monitor: Split out monitor/qmp.c

Move QMP infrastructure from monitor/misc.c to monitor/qmp.c. This is
code that can be shared for all targets, so compile it only once.

The amount of function and particularly extern variables in
monitor_int.h is probably a bit larger than it needs to be, but this way
no non-trivial code modifications are needed. The interfaces between QMP
and the monitor core can be cleaned up later.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190613153405.24769-11-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[monitor_is_qmp() tidied up to make checkpatch.pl happy,
superfluous #include dropped]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

5 files changed, 449 insertions, 393 deletions

diff --git a/monitor/Makefile.objs b/monitor/Makefile.objs
index a7170af6e1..1037c09ff8 100644
--- a/monitor/Makefile.objs
+++ b/monitor/Makefile.objs
@@ -1,2 +1,3 @@
 obj-y += misc.o
+common-obj-y += qmp.o
 common-obj-y += qmp-cmds.o hmp-cmds.o
diff --git a/monitor/misc.c b/monitor/misc.c
index 3cdbb681fb..824d28d717 100644
--- a/monitor/misc.c
+++ b/monitor/misc.c
@@ -36,7 +36,6 @@
 #include "exec/gdbstub.h"
 #include "net/net.h"
 #include "net/slirp.h"
-#include "chardev/char-io.h"
 #include "chardev/char-mux.h"
 #include "ui/qemu-spice.h"
 #include "sysemu/numa.h"
@@ -58,8 +57,6 @@
 #include "qapi/qmp/qerror.h"
 #include "qapi/qmp/qnum.h"
 #include "qapi/qmp/qstring.h"
-#include "qapi/qmp/qjson.h"
-#include "qapi/qmp/qlist.h"
 #include "qom/object_interfaces.h"
 #include "trace.h"
 #include "trace/control.h"
@@ -81,7 +78,6 @@
 #include "qapi/qapi-introspect.h"
 #include "sysemu/qtest.h"
 #include "sysemu/cpus.h"
-#include "sysemu/iothread.h"
 #include "qemu/cutils.h"
 #include "tcg/tcg.h"
 
@@ -138,52 +134,30 @@ IOThread *mon_iothread;
 /* Bottom half to dispatch the requests received from I/O thread */
 QEMUBH *qmp_dispatcher_bh;
 
-struct QMPRequest {
-    /* Owner of the request */
-    MonitorQMP *mon;
-    /*
-     * Request object to be handled or Error to be reported
-     * (exactly one of them is non-null)
-     */
-    QObject *req;
-    Error *err;
-};
-typedef struct QMPRequest QMPRequest;
-
 /* QMP checker flags */
 #define QMP_ACCEPT_UNKNOWNS 1
 
 /* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
-static QemuMutex monitor_lock;
+QemuMutex monitor_lock;
 static GHashTable *monitor_qapi_event_state;
-static QTAILQ_HEAD(, Monitor) mon_list;
+MonitorList mon_list;
 static bool monitor_destroyed;
 
 /* Protects mon_fdsets */
 static QemuMutex mon_fdsets_lock;
 static QLIST_HEAD(, MonFdset) mon_fdsets;
 
-static int mon_refcount;
+int mon_refcount;
 
 static HMPCommand hmp_cmds[];
 static HMPCommand hmp_info_cmds[];
 
-QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
-
 __thread Monitor *cur_mon;
 
 static void monitor_command_cb(void *opaque, const char *cmdline,
                                void *readline_opaque);
 
 /**
- * Is @mon a QMP monitor?
- */
-static inline bool monitor_is_qmp(const Monitor *mon)
-{
-    return (mon->flags & MONITOR_USE_CONTROL);
-}
-
-/**
  * Is @mon is using readline?
  * Note: not all HMP monitors use readline, e.g., gdbserver has a
  * non-interactive HMP monitor, so readline is not used there.
@@ -241,28 +215,6 @@ int monitor_read_password(MonitorHMP *mon, ReadLineFunc *readline_func,
     }
 }
 
