[Qemu-devel] [RFC] docs: Add writing-qmp-commands.txt

[Luiz Capitulino]

Explains how to write QMP commands using the QAPI.

TODO:
    - write "returning lists" chapter
    - review it

Signed-off-by: Luiz Capitulino <address@hidden>
---

This is incomplete, but I figured I should send it anyway as there are people
who want to add new QMP commands but are still using the old interface. Review
is really appreciated.

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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,
+which do QMP calls.
+
+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/qapi-code-gen.txt. For documentation about the QMP protocol, please
+check the files in QMP/.
+
+== Overview ==
+
+Generally speaking, the following steps should be taken in order to write a
+new QMP command.
+
+1. Write the command and type(s) specification in the QAPI schema file
+   (qapi-schema.json in the root directory)
+
+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 qmp.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 commands implementations in the next sections, the test setup is
+the same and is shown here.
+
+First, QEMU should be started as:
+
+# /path/to/your/source/qemu [...] \
+    -chardev socket,id=qmp,port=4444,host=localhost,server \
+    -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 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 suggested
+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 to add the following line to the bottom of the
+qapi-schema.json file:
+
+{ 'command': 'hello-world' }
+
+This 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 add this to qmp.c:
+
+void qmp_hello_world(Error **errp)
+{
+    printf("Hello, world!\n");
+}
+
+There are a few things to be noted:
+
+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
+
+Now a little hack is needed. As we're still using the old QMP server we need
+to add the new command to its internal dispatch table. This step won't be
+required in the near future. Open the qmp-commands.hx file and add the
+following in the botton:
+
+    {
+        .name       = "hello-world",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_marshal_input_hello_world,
+    },
+
+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 change 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 a Python dictionary 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. In this case
+it's a string. The QAPI also supports 'int' for integers and user defined 
types.
+
+Now, let's update our C implementation in qmp.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 noted:
+
+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.
+   In other words, the arguments must be in the same order of the arguments
+   defined in the 'data' dictionary entry in the schema file
+
+The last step is to update the qmp-commands.hx file:
+
+    {
+        .name       = "hello-world",
+        .args_type  = "message:s?",
+        .mhandler.cmd_new = qmp_marshal_input_hello_world,
+    },
+
+Notice that the "args_type" member got our "message" argument. The character
+"s" stands for "string" and "?" means it's optional. This too must be ordered
+according to the C implementation and schema file. You can look for more
+examples in the qmp-commands.hx file if you need to define more arguments.
+
+Again, this step won't be required in the future.
+
+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. The basic function used to set an error is the error_set() one.
+
+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 the return the
+InvalidParameter error.
+
+Only one change is required, and it's in the C implementation:
+
+void qmp_hello_world(bool has_message, const char *message, Error **errp)
+{
+    if (has_message) {
+        if (strstr(message, "love")) {
+            error_set(errp, QERR_INVALID_PARAMETER, "message");
+            return;
+        }
+        printf("%s\n", message);
+    } else {
+        printf("Hello, world\n");
+    }
+}
+
+Let's test it. Build qemu, run it as defined in the "Testing" section, and
+then issue the following command:
+
+{ "execute": "hello-world", "arguments": { "message": "we love qemu" } }
+
+The QMP server's response should be:
+
+{
+    "error": {
+        "class": "InvalidParameter",
+        "desc": "Invalid parameter 'message'",
+        "data": {
+            "name": "message"
+        }
+    }
+}
+
+Which is the InvalidParameter parameter error.
+
+When you have to return an error but you're unsure what error to return or
+which arguments an error takes, you should look at the qerror.h file. Note
+that you might be required to add new errors if needed.
+
+FIXME: describe better the error API and how to add new errors.
+
+=== Command Documentation ===
+
+There's only one step missing to make 'hello-world's implementation complete,
+and that's its documentation in the schema file.
+
+This is very important. No QMP command will be accepted in QEMU without proper
+documentation.
+
+There are many examples of such documentation in the schema file already, but
+here goes 'hello-world's new entry for the qapi-schema.json file:
+
+##
+# @hello-world
+#
+# Print a client provided string to the standard output stream.
+#
+# @message: #optional string to be printed
+#
+# Returns: Nothing on success.
+#          If @message contains "love", InvalidParameter
+#
+# 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
+HMP commands are simple wrappers. All HMP commands implementation exist in
+the hmp.c file.
+
+Here's the implementation of the 'hello-world' HMP command:
+
+void hmp_hello_world(Monitor *mon, const QDict *qdict)
+{
+    Error *errp = NULL;
+    const char *message = qdict_get_str(qdict, "message");
+
+    qmp_hello_world(!!message, message, &errp);
+    if (error_is_set(&errp)) {
+        monitor_printf(mon, "%s\n", error_get_pretty(errp));
+        error_free(errp);
+        return;
+    }
+}
+
+Also, you have to add the function's prototype to the hmp.h file.
+
+There are three important points to be noted:
+
+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() returned
+3. The 'errp' variable must be initialized to NULL
+
+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",
+        .mhandler.cmd = hmp_hello_world,
+    },
+
+STEXI
address@hidden hello_world @var{message}
address@hidden hello_world
+Print message to the standard output
+ETEXI
+
+To test this you need 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.
+
+== Writing a command that returns data ==
+
+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's name.
+#
+# @next-deadline: #optional 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 QAPI type. Its 'data' dictionary 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() command, 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) {
+        clock->has_next_deadline = true;
+        clock->next_deadline = deadline;
+    }
+    clock->clock_name = g_strdup(alarm_timer->name);
+
+    return clock;
+}
+
+There are five things to be noticed here:
+
+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 recomended for all QAPI types, as
+   it avoid 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 in the example
+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
+
+The last step is to add the correspoding entry in the qmp-commands.hx file:
+
+    {
+        .name       = "qmp-alarm-clock",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_marshal_input_query_alarm_clock,
+    }
+
+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, const QDict *qdict)
+{
+    QemuAlarmClock *clock;
+
+    clock = qmp_query_alarm_clock(NULL);
+    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); 
+}
+
+The most important thing to note about hmp_info_alarm_clock() is that HMP
+functions have to use the qapi_free_QAPI_TYPE() function (provided by the QAPI)
+to free the data returned by the QMP functions.
+
+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.c file. The entry for the "info alarmclock" follows:
+
+    {
+        .name       = "alarmclock",
+        .args_type  = "",
+        .params     = "",
+        .help       = "show information about the alarm clock",
+        .mhandler.info = hmp_info_alarm_clock,
+    },
+
+=== Returning Lists ===
-- 
1.7.8.rc0.32.g87bf9.dirty