return:

>     `[``string``]` – A lovely computer-written poem for you.

### qapi:event

This directive documents a QMP event. It may use any of the standard Sphinx or QAPI options, and the documentation body may contain `:memb:` or `:feat:` info field list entries.

Example:

    .. qapi:event:: COMPUTER_IS_RUINED
       :since: 0.1
       :deprecated:

       This event is emitted when your computer is *extremely* ruined.

       :memb string reason: Diagnostics as to what caused your computer to
          be ruined.
       :feat sadness: When present, the diagnostic message will also
          explain how sad the computer is as a result of your wrongdoings.

Will be rendered as:

Event COMPUTER_IS_RUINED (Since: 0.1)
  This event is deprecated.

  This event is emitted when your computer is extremely ruined.

  Members:

    - reason (`string`) – Diagnostics as to what caused your computer to be ruined.

  Features:

    - sadness – When present, the diagnostic message will also explain how sad the computer is as a result of your wrongdoings.

### qapi:enum

This directive documents a QAPI enum. It may use any of the standard Sphinx or QAPI options, and the documentation body may contain `:value:` or `:feat:` info field list entries.

Example:

    .. qapi:enum:: Mood
       :ifcond: LIB_PERSONALITY

       This enum represents your virtual machine's current mood!

       :value Happy: Your VM is content and well-fed.
       :value Hungry: Your VM needs food.
       :value Melancholic: Your VM is experiencing existential angst.
       :value Petulant: Your VM is throwing a temper tantrum.

Will be rendered as:

Enum Mood
  Availability: `LIB_PERSONALITY`

  This enum represents your virtual machine’s current mood!

  Values:

    - Happy – Your VM is content and well-fed.

    - Hungry – Your VM needs food.

    - Melancholic – Your VM is experiencing existential angst.

    - Petulant – Your VM is throwing a temper tantrum.

### qapi:object

This directive documents a QAPI structure or union and represents a QMP object. It may use any of the standard Sphinx or QAPI options, and the documentation body may contain `:memb:` or `:feat:` info field list entries.

Example:

    .. qapi:object:: BigBlobOfStuff

       This object has a bunch of disparate and unrelated things in it.

       :memb int Birthday: Your birthday, represented in seconds since the
                           UNIX epoch.
       :memb [string] Fav-Foods: A list of your favorite foods.
       :memb boolean? Bizarre-Docs: True if the documentation reference
          should be strange.

Will be rendered as:

Object BigBlobOfStuff

  This object has a bunch of disparate and unrelated things in it.

  Members:

    - Birthday (`int`) – Your birthday, represented in seconds since the UNIX epoch.

    - Fav-Foods (`[``string``]`) – A list of your favorite foods.

    - Bizarre-Docs (`boolean`, optional) – True if the documentation reference should be strange.

### qapi:alternate

This directive documents a QAPI alternate. It may use any of the standard Sphinx or QAPI options, and the documentation body may contain `:alt:` or `:feat:` info field list entries.

Example:

    .. qapi:alternate:: ErrorCode

       This alternate represents an Error Code from the VM.

       :alt int ec: An error code, like the type you're used to.
       :alt string em: An expletive-laced error message, if your
          computer is feeling particularly cranky and tired of your
          antics.

Will be rendered as:

Alternate ErrorCode

  This alternate represents an Error Code from the VM.

  Alternatives:

    - ec (`int`) – An error code, like the type you’re used to.

    - em (`string`) – An expletive-laced error message, if your computer is feeling particularly cranky and tired of your antics.