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docs/devel/qapi-code-gen: Improve QAPI schema language doc

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We document the language by giving patterns of valid JSON objects.
The patterns contain placeholders we don't define anywhere; their
names have to speak for themselves.  I guess they do, but I'd prefer a
bit more rigor.  Provide a grammar instead, and rework the text
accordingly.

Documentation for QAPI schema conditionals (commit 967c885108,
6cc32b0e14, 87adbbffd4..3e270dcacc) and feature flags (commit
6a8c0b5102) was bolted on.  The sections documenting types, commands
and events don't mention them.  Section "Features" and "Configuring
the schema" then provide additional syntax for types, commands and
events.  I hate that.  Fix the sections documenting types, commands
and events to provide accurate syntax, and point to "Features" and
"Configuring the schema" for details.

We talk about "(top-level) expressions other than include and pragma".
Adopt more convenient terminology: a (top-level) expression is either
a directive (include or pragma) or a definition (anything else).

Avoid the terms "dictionary" and "key".  Stick to JSON terminology
"object" and "member name" instead.

While there, make spacing more consistent.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-16-armbru@redhat.com>
This commit is contained in:
Markus Armbruster 2019-09-13 22:13:48 +02:00
parent 634c82c163
commit b6c37ebaaf
1 changed files with 366 additions and 218 deletions
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584
docs/devel/qapi-code-gen.txt
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@ -4,12 +4,12 @@ Copyright IBM Corp. 2011
Copyright (C) 2012-2016 Red Hat, Inc. Copyright (C) 2012-2016 Red Hat, Inc.
This work is licensed under the terms of the GNU GPL, version 2 or This work is licensed under the terms of the GNU GPL, version 2 or
later. See the COPYING file in the top-level directory. later. See the COPYING file in the top-level directory.
== Introduction == == Introduction ==
QAPI is a native C API within QEMU which provides management-level QAPI is a native C API within QEMU which provides management-level
functionality to internal and external users. For external functionality to internal and external users. For external
users/processes, this interface is made available by a JSON-based wire users/processes, this interface is made available by a JSON-based wire
format for the QEMU Monitor Protocol (QMP) for controlling qemu, as format for the QEMU Monitor Protocol (QMP) for controlling qemu, as
well as the QEMU Guest Agent (QGA) for communicating with the guest. well as the QEMU Guest Agent (QGA) for communicating with the guest.
@ -54,24 +54,49 @@ Differences:
* Numbers are not supported. * Numbers are not supported.
A QAPI schema consists of a series of top-level expressions (JSON A second layer of syntax defines the sequences of JSON texts that are
objects). Code and documentation is generated in schema definition a correctly structured QAPI schema. We provide a grammar for this
order. Code order should not matter. syntax in an EBNF-like notation:
The order of keys within JSON objects does not matter unless * Production rules look like non-terminal = expression
* Concatenation: expression A B matches expression A, then B
* Alternation: expression A | B matches expression A or B
* Repetition: expression A... matches zero or more occurrences of
expression A
* Repetition: expression A, ... matches zero or more occurrences of
expression A separated by ,
* Grouping: expression ( A ) matches expression A
* JSON's structural characters are terminals: { } [ ] : ,
* JSON's literal names are terminals: false true null
* String literals enclosed in 'single quotes' are terminal, and match
this JSON string, with a leading '*' stripped off
* When JSON object member's name starts with '*', the member is
optional.
* The symbol STRING is a terminal, and matches any JSON string
* The symbol BOOL is a terminal, and matches JSON false or true
* ALL-CAPS words other than STRING are non-terminals
The order of members within JSON objects does not matter unless
explicitly noted. explicitly noted.
There are eight kinds of top-level expressions: 'include', 'pragma', A QAPI schema consists of a series of top-level expressions:
'command', 'struct', 'enum', 'union', 'alternate', and 'event'. These
are discussed in detail below.
In the rest of this document, usage lines are given for each SCHEMA = TOP-LEVEL-EXPR...
expression type, with literal strings written in lower case and
placeholders written in capitals. If a literal string includes a The top-level expressions are all JSON objects. Code and
prefix of '*', that key/value pair can be omitted from the expression. documentation is generated in schema definition order. Code order
For example, a usage statement that includes '*base':STRUCT-NAME should not matter.
means that an expression has an optional key 'base', which if present
must have a value that forms a struct name. A top-level expressions is either a directive or a definition:
TOP-LEVEL-EXPR = DIRECTIVE | DEFINITION
There are two kinds of directives and six kinds of definitions:
DIRECTIVE = INCLUDE | PRAGMA
DEFINITION = ENUM | STRUCT | UNION | ALTERNATE | COMMAND | EVENT
These are discussed in detail below.
