docs: convert writing-qmp-commands.txt to writing-qmp-commands.rst

This does about the bare minimum, converting section headers to ReST
ones and adding an indent for code blocks.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210721165015.2180311-1-jsnow@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 0 insertions, 597 deletions

diff --git a/docs/devel/writing-qmp-commands.txt b/docs/devel/writing-qmp-commands.txt
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-= How to write QMP commands using the QAPI framework =
-
-This document is a step-by-step guide on how to write new QMP commands using
-the QAPI framework. It also shows how to implement new style HMP commands.
-
-This document doesn't discuss QMP protocol level details, nor does it dive
-into the QAPI framework implementation.
-
-For an in-depth introduction to the QAPI framework, please refer to
-docs/devel/qapi-code-gen.txt. For documentation about the QMP protocol,
-start with docs/interop/qmp-intro.txt.
-
-== Overview ==
-
-Generally speaking, the following steps should be taken in order to write a
-new QMP command.
-
-1. Define the command and any types it needs in the appropriate QAPI
-   schema module.
-
-2. Write the QMP command itself, which is a regular C function. Preferably,
-   the command should be exported by some QEMU subsystem. But it can also be
-   added to the monitor/qmp-cmds.c file
-
-3. At this point the command can be tested under the QMP protocol
-
-4. Write the HMP command equivalent. This is not required and should only be
-   done if it does make sense to have the functionality in HMP. The HMP command
-   is implemented in terms of the QMP command
-
-The following sections will demonstrate each of the steps above. We will start
-very simple and get more complex as we progress.
-
-=== Testing ===
-
-For all the examples in the next sections, the test setup is the same and is
-shown here.
-
-First, QEMU should be started like this:
-
-# qemu-system-TARGET [...] \
-    -chardev socket,id=qmp,port=4444,host=localhost,server=on \
-    -mon chardev=qmp,mode=control,pretty=on
-
-Then, in a different terminal:
-
-$ telnet localhost 4444
-Trying 127.0.0.1...
-Connected to localhost.
-Escape character is '^]'.
-{
-    "QMP": {
-        "version": {
-            "qemu": {
-                "micro": 50, 
-                "minor": 15, 
-                "major": 0
-            }, 
-            "package": ""
-        }, 
-        "capabilities": [
-        ]
-    }
-}
-
-The above output is the QMP server saying you're connected. The server is
-actually in capabilities negotiation mode. To enter in command mode type:
-
-{ "execute": "qmp_capabilities" }
-
-Then the server should respond:
-
-{
-    "return": {
-    }
-}
-
-Which is QMP's way of saying "the latest command executed OK and didn't return
-any data". Now you're ready to enter the QMP example commands as explained in
-the following sections.
-
-== Writing a command that doesn't return data ==
-
-That's the most simple QMP command that can be written. Usually, this kind of
-command carries some meaningful action in QEMU but here it will just print
-"Hello, world" to the standard output.
-
-Our command will be called "hello-world". It takes no arguments, nor does it
-return any data.
-
-The first step is defining the command in the appropriate QAPI schema
-module.  We pick module qapi/misc.json, and add the following line at
-the bottom:
-
-{ 'command': 'hello-world' }
-
-The "command" keyword defines a new QMP command. It's an JSON object. All
-schema entries are JSON objects. The line above will instruct the QAPI to
-generate any prototypes and the necessary code to marshal and unmarshal
-protocol data.
