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

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On 11/03/2011 01:36 PM, Luiz Capitulino wrote:
> Explains how to write QMP commands using the QAPI.
>
> TODO:
>      - write "returning lists" chapter
>      - review it
>
> Signed-off-by: Luiz Capitulino<lcapitulino@redhat.com>
> ---
>
> 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.
>
>   docs/writing-qmp-commands.txt |  488 +++++++++++++++++++++++++++++++++++++++++
>   1 files changed, 488 insertions(+), 0 deletions(-)
>   create mode 100644 docs/writing-qmp-commands.txt
>
> diff --git a/docs/writing-qmp-commands.txt b/docs/writing-qmp-commands.txt
> new file mode 100644
> index 0000000..26c8d15
> --- /dev/null
> +++ b/docs/writing-qmp-commands.txt
> @@ -0,0 +1,488 @@
> += 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
> +@item hello_world @var{message}
> +@findex 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);

For clarity, and to discourage bad practices, it might be best to pass 
in an Error* here and do the error check, then maybe add a note that if 
there's guaranteed to be no error set by the qmp command, you could pass 
in a NULL instead to avoid the check.

>
> +    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.
Only in the case of schema-defined dict/list types; if the qmp command 
returned a 'str' type (i.e. char*) you'd just do the normal g_free(), 
for instance. Also worth noting that if the return value is a 
QAPI-defined linked-list (returns: [MyType]), you'd use 
qapi_free_MyTypeList()

>
> +
> +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 ===
...actually I suppose you were saving the list-handling stuff for here :)

Looks good so far. Thanks.