=== Built-in Types === === Built-in Types ===
@ -101,16 +126,16 @@ The following types are predefined, and map to C as follows:
=== Include directives === === Include directives ===
Usage: { 'include': STRING } Syntax:
INCLUDE = { 'include': STRING }
The QAPI schema definitions can be modularized using the 'include' directive: The QAPI schema definitions can be modularized using the 'include' directive:
{ 'include': 'path/to/file.json' } { 'include': 'path/to/file.json' }
The directive is evaluated recursively, and include paths are relative to the The directive is evaluated recursively, and include paths are relative
file using the directive. Multiple includes of the same file are to the file using the directive. Multiple includes of the same file
idempotent. No other keys should appear in the expression, and the include are idempotent.
value should be a string.
As a matter of style, it is a good idea to have all files be As a matter of style, it is a good idea to have all files be
self-contained, but at the moment, nothing prevents an included file self-contained, but at the moment, nothing prevents an included file
@ -121,10 +146,12 @@ prevent incomplete include files.
=== Pragma directives === === Pragma directives ===
Usage: { 'pragma': DICT } Syntax:
PRAGMA = { 'pragma': { '*doc-required': BOOL,
'*returns-whitelist': [ STRING, ... ],
'*name-case-whitelist': [ STRING, ... ] } }
The pragma directive lets you control optional generator behavior. The pragma directive lets you control optional generator behavior.
The dictionary's entries are pragma names and values.
Pragma's scope is currently the complete schema. Setting the same Pragma's scope is currently the complete schema. Setting the same
pragma to different values in parts of the schema doesn't work. pragma to different values in parts of the schema doesn't work.
@ -141,60 +168,95 @@ rules on use of upper- vs. lower-case letters. Default is none.
=== Enumeration types === === Enumeration types ===
Usage: { 'enum': STRING, 'data': ARRAY-OF-STRING } Syntax:
{ 'enum': STRING, '*prefix': STRING, 'data': ARRAY-OF-STRING } ENUM = { 'enum': STRING,
'data': [ ENUM-VALUE, ... ],
'*prefix': STRING,
'*if': COND }
ENUM-VALUE = STRING
| { 'name': STRING, '*if': COND }
An enumeration type is a dictionary containing a single 'data' key Member 'enum' names the enum type.
whose value is a list of strings. An example enumeration is:
Each member of the 'data' array defines a value of the enumeration
type. The form STRING is shorthand for { 'name': STRING }. The
'name' values must be be distinct.
Example:
{ 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] } { 'enum': 'MyEnum', 'data': [ 'value1', 'value2', 'value3' ] }
Nothing prevents an empty enumeration, although it is probably not Nothing prevents an empty enumeration, although it is probably not
useful. The list of strings should be lower case; if an enum name useful.
represents multiple words, use '-' between words. The string 'max' is
not allowed as an enum value, and values should not be repeated.
The enum constants will be named by using a heuristic to turn the On the wire, an enumeration type's value is represented by its
type name into a set of underscore separated words. For the example (string) name. In C, it's represented by an enumeration constant.
above, 'MyEnum' will turn into 'MY_ENUM' giving a constant name These are of the form PREFIX_NAME, where PREFIX is derived from the
of 'MY_ENUM_VALUE1' for the first value. If the default heuristic enumeration type's name, and NAME from the value's name. For the
does not result in a desirable name, the optional 'prefix' member example above, the generator maps 'MyEnum' to MY_ENUM and 'value1' to
can be used when defining the enum. VALUE1, resulting in the enumeration constant MY_ENUM_VALUE1. The
optional 'prefix' member overrides PREFIX.
The enumeration values are passed as strings over the Client JSON The generated C enumeration constants have values 0, 1, ..., N-1 (in
Protocol, but are encoded as C enum integral values in generated code. QAPI schema order), where N is the number of values. There is an
While the C code starts numbering at 0, it is better to use explicit additional enumeration constant PREFIX__MAX with value N.
comparisons to enum values than implicit comparisons to 0; the C code
will also include a generated enum member ending in _MAX for tracking Do not use string or an integer type when an enumeration type can do
the size of the enum, useful when using common functions for the job satisfactorily.
converting between strings and enum values.
The optional 'if' member specifies a conditional. See "Configuring
the schema" below for more on this.
=== Type references and array types ===
Syntax:
TYPE-REF = STRING | ARRAY-TYPE
ARRAY-TYPE = [ STRING ]
A string denotes the type named by the string.
A one-element array containing a string denotes an array of the type
named by the string. Example: ['int'] denotes an array of 'int'.