-
-The next step is to write the "hello-world" implementation. As explained
-earlier, it's preferable for commands to live in QEMU subsystems. But
-"hello-world" doesn't pertain to any, so we put its implementation in
-monitor/qmp-cmds.c:
-
-void qmp_hello_world(Error **errp)
-{
-    printf("Hello, world!\n");
-}
-
-There are a few things to be noticed:
-
-1. QMP command implementation functions must be prefixed with "qmp_"
-2. qmp_hello_world() returns void, this is in accordance with the fact that the
-   command doesn't return any data
-3. It takes an "Error **" argument. This is required. Later we will see how to
-   return errors and take additional arguments. The Error argument should not
-   be touched if the command doesn't return errors
-4. We won't add the function's prototype. That's automatically done by the QAPI
-5. Printing to the terminal is discouraged for QMP commands, we do it here
-   because it's the easiest way to demonstrate a QMP command
-
-You're done. Now build qemu, run it as suggested in the "Testing" section,
-and then type the following QMP command:
-
-{ "execute": "hello-world" }
-
-Then check the terminal running qemu and look for the "Hello, world" string. If
-you don't see it then something went wrong.
-
-=== Arguments ===
-
-Let's add an argument called "message" to our "hello-world" command. The new
-argument will contain the string to be printed to stdout. It's an optional
-argument, if it's not present we print our default "Hello, World" string.
-
-The first change we have to do is to modify the command specification in the
-schema file to the following:
-
-{ 'command': 'hello-world', 'data': { '*message': 'str' } }
-
-Notice the new 'data' member in the schema. It's an JSON object whose each
-element is an argument to the command in question. Also notice the asterisk,
-it's used to mark the argument optional (that means that you shouldn't use it
-for mandatory arguments). Finally, 'str' is the argument's type, which
-stands for "string". The QAPI also supports integers, booleans, enumerations
-and user defined types.
-
-Now, let's update our C implementation in monitor/qmp-cmds.c:
-
-void qmp_hello_world(bool has_message, const char *message, Error **errp)
-{
-    if (has_message) {
-        printf("%s\n", message);
-    } else {
-        printf("Hello, world\n");
-    }
-}
-
-There are two important details to be noticed:
-
-1. All optional arguments are accompanied by a 'has_' boolean, which is set
-   if the optional argument is present or false otherwise
-2. The C implementation signature must follow the schema's argument ordering,
-   which is defined by the "data" member
-
-Time to test our new version of the "hello-world" command. Build qemu, run it as
-described in the "Testing" section and then send two commands:
-
-{ "execute": "hello-world" }
-{
-    "return": {
-    }
-}
-
-{ "execute": "hello-world", "arguments": { "message": "We love qemu" } }
-{
-    "return": {
-    }
-}
-
-You should see "Hello, world" and "We love qemu" in the terminal running qemu,
-if you don't see these strings, then something went wrong.
-
-=== Errors ===
-
-QMP commands should use the error interface exported by the error.h header
-file. Basically, most errors are set by calling the error_setg() function.
-
-Let's say we don't accept the string "message" to contain the word "love". If
-it does contain it, we want the "hello-world" command to return an error:
-
-void qmp_hello_world(bool has_message, const char *message, Error **errp)
-{
-    if (has_message) {
-        if (strstr(message, "love")) {
-            error_setg(errp, "the word 'love' is not allowed");
-            return;
-        }
-        printf("%s\n", message);
-    } else {
-        printf("Hello, world\n");
-    }
-}
-
-The first argument to the error_setg() function is the Error pointer
-to pointer, which is passed to all QMP functions. The next argument is a human
-description of the error, this is a free-form printf-like string.
-
-Let's test the example above. Build qemu, run it as defined in the "Testing"
-section, and then issue the following command:
-
-{ "execute": "hello-world", "arguments": { "message": "all you need is love" } }
-
-The QMP server's response should be:
-
-{
-    "error": {
-        "class": "GenericError",
-        "desc": "the word 'love' is not allowed"
-    }
-}
-
-Note that error_setg() produces a "GenericError" class.  In general,
-all QMP errors should have that error class.  There are two exceptions
-to this rule:
-
- 1. To support a management application's need to recognize a specific
-    error for special handling
-
- 2. Backward compatibility
-
-If the failure you want to report falls into one of the two cases above,
-use error_set() with a second argument of an ErrorClass value.
-
-=== Command Documentation ===
-
-There's only one step missing to make "hello-world"'s implementation complete,
-and that's its documentation in the schema file.