-static void qmp_request_free(QMPRequest *req)
-{
-    qobject_unref(req->req);
-    error_free(req->err);
-    g_free(req);
-}
-
-/* Caller must hold mon->qmp.qmp_queue_lock */
-static void monitor_qmp_cleanup_req_queue_locked(MonitorQMP *mon)
-{
-    while (!g_queue_is_empty(mon->qmp_requests)) {
-        qmp_request_free(g_queue_pop_head(mon->qmp_requests));
-    }
-}
-
-static void monitor_qmp_cleanup_queues(MonitorQMP *mon)
-{
-    qemu_mutex_lock(&mon->qmp_queue_lock);
-    monitor_qmp_cleanup_req_queue_locked(mon);
-    qemu_mutex_unlock(&mon->qmp_queue_lock);
-}
-
 
 static void monitor_flush_locked(Monitor *mon);
 
@@ -322,7 +274,7 @@ void monitor_flush(Monitor *mon)
 }
 
 /* flush at every end of line */
-static int monitor_puts(Monitor *mon, const char *str)
+int monitor_puts(Monitor *mon, const char *str)
 {
     int i;
     char c;
@@ -372,21 +324,6 @@ int monitor_printf(Monitor *mon, const char *fmt, ...)
     return ret;
 }
 
-static void qmp_send_response(MonitorQMP *mon, const QDict *rsp)
-{
-    const QObject *data = QOBJECT(rsp);
-    QString *json;
-
-    json = mon->common.flags & MONITOR_USE_PRETTY ?
-           qobject_to_json_pretty(data) : qobject_to_json(data);
-    assert(json != NULL);
-
-    qstring_append_chr(json, '\n');
-    monitor_puts(&mon->common, qstring_get_str(json));
-
-    qobject_unref(json);
-}
-
 static MonitorQAPIEventConf monitor_qapi_event_conf[QAPI_EVENT__MAX] = {
     /* Limit guest-triggerable events to 1 per second */
     [QAPI_EVENT_RTC_CHANGE]        = { 1000 * SCALE_MS },
@@ -601,8 +538,8 @@ static void handle_hmp_command(MonitorHMP *mon, const char *cmdline);
 
 static void monitor_iothread_init(void);
 
-static void monitor_data_init(Monitor *mon, int flags, bool skip_flush,
-                              bool use_io_thread)
+void monitor_data_init(Monitor *mon, int flags, bool skip_flush,
+                       bool use_io_thread)
 {
     if (use_io_thread && !mon_iothread) {
         monitor_iothread_init();
@@ -614,14 +551,6 @@ static void monitor_data_init(Monitor *mon, int flags, bool skip_flush,
     mon->flags = flags;
 }
 
-static void monitor_data_destroy_qmp(MonitorQMP *mon)
-{
-    json_message_parser_destroy(&mon->parser);
-    qemu_mutex_destroy(&mon->qmp_queue_lock);
-    monitor_qmp_cleanup_req_queue_locked(mon);
-    g_queue_free(mon->qmp_requests);
-}
-
 static void monitor_data_destroy(Monitor *mon)
 {
     g_free(mon->mon_cpu_path);
@@ -1058,18 +987,6 @@ static void monitor_init_qmp_commands(void)
                          qmp_marshal_qmp_capabilities, QCO_ALLOW_PRECONFIG);
 }
 
-static bool qmp_oob_enabled(MonitorQMP *mon)
-{
-    return mon->capab[QMP_CAPABILITY_OOB];
-}
-
-static void monitor_qmp_caps_reset(MonitorQMP *mon)
-{
-    memset(mon->capab_offered, 0, sizeof(mon->capab_offered));
-    memset(mon->capab, 0, sizeof(mon->capab));
-    mon->capab_offered[QMP_CAPABILITY_OOB] = mon->common.use_io_thread;
-}
-
 /*
  * Accept QMP capabilities in @list for @mon.
  * On success, set mon->qmp.capab[], and return true.
@@ -2245,7 +2162,7 @@ static void monitor_fdset_cleanup(MonFdset *mon_fdset)
     }
 }
 