For any struct that has a member that will only contain a finite set
of string values, using an enum type for that member is better than
open-coding the member to be type 'str'.
=== Struct types === === Struct types ===
Usage: { 'struct': STRING, 'data': DICT, '*base': STRUCT-NAME } Syntax:
STRUCT = { 'struct': STRING,
'data': MEMBERS,
'*base': STRING,
'*if': COND,
'*features': FEATURES }
MEMBERS = { MEMBER, ... }
MEMBER = STRING : TYPE-REF
| STRING : { 'type': TYPE-REF, '*if': COND }
A struct is a dictionary containing a single 'data' key whose value is Member 'struct' names the struct type.
a dictionary; the dictionary may be empty. This corresponds to a
struct in C or an Object in JSON. Each value of the 'data' dictionary Each MEMBER of the 'data' object defines a member of the struct type.
must be the name of a type, or a one-element array containing a type
name. An example of a struct is: The MEMBER's STRING name consists of an optional '*' prefix and the
struct member name. If '*' is present, the member is optional.
The MEMBER's value defines its properties, in particular its type.
The form TYPE-REF is shorthand for { 'type': TYPE-REF }.
Example:
{ 'struct': 'MyType', { 'struct': 'MyType',
'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } } 'data': { 'member1': 'str', 'member2': ['int'], '*member3': 'str' } }
The use of '*' as a prefix to the name means the member is optional in A struct type corresponds to a struct in C, and an object in JSON.
the corresponding JSON protocol usage. The C struct's members are generated in QAPI schema order.
A struct definition can specify another struct as its base. The optional 'base' member names a struct type whose members are to be
In this case, the members of the base type are included as top-level members included in this type. They go first in the C struct.
of the new struct's dictionary in the Client JSON Protocol wire
format. An example definition is:
{ 'struct': 'BlockdevOptionsGenericFormat', 'data': { 'file': 'str' } } Example:
{ 'struct': 'BlockdevOptionsGenericFormat',
'data': { 'file': 'str' } }
{ 'struct': 'BlockdevOptionsGenericCOWFormat', { 'struct': 'BlockdevOptionsGenericCOWFormat',
'base': 'BlockdevOptionsGenericFormat', 'base': 'BlockdevOptionsGenericFormat',
'data': { '*backing': 'str' } } 'data': { '*backing': 'str' } }
@ -205,18 +267,40 @@ both members like this:
{ "file": "/some/place/my-image", { "file": "/some/place/my-image",
"backing": "/some/place/my-backing-file" } "backing": "/some/place/my-backing-file" }
The optional 'if' member specifies a conditional. See "Configuring
the schema" below for more on this.
The optional 'features' member specifies features. See "Features"
below for more on this.
=== Union types === === Union types ===
Usage: { 'union': STRING, 'data': DICT } Syntax:
or: { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME-OR-DICT, UNION = { 'union': STRING,
'discriminator': ENUM-MEMBER-OF-BASE } 'data': BRANCHES,
'*if': COND }
| { 'union': STRING,
'data': BRANCHES,
'base': ( MEMBERS | STRING ),
'discriminator': STRING,
'*if': COND }
BRANCHES = { BRANCH, ... }
BRANCH = STRING : TYPE-REF
| STRING : { 'type': TYPE-REF, '*if': COND }
Union types are used to let the user choose between several different Member 'union' names the union type.
variants for an object. There are two flavors: simple (no
discriminator or base), and flat (both discriminator and base). A union There are two flavors of union types: simple (no discriminator or
type is defined using a data dictionary as explained in the following base), and flat (both discriminator and base).
paragraphs. Unions must have at least one branch.
Each BRANCH of the 'data' object defines a branch of the union. A
union must have at least one branch.
The BRANCH's STRING name is the branch name.
The BRANCH's value defines the branch's properties, in particular its
type. The form TYPE-REF is shorthand for { 'type': TYPE-REF }.
A simple union type defines a mapping from automatic discriminator A simple union type defines a mapping from automatic discriminator
values to data types like in this example: values to data types like in this example:
@ -229,8 +313,8 @@ values to data types like in this example:
'data': { 'file': 'BlockdevOptionsFile', 'data': { 'file': 'BlockdevOptionsFile',
'qcow2': 'BlockdevOptionsQcow2' } } 'qcow2': 'BlockdevOptionsQcow2' } }
In the Client JSON Protocol, a simple union is represented by a In the Client JSON Protocol, a simple union is represented by an
dictionary that contains the 'type' member as a discriminator, and a object that contains the 'type' member as a discriminator, and a
'data' member that is of the specified data type corresponding to the 'data' member that is of the specified data type corresponding to the
discriminator value, as in these examples: discriminator value, as in these examples:
@ -238,21 +322,27 @@ discriminator value, as in these examples:
{ "type": "qcow2", "data": { "backing": "/some/place/my-image", { "type": "qcow2", "data": { "backing": "/some/place/my-image",
"lazy-refcounts": true } } "lazy-refcounts": true } }
The generated C code uses a struct containing a union. Additionally, The generated C code uses a struct containing a union. Additionally,
an implicit C enum 'NameKind' is created, corresponding to the union an implicit C enum 'NameKind' is created, corresponding to the union
'Name', for accessing the various branches of the union. The value 'Name', for accessing the various branches of the union. The value
for each branch can be of any type. for each branch can be of any type.