-
-There are many examples of such documentation in the schema file already, but
-here goes "hello-world"'s new entry for qapi/misc.json:
-
-##
-# @hello-world:
-#
-# Print a client provided string to the standard output stream.
-#
-# @message: string to be printed
-#
-# Returns: Nothing on success.
-#
-# Notes: if @message is not provided, the "Hello, world" string will
-#        be printed instead
-#
-# Since: <next qemu stable release, eg. 1.0>
-##
-{ 'command': 'hello-world', 'data': { '*message': 'str' } }
-
-Please, note that the "Returns" clause is optional if a command doesn't return
-any data nor any errors.
-
-=== Implementing the HMP command ===
-
-Now that the QMP command is in place, we can also make it available in the human
-monitor (HMP).
-
-With the introduction of the QAPI, HMP commands make QMP calls. Most of the
-time HMP commands are simple wrappers. All HMP commands implementation exist in
-the monitor/hmp-cmds.c file.
-
-Here's the implementation of the "hello-world" HMP command:
-
-void hmp_hello_world(Monitor *mon, const QDict *qdict)
-{
-    const char *message = qdict_get_try_str(qdict, "message");
-    Error *err = NULL;
-
-    qmp_hello_world(!!message, message, &err);
-    if (err) {
-        monitor_printf(mon, "%s\n", error_get_pretty(err));
-        error_free(err);
-        return;
-    }
-}
-
-Also, you have to add the function's prototype to the hmp.h file.
-
-There are three important points to be noticed:
-
-1. The "mon" and "qdict" arguments are mandatory for all HMP functions. The
-   former is the monitor object. The latter is how the monitor passes
-   arguments entered by the user to the command implementation
-2. hmp_hello_world() performs error checking. In this example we just print
-   the error description to the user, but we could do more, like taking
-   different actions depending on the error qmp_hello_world() returns
-3. The "err" variable must be initialized to NULL before performing the
-   QMP call
-
-There's one last step to actually make the command available to monitor users,
-we should add it to the hmp-commands.hx file:
-
-    {
-        .name       = "hello-world",
-        .args_type  = "message:s?",
-        .params     = "hello-world [message]",
-        .help       = "Print message to the standard output",
-        .cmd        = hmp_hello_world,
-    },
-
-STEXI
-@item hello_world @var{message}
-@findex hello_world
-Print message to the standard output
-ETEXI
-
-To test this you have to open a user monitor and issue the "hello-world"
-command. It might be instructive to check the command's documentation with
-HMP's "help" command.
-
-Please, check the "-monitor" command-line option to know how to open a user
-monitor.
-
-== Writing a command that returns data ==
-
-A QMP command is capable of returning any data the QAPI supports like integers,
-strings, booleans, enumerations and user defined types.
-
-In this section we will focus on user defined types. Please, check the QAPI
-documentation for information about the other types.
-
-=== User Defined Types ===
-
-FIXME This example needs to be redone after commit 6d32717
-
-For this example we will write the query-alarm-clock command, which returns
-information about QEMU's timer alarm. For more information about it, please
-check the "-clock" command-line option.
-
-We want to return two pieces of information. The first one is the alarm clock's
-name. The second one is when the next alarm will fire. The former information is
-returned as a string, the latter is an integer in nanoseconds (which is not
-very useful in practice, as the timer has probably already fired when the
-information reaches the client).
-
-The best way to return that data is to create a new QAPI type, as shown below:
-
-##
-# @QemuAlarmClock
-#
-# QEMU alarm clock information.
-#
-# @clock-name: The alarm clock method's name.
-#
-# @next-deadline: The time (in nanoseconds) the next alarm will fire.
-#
-# Since: 1.0
-##
-{ 'type': 'QemuAlarmClock',
-  'data': { 'clock-name': 'str', '*next-deadline': 'int' } }
-
-The "type" keyword defines a new QAPI type. Its "data" member contains the
-type's members. In this example our members are the "clock-name" and the
-"next-deadline" one, which is optional.
