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diff --git a/src/docs/writing-qmp-commands.txt b/src/docs/writing-qmp-commands.txt new file mode 100644 index 0000000..59aa77a --- /dev/null +++ b/src/docs/writing-qmp-commands.txt @@ -0,0 +1,649 @@ += 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/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's and type(s) specification in the QAPI schema file + (qapi-schema.json in the root source 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 examples 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'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 to add the following line to the bottom of the +qapi-schema.json file: + +{ '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 qmp.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 + +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 at the bottom: + + { + .name = "hello-world", + .args_type = "", + .mhandler.cmd_new = qmp_marshal_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 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 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 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 + +The last step is to update the qmp-commands.hx file: + + { + .name = "hello-world", + .args_type = "message:s?", + .mhandler.cmd_new = qmp_marshal_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. 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" + } +} + +As a general rule, all QMP errors should use ERROR_CLASS_GENERIC_ERROR +(done by default when using error_setg()). There are two exceptions to +this rule: + + 1. A non-generic ErrorClass value exists* for the failure you want to report + (eg. DeviceNotFound) + + 2. Management applications have to take special action on the failure you + want to report, hence you have to add a new ErrorClass value so that they + can check for it + +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. + + * All existing ErrorClass values are defined in the qapi-schema.json file + +=== 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. +# +# 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 hmp.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", + .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 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: #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 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 the "qmp-commands.h" header file in qemu-timer.c, + otherwise qemu won't build + +The last step is to add the correspoding entry in the qmp-commands.hx file: + + { + .name = "query-alarm-clock", + .args_type = "", + .mhandler.cmd_new = qmp_marshal_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) +{ + 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.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, + }, + +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++) { + TimerAlarmMethodList *info = g_malloc0(sizeof(*info)); + info->value = g_malloc0(sizeof(*info->value)); + info->value->method_name = g_strdup(p->name); + info->value->current = current; + + current = false; + + info->next = method_list; + method_list = info; + } + + 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. + +To test this you have to add the corresponding qmp-commands.hx entry: + + { + .name = "query-alarm-methods", + .args_type = "", + .mhandler.cmd_new = qmp_marshal_query_alarm_methods, + }, + +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); +} |