A flat union definition avoids nesting on the wire, and specifies a Flat unions permit arbitrary common members that occur in all variants
set of common members that occur in all variants of the union. The of the union, not just a discriminator. Their discriminators need not
'base' key must specify either a type name (the type must be a be named 'type'. They also avoid nesting on the wire.
struct, not a union), or a dictionary representing an anonymous type.
All branches of the union must be struct types, and the top-level The 'base' member defines the common members. If it is a MEMBERS
members of the union dictionary on the wire will be combination of object, it defines common members just like a struct type's 'data'
members from both the base type and the appropriate branch type (when member defines struct type members. If it is a STRING, it names a
merging two dictionaries, there must be no keys in common). The struct type whose members are the common members.
'discriminator' member must be the name of a non-optional enum-typed
member of the base struct. All flat union branches must be of struct type.
In the Client JSON Protocol, a flat union is represented by an object
with the common members (from the base type) and the selected branch's
members. The two sets of member names must be disjoint. Member
'discriminator' must name a non-optional enum-typed member of the base
struct.
The following example enhances the above simple union example by The following example enhances the above simple union example by
adding an optional common member 'read-only', renaming the adding an optional common member 'read-only', renaming the
@ -276,12 +366,13 @@ Resulting in these JSON objects:
Notice that in a flat union, the discriminator name is controlled by Notice that in a flat union, the discriminator name is controlled by
the user, but because it must map to a base member with enum type, the the user, but because it must map to a base member with enum type, the
code generator ensures that branches match the existing values of the code generator ensures that branches match the existing values of the
enum. The order of the keys need not match the declaration of the enum. enum. The order of branches need not match the order of the enum
The keys need not cover all possible enum values. Omitted enum values values. The branches need not cover all possible enum values.
are still valid branches that add no additional members to the data type. Omitted enum values are still valid branches that add no additional
In the resulting generated C data types, a flat union is members to the data type. In the resulting generated C data types, a
represented as a struct with the base members included directly, and flat union is represented as a struct with the base members in QAPI
then a union of structures for each branch of the struct. schema order, and then a union of structures for each branch of the
struct.
A simple union can always be re-written as a flat union where the base A simple union can always be re-written as a flat union where the base
class has a single member named 'type', and where each branch of the class has a single member named 'type', and where each branch of the
@ -297,32 +388,47 @@ is identical on the wire to:
{ 'union': 'Flat': 'base': { 'type': 'Enum' }, 'discriminator': 'type', { 'union': 'Flat': 'base': { 'type': 'Enum' }, 'discriminator': 'type',
'data': { 'one': 'Branch1', 'two': 'Branch2' } } 'data': { 'one': 'Branch1', 'two': 'Branch2' } }
The optional 'if' member specifies a conditional. See "Configuring
the schema" below for more on this.
=== Alternate types === === Alternate types ===
Usage: { 'alternate': STRING, 'data': DICT } Syntax:
ALTERNATE = { 'alternate': STRING,
'data': ALTERNATIVES,
'*if': COND }
ALTERNATIVES = { ALTERNATIVE, ... }
ALTERNATIVE = STRING : TYPE-REF
| STRING : { 'type': STRING, '*if': COND }
An alternate type is one that allows a choice between two or more JSON Member 'alternate' names the alternate type.
data types (string, integer, number, or object, but currently not
array) on the wire. The definition is similar to a simple union type, Each ALTERNATIVE of the 'data' object defines a branch of the
where each branch of the union names a QAPI type. For example: alternate. An alternate must have at least one branch.
The ALTERNATIVE's STRING name is the branch name.
The ALTERNATIVE's value defines the branch's properties, in particular
its type. The form STRING is shorthand for { 'type': STRING }.