-
-Now let's define the query-alarm-clock command:
-
-##
-# @query-alarm-clock
-#
-# Return information about QEMU's alarm clock.
-#
-# Returns a @QemuAlarmClock instance describing the alarm clock method
-# being currently used by QEMU (this is usually set by the '-clock'
-# command-line option).
-#
-# Since: 1.0
-##
-{ 'command': 'query-alarm-clock', 'returns': 'QemuAlarmClock' }
-
-Notice the "returns" keyword. As its name suggests, it's used to define the
-data returned by a command.
-
-It's time to implement the qmp_query_alarm_clock() function, you can put it
-in the qemu-timer.c file:
-
-QemuAlarmClock *qmp_query_alarm_clock(Error **errp)
-{
-    QemuAlarmClock *clock;
-    int64_t deadline;
-
-    clock = g_malloc0(sizeof(*clock));
-
-    deadline = qemu_next_alarm_deadline();
-    if (deadline > 0) {
-        clock->has_next_deadline = true;
-        clock->next_deadline = deadline;
-    }
-    clock->clock_name = g_strdup(alarm_timer->name);
-
-    return clock;
-}
-
-There are a number of things to be noticed:
-
-1. The QemuAlarmClock type is automatically generated by the QAPI framework,
-   its members correspond to the type's specification in the schema file
-2. As specified in the schema file, the function returns a QemuAlarmClock
-   instance and takes no arguments (besides the "errp" one, which is mandatory
-   for all QMP functions)
-3. The "clock" variable (which will point to our QAPI type instance) is
-   allocated by the regular g_malloc0() function. Note that we chose to
-   initialize the memory to zero. This is recommended for all QAPI types, as
-   it helps avoiding bad surprises (specially with booleans)
-4. Remember that "next_deadline" is optional? All optional members have a
-   'has_TYPE_NAME' member that should be properly set by the implementation,
-   as shown above
-5. Even static strings, such as "alarm_timer->name", should be dynamically
-   allocated by the implementation. This is so because the QAPI also generates
-   a function to free its types and it cannot distinguish between dynamically
-   or statically allocated strings
-6. You have to include "qapi/qapi-commands-misc.h" in qemu-timer.c
-
-Time to test the new command. Build qemu, run it as described in the "Testing"
-section and try this:
-
-{ "execute": "query-alarm-clock" }
-{
-    "return": {
-        "next-deadline": 2368219,
-        "clock-name": "dynticks"
-    }
-}
-
-==== The HMP command ====
-
-Here's the HMP counterpart of the query-alarm-clock command:
-
-void hmp_info_alarm_clock(Monitor *mon)
-{
-    QemuAlarmClock *clock;
-    Error *err = NULL;
-
-    clock = qmp_query_alarm_clock(&err);
-    if (err) {
-        monitor_printf(mon, "Could not query alarm clock information\n");
-        error_free(err);
-        return;
-    }
-
-    monitor_printf(mon, "Alarm clock method in use: '%s'\n", clock->clock_name);
-    if (clock->has_next_deadline) {
-        monitor_printf(mon, "Next alarm will fire in %" PRId64 " nanoseconds\n",
-                       clock->next_deadline);
-    }
-
-   qapi_free_QemuAlarmClock(clock); 
-}
-
-It's important to notice that hmp_info_alarm_clock() calls
-qapi_free_QemuAlarmClock() to free the data returned by qmp_query_alarm_clock().
-For user defined types, the QAPI will generate a qapi_free_QAPI_TYPE_NAME()
-function and that's what you have to use to free the types you define and
-qapi_free_QAPI_TYPE_NAMEList() for list types (explained in the next section).
-If the QMP call returns a string, then you should g_free() to free it.
-
-Also note that hmp_info_alarm_clock() performs error handling. That's not
-strictly required if you're sure the QMP function doesn't return errors, but
-it's good practice to always check for errors.