Example:
{ 'alternate': 'BlockdevRef', { 'alternate': 'BlockdevRef',
'data': { 'definition': 'BlockdevOptions', 'data': { 'definition': 'BlockdevOptions',
'reference': 'str' } } 'reference': 'str' } }
Unlike a union, the discriminator string is never passed on the wire An alternate type is like a union type, except there is no
for the Client JSON Protocol. Instead, the value's JSON type serves discriminator on the wire. Instead, the branch to use is inferred
as an implicit discriminator, which in turn means that an alternate from the value. An alternate can only express a choice between types
can only express a choice between types represented differently in represented differently on the wire.
JSON. If a branch is typed as the 'bool' built-in, the alternate
accepts true and false; if it is typed as any of the various numeric If a branch is typed as the 'bool' built-in, the alternate accepts
true and false; if it is typed as any of the various numeric
built-ins, it accepts a JSON number; if it is typed as a 'str' built-ins, it accepts a JSON number; if it is typed as a 'str'
built-in or named enum type, it accepts a JSON string; if it is typed built-in or named enum type, it accepts a JSON string; if it is typed
as the 'null' built-in, it accepts JSON null; and if it is typed as a as the 'null' built-in, it accepts JSON null; and if it is typed as a
complex type (struct or union), it accepts a JSON object. Two complex type (struct or union), it accepts a JSON object.
different complex types, for instance, aren't permitted, because both
are represented as a JSON object.
The example alternate declaration above allows using both of the The example alternate declaration above allows using both of the
following example objects: following example objects:
@ -332,43 +438,52 @@ following example objects:
"read-only": false, "read-only": false,
"filename": "/tmp/mydisk.qcow2" } } "filename": "/tmp/mydisk.qcow2" } }
The optional 'if' member specifies a conditional. See "Configuring
the schema" below for more on this.
=== Commands === === Commands ===
--- General Command Layout --- Syntax:
COMMAND = { 'command': STRING,
(
'*data': ( MEMBERS | STRING ),
|
'data': STRING,
'boxed': true,
)
'*returns': TYPE-REF,
'*success-response': false,
'*gen': false,
'*allow-oob': true,
'*allow-preconfig': true,
'*if': COND }
Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT, Member 'command' names the command.
'*returns': TYPE-NAME, '*boxed': true,
'*gen': false, '*success-response': false,
'*allow-oob': true, '*allow-preconfig': true }
Commands are defined by using a dictionary containing several members, Member 'data' defines the arguments. It defaults to an empty MEMBERS
where three members are most common. The 'command' member is a object.
mandatory string, and determines the "execute" value passed in a
Client JSON Protocol command exchange.
The 'data' argument maps to the "arguments" dictionary passed in as If 'data' is a MEMBERS object, then MEMBERS defines arguments just
part of a Client JSON Protocol command. The 'data' member is optional like a struct type's 'data' defines struct type members.
and defaults to {} (an empty dictionary). If present, it must be the
string name of a complex type, or a dictionary that declares an
anonymous type with the same semantics as a 'struct' expression.
The 'returns' member describes what will appear in the "return" member If 'data' is a STRING, then STRING names a complex type whose members
of a Client JSON Protocol reply on successful completion of a command. are the arguments. A union type requires 'boxed': true.
The member is optional from the command declaration; if absent, the
"return" member will be an empty dictionary. If 'returns' is present, Member 'returns' defines the command's return type. It defaults to an
it must be the string name of a complex type, or a empty struct type. It must normally be a complex type or an array of
one-element array containing the name of a complex type. a complex type. To return anything else, the command must be listed
To return anything else, you have to list the command in pragma in pragma 'returns-whitelist'. If you do this, extending the command
'returns-whitelist'. If you do this, the command cannot be extended to return additional information will be harder. Use of
to return additional information in the future. Use of
'returns-whitelist' for new commands is strongly discouraged. 'returns-whitelist' for new commands is strongly discouraged.
All commands in Client JSON Protocol use a dictionary to report A command's error responses are not specified in the QAPI schema.
failure, with no way to specify that in QAPI. Where the error return Error conditions should be documented in comments.
is different than the usual GenericError class in order to help the
client react differently to certain error conditions, it is worth In the Client JSON Protocol, the value of the "execute" or "exec-oob"
documenting this in the comments before the command declaration. member is the command name. The value of the "arguments" member then
has to conform to the arguments, and the value of the success
response's "return" member will conform to the return type.