-
-Another important detail is that HMP's "info" commands don't go into the
-hmp-commands.hx. Instead, they go into the info_cmds[] table, which is defined
-in the monitor/misc.c file. The entry for the "info alarmclock" follows:
-
-    {
-        .name       = "alarmclock",
-        .args_type  = "",
-        .params     = "",
-        .help       = "show information about the alarm clock",
-        .cmd        = hmp_info_alarm_clock,
-    },
-
-To test this, run qemu and type "info alarmclock" in the user monitor.
-
-=== Returning Lists ===
-
-For this example, we're going to return all available methods for the timer
-alarm, which is pretty much what the command-line option "-clock ?" does,
-except that we're also going to inform which method is in use.
-
-This first step is to define a new type:
-
-##
-# @TimerAlarmMethod
-#
-# Timer alarm method information.
-#
-# @method-name: The method's name.
-#
-# @current: true if this alarm method is currently in use, false otherwise
-#
-# Since: 1.0
-##
-{ 'type': 'TimerAlarmMethod',
-  'data': { 'method-name': 'str', 'current': 'bool' } }
-
-The command will be called "query-alarm-methods", here is its schema
-specification:
-
-##
-# @query-alarm-methods
-#
-# Returns information about available alarm methods.
-#
-# Returns: a list of @TimerAlarmMethod for each method
-#
-# Since: 1.0
-##
-{ 'command': 'query-alarm-methods', 'returns': ['TimerAlarmMethod'] }
-
-Notice the syntax for returning lists "'returns': ['TimerAlarmMethod']", this
-should be read as "returns a list of TimerAlarmMethod instances".
-
-The C implementation follows:
-
-TimerAlarmMethodList *qmp_query_alarm_methods(Error **errp)
-{
-    TimerAlarmMethodList *method_list = NULL;
-    const struct qemu_alarm_timer *p;
-    bool current = true;
-
-    for (p = alarm_timers; p->name; p++) {
-        TimerAlarmMethod *value = g_malloc0(*value);
-        value->method_name = g_strdup(p->name);
-        value->current = current;
-        QAPI_LIST_PREPEND(method_list, value);
-        current = false;
-    }
-
-    return method_list;
-}
-
-The most important difference from the previous examples is the
-TimerAlarmMethodList type, which is automatically generated by the QAPI from
-the TimerAlarmMethod type.
-
-Each list node is represented by a TimerAlarmMethodList instance. We have to
-allocate it, and that's done inside the for loop: the "info" pointer points to
-an allocated node. We also have to allocate the node's contents, which is
-stored in its "value" member. In our example, the "value" member is a pointer
-to an TimerAlarmMethod instance.
-
-Notice that the "current" variable is used as "true" only in the first
-iteration of the loop. That's because the alarm timer method in use is the
-first element of the alarm_timers array. Also notice that QAPI lists are handled
-by hand and we return the head of the list.
-
-Now Build qemu, run it as explained in the "Testing" section and try our new
-command:
-
-{ "execute": "query-alarm-methods" }
-{
-    "return": [
-        {
-            "current": false, 
-            "method-name": "unix"
-        }, 
-        {
-            "current": true, 
-            "method-name": "dynticks"
-        }
-    ]
-}
-
-The HMP counterpart is a bit more complex than previous examples because it
-has to traverse the list, it's shown below for reference:
-
-void hmp_info_alarm_methods(Monitor *mon)
-{
-    TimerAlarmMethodList *method_list, *method;
-    Error *err = NULL;
-
-    method_list = qmp_query_alarm_methods(&err);
-    if (err) {
-        monitor_printf(mon, "Could not query alarm methods\n");
-        error_free(err);
-        return;
-    }
-
-    for (method = method_list; method; method = method->next) {
-        monitor_printf(mon, "%c %s\n", method->value->current ? '*' : ' ',
-                                       method->value->method_name);
-    }
-
-    qapi_free_TimerAlarmMethodList(method_list);
-}