Some example commands: Some example commands:
@ -386,23 +501,24 @@ which would validate this Client JSON Protocol transaction:
=> { "execute": "my-second-command" } => { "execute": "my-second-command" }
<= { "return": [ { "value": "one" }, { } ] } <= { "return": [ { "value": "one" }, { } ] }
The generator emits a prototype for the user's function implementing The generator emits a prototype for the C function implementing the
the command. Normally, 'data' is a dictionary for an anonymous type, command. The function itself needs to be written by hand. See
or names a struct type (possibly empty, but not a union), and its section "Code generated for commands" for examples.
members are passed as separate arguments to this function. If the
command definition includes a key 'boxed' with the boolean value true, The function returns the return type. When member 'boxed' is absent,
then 'data' is instead the name of any non-empty complex type (struct it takes the command arguments as arguments one by one, in QAPI schema
or union), and a pointer to that QAPI type is passed as a single order. Else it takes them wrapped in the C struct generated for the
argument. complex argument type. It takes an additional Error ** argument in
either case.
The generator also emits a marshalling function that extracts The generator also emits a marshalling function that extracts
arguments for the user's function out of an input QDict, calls the arguments for the user's function out of an input QDict, calls the
user's function, and if it succeeded, builds an output QObject from user's function, and if it succeeded, builds an output QObject from
its return value. its return value. This is for use by the QMP monitor core.
In rare cases, QAPI cannot express a type-safe representation of a In rare cases, QAPI cannot express a type-safe representation of a
corresponding Client JSON Protocol command. You then have to suppress corresponding Client JSON Protocol command. You then have to suppress
generation of a marshalling function by including a key 'gen' with generation of a marshalling function by including a member 'gen' with
boolean value false, and instead write your own function. For boolean value false, and instead write your own function. For
example: example:
@ -416,13 +532,12 @@ use type-safe unions.
Normally, the QAPI schema is used to describe synchronous exchanges, Normally, the QAPI schema is used to describe synchronous exchanges,
where a response is expected. But in some cases, the action of a where a response is expected. But in some cases, the action of a
command is expected to change state in a way that a successful command is expected to change state in a way that a successful
response is not possible (although the command will still return a response is not possible (although the command will still return an
normal dictionary error on failure). When a successful reply is not error object on failure). When a successful reply is not possible,
possible, the command expression includes the optional key the command definition includes the optional member 'success-response'
'success-response' with boolean value false. So far, only QGA makes with boolean value false. So far, only QGA makes use of this member.
use of this member.
Key 'allow-oob' declares whether the command supports out-of-band Member 'allow-oob' declares whether the command supports out-of-band
(OOB) execution. It defaults to false. For example: (OOB) execution. It defaults to false. For example:
{ 'command': 'migrate_recover', { 'command': 'migrate_recover',
@ -455,8 +570,8 @@ other "slow" lock.
When in doubt, do not implement OOB execution support. When in doubt, do not implement OOB execution support.
Key 'allow-preconfig' declares whether the command is available before Member 'allow-preconfig' declares whether the command is available
the machine is built. It defaults to false. For example: before the machine is built. It defaults to false. For example:
{ 'command': 'qmp_capabilities', { 'command': 'qmp_capabilities',
'data': { '*enable': [ 'QMPCapability' ] }, 'data': { '*enable': [ 'QMPCapability' ] },
@ -465,16 +580,33 @@ the machine is built. It defaults to false. For example:
QMP is available before the machine is built only when QEMU was QMP is available before the machine is built only when QEMU was
started with --preconfig. started with --preconfig.
The optional 'if' member specifies a conditional. See "Configuring
the schema" below for more on this.
=== Events === === Events ===
Usage: { 'event': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT, Syntax:
'*boxed': true } EVENT = { 'event': STRING,
(
'*data': ( MEMBERS | STRING ),
|
'data': STRING,
'boxed': true,
)
'*if': COND }
Events are defined with the keyword 'event'. When 'data' is also Member 'event' names the event. This is the event name used in the
specified, additional info will be included in the event, with similar Client JSON Protocol.
semantics to a 'struct' expression. Finally there will be C API
generated in qapi-events.h; when called by QEMU code, a message with Member 'data' defines the event-specific data. It defaults to an
timestamp will be emitted on the wire. empty MEMBERS object.
If 'data' is a MEMBERS object, then MEMBERS defines event-specific
data just like a struct type's 'data' defines struct type members.
If 'data' is a STRING, then STRING names a complex type whose members
are the event-specific data. A union type requires 'boxed': true.
An example event is: An example event is:
@ -487,42 +619,44 @@ Resulting in this JSON object:
"data": { "b": "test string" }, "data": { "b": "test string" },
"timestamp": { "seconds": 1267020223, "microseconds": 435656 } } "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
The generator emits a function to send the event. Normally, 'data' is The generator emits a function to send the event. When member 'boxed'
a dictionary for an anonymous type, or names a struct type (possibly is absent, it takes event-specific data one by one, in QAPI schema
empty, but not a union), and its members are passed as separate order. Else it takes them wrapped in the C struct generated for the
arguments to this function. If the event definition includes a key complex type. See section "Code generated for events" for examples.
'boxed' with the boolean value true, then 'data' is instead the name
of any non-empty complex type (struct or union), and a pointer to that The optional 'if' member specifies a conditional. See "Configuring
QAPI type is passed as a single argument. the schema" below for more on this.
=== Features === === Features ===
Syntax:
FEATURES = [ FEATURE, ... ]
FEATURE = STRING
| { 'name': STRING, '*if': COND }
Sometimes, the behaviour of QEMU changes compatibly, but without a Sometimes, the behaviour of QEMU changes compatibly, but without a
change in the QMP syntax (usually by allowing values or operations that change in the QMP syntax (usually by allowing values or operations
previously resulted in an error). QMP clients may still need to know that previously resulted in an error). QMP clients may still need to
whether the extension is available. know whether the extension is available.
For this purpose, a list of features can be specified for a struct type. For this purpose, a list of features can be specified for a struct type.
This is exposed to the client as a list of string, where each string This is exposed to the client as a list of string, where each string
signals that this build of QEMU shows a certain behaviour. signals that this build of QEMU shows a certain behaviour.
In the schema, features can be specified as simple strings, for example: Each member of the 'features' array defines a feature. It can either
be { 'name': STRING, '*if': COND }, or STRING, which is shorthand for
{ 'name': STRING }.
The optional 'if' member specifies a conditional. See "Configuring
the schema" below for more on this.
Example:
{ 'struct': 'TestType', { 'struct': 'TestType',
'data': { 'number': 'int' }, 'data': { 'number': 'int' },
'features': [ 'allow-negative-numbers' ] } 'features': [ 'allow-negative-numbers' ] }
Another option is to specify features as dictionaries, where the key
'name' specifies the feature string to be exposed to clients:
{ 'struct': 'TestType',
'data': { 'number': 'int' },
'features': [ { 'name': 'allow-negative-numbers' } ] }
This expanded form is necessary if you want to make the feature
conditional (see below in "Configuring the schema").
=== Naming rules and reserved names === === Naming rules and reserved names ===
@ -546,9 +680,8 @@ respectively.
Command names, and member names within a type, should be all lower Command names, and member names within a type, should be all lower
case with words separated by a hyphen. However, some existing older case with words separated by a hyphen. However, some existing older
commands and complex types use underscore; when extending such commands and complex types use underscore; when extending them,
expressions, consistency is preferred over blindly avoiding consistency is preferred over blindly avoiding underscore.
underscore.
Event names should be ALL_CAPS with words separated by underscore. Event names should be ALL_CAPS with words separated by underscore.
@ -578,11 +711,14 @@ downstream command __com.redhat_drive-mirror.
=== Configuring the schema === === Configuring the schema ===
The 'struct', 'enum', 'union', 'alternate', 'command' and 'event' Syntax:
top-level expressions can take an 'if' key. Its value must be a string COND = STRING
or a list of strings. A string is shorthand for a list containing just | [ STRING, ... ]
that string. The code generated for the top-level expression will then
be guarded by #if COND for each COND in the list. All definitions take an optional 'if' member. Its value must be a
string or a list of strings. A string is shorthand for a list
containing just that string. The code generated for the definition
will then be guarded by #if STRING for each STRING in the COND list.
Example: a conditional struct Example: a conditional struct
@ -597,29 +733,33 @@ gets its generated code guarded like this:
#endif /* defined(HAVE_BAR) */ #endif /* defined(HAVE_BAR) */
#endif /* defined(CONFIG_FOO) */ #endif /* defined(CONFIG_FOO) */
Where a member can be defined with a single string value for its type, Individual members of complex types, commands arguments, and
it is also possible to supply a dictionary instead with both 'type' event-specific data can also be made conditional. This requires the
and 'if' keys. longhand form of MEMBER.
Example: a conditional 'bar' member Example: a struct type with unconditional member 'foo' and conditional
member 'bar'
{ 'struct': 'IfStruct', 'data': { 'struct': 'IfStruct', 'data':
{ 'foo': 'int', { 'foo': 'int',
'bar': { 'type': 'int', 'if': 'defined(IFCOND)'} } } 'bar': { 'type': 'int', 'if': 'defined(IFCOND)'} } }
An enum value can be replaced by a dictionary with a 'name' and a 'if' A union's discriminator may not be conditional.
key.
Example: a conditional 'bar' enum member. Likewise, individual enumeration values be conditional. This requires
the longhand form of ENUM-VALUE.
Example: an enum type with unconditional value 'foo' and conditional
value 'bar'
{ 'enum': 'IfEnum', 'data': { 'enum': 'IfEnum', 'data':
[ 'foo', [ 'foo',
{ 'name' : 'bar', 'if': 'defined(IFCOND)' } ] } { 'name' : 'bar', 'if': 'defined(IFCOND)' } ] }
Similarly, features can be specified as a dictionary with a 'name' and Likewise, features can be conditional. This requires the longhand
an 'if' key. form of FEATURE.
Example: a conditional 'allow-negative-numbers' feature Example: a struct with conditional feature 'allow-negative-numbers'
{ 'struct': 'TestType', { 'struct': 'TestType',
'data': { 'number': 'int' }, 'data': { 'number': 'int' },
@ -628,17 +768,30 @@ Example: a conditional 'allow-negative-numbers' feature
Please note that you are responsible to ensure that the C code will Please note that you are responsible to ensure that the C code will
compile with an arbitrary combination of conditions, since the compile with an arbitrary combination of conditions, since the
generators are unable to check it at this point. generator is unable to check it at this point.
The presence of 'if' keys in the schema is reflected through to the The conditions apply to introspection as well, i.e. introspection
introspection output depending on the build configuration. shows a conditional entity only when the condition is satisfied in
this particular build.
=== Documentation comments === === Documentation comments ===
A multi-line comment that starts and ends with a '##' line is a A multi-line comment that starts and ends with a '##' line is a
documentation comment. These are parsed by the documentation documentation comment.
generator, which recognizes certain markup detailed below.
If the documentation comment starts like
##
# @SYMBOL:
it documents the definition if SYMBOL, else it's free-form
documentation.
See below for more on definition documentation.
Free-form documentation may be used to provide additional text and
structuring content.
==== Documentation markup ==== ==== Documentation markup ====
@ -701,23 +854,24 @@ Example:
## ##
==== Expression documentation ==== ==== Definition documentation ====
Expressions other than include and pragma directives may be preceded Definition documentation, if present, must immediately precede the
by a documentation block. Such blocks are called expression definition it documents.
documentation blocks.
When documentation is required (see pragma 'doc-required'), expression When documentation is required (see pragma 'doc-required'), every
documentation blocks are mandatory. definition must have documentation.
The documentation block consists of a first line naming the Definition documentation starts with a line naming the definition,
expression, an optional overview, a description of each argument (for followed by an optional overview, a description of each argument (for
commands and events) or member (for structs, unions and alternates), commands and events), member (for structs and unions), branch (for
and optional tagged sections. alternates), or value (for enums), and finally optional tagged
sections.
FIXME: the parser accepts these things in almost any order. FIXME: the parser accepts these things in almost any order.
FIXME: union branches should be described, too.
Extensions added after the expression was first released carry a Extensions added after the definition was first released carry a
'(since x.y.z)' comment. '(since x.y.z)' comment.
A tagged section starts with one of the following words: A tagged section starts with one of the following words:
@ -725,7 +879,7 @@ A tagged section starts with one of the following words:
The section ends with the start of a new section. The section ends with the start of a new section.
A 'Since: x.y.z' tagged section lists the release that introduced the A 'Since: x.y.z' tagged section lists the release that introduced the
expression. definition.
For example: For example:
@ -771,12 +925,6 @@ For example:
'data': { '*query-nodes': 'bool' }, 'data': { '*query-nodes': 'bool' },
'returns': ['BlockStats'] } 'returns': ['BlockStats'] }
==== Free-form documentation ====
A documentation block that isn't an expression documentation block is
a free-form documentation block. These may be used to provide
additional text and structuring content.
== Client JSON Protocol introspection == == Client JSON Protocol introspection ==
@ -862,7 +1010,7 @@ If the event carries no additional information, "arg-type" names an
object type without members. The event may not have a data member on object type without members. The event may not have a data member on
the wire then. the wire then.
Each command or event defined with dictionary-valued 'data' in the Each command or event defined with 'data' as MEMBERS object in the
QAPI schema implicitly defines an object type. QAPI schema implicitly defines an object type.
Example: the SchemaInfo for EVENT_C from section Events Example: the SchemaInfo for EVENT_C from section Events
@ -1044,7 +1192,7 @@ receive direction compatibility.
Any change to types used in both contexts need to consider both. Any change to types used in both contexts need to consider both.
Members of enumeration types, complex types and alternate types may be Enumeration type values and complex and alternate type members may be
reordered freely. For enumerations and alternate types, this doesn't reordered freely. For enumerations and alternate types, this doesn't
affect the wire encoding. For complex types, this might make the affect the wire encoding. For complex types, this might make the
implementation emit JSON object members in a different order, which implementation emit JSON object members in a different order, which