-static void monitor_fdsets_cleanup(void)
+void monitor_fdsets_cleanup(void)
 {
     MonFdset *mon_fdset;
     MonFdset *mon_fdset_next;
@@ -4023,209 +3940,13 @@ cleanup:
     free_cmdline_args(args, nb_args);
 }
 
-static int monitor_can_read(void *opaque)
+int monitor_can_read(void *opaque)
 {
     Monitor *mon = opaque;
 
     return !atomic_mb_read(&mon->suspend_cnt);
 }
 
-/*
- * Emit QMP response @rsp with ID @id to @mon.
- * Null @rsp can only happen for commands with QCO_NO_SUCCESS_RESP.
- * Nothing is emitted then.
- */
-static void monitor_qmp_respond(MonitorQMP *mon, QDict *rsp)
-{
-    if (rsp) {
-        qmp_send_response(mon, rsp);
-    }
-}
-
-static void monitor_qmp_dispatch(MonitorQMP *mon, QObject *req)
-{
-    Monitor *old_mon;
-    QDict *rsp;
-    QDict *error;
-
-    old_mon = cur_mon;
-    cur_mon = &mon->common;
-
-    rsp = qmp_dispatch(mon->commands, req, qmp_oob_enabled(mon));
-
-    cur_mon = old_mon;
-
-    if (mon->commands == &qmp_cap_negotiation_commands) {
-        error = qdict_get_qdict(rsp, "error");
-        if (error
-            && !g_strcmp0(qdict_get_try_str(error, "class"),
-                    QapiErrorClass_str(ERROR_CLASS_COMMAND_NOT_FOUND))) {
-            /* Provide a more useful error message */
-            qdict_del(error, "desc");
-            qdict_put_str(error, "desc", "Expecting capabilities negotiation"
-                          " with 'qmp_capabilities'");
-        }
-    }
-
-    monitor_qmp_respond(mon, rsp);
-    qobject_unref(rsp);
-}
-
-/*
- * Pop a QMP request from a monitor request queue.
- * Return the request, or NULL all request queues are empty.
- * We are using round-robin fashion to pop the request, to avoid
- * processing commands only on a very busy monitor.  To achieve that,
- * when we process one request on a specific monitor, we put that
- * monitor to the end of mon_list queue.
- *
- * Note: if the function returned with non-NULL, then the caller will
- * be with qmp_mon->qmp_queue_lock held, and the caller is responsible
- * to release it.
- */
-static QMPRequest *monitor_qmp_requests_pop_any_with_lock(void)
-{
-    QMPRequest *req_obj = NULL;
-    Monitor *mon;
-    MonitorQMP *qmp_mon;
-
-    qemu_mutex_lock(&monitor_lock);
-
-    QTAILQ_FOREACH(mon, &mon_list, entry) {
-        if (!monitor_is_qmp(mon)) {
-            continue;
-        }
-
-        qmp_mon = container_of(mon, MonitorQMP, common);
-        qemu_mutex_lock(&qmp_mon->qmp_queue_lock);
-        req_obj = g_queue_pop_head(qmp_mon->qmp_requests);
-        if (req_obj) {
-            /* With the lock of corresponding queue held */
-            break;
-        }
-        qemu_mutex_unlock(&qmp_mon->qmp_queue_lock);
-    }
-
-    if (req_obj) {
-        /*
-         * We found one request on the monitor. Degrade this monitor's
-         * priority to lowest by re-inserting it to end of queue.
-         */
-        QTAILQ_REMOVE(&mon_list, mon, entry);
-        QTAILQ_INSERT_TAIL(&mon_list, mon, entry);
-    }
-
-    qemu_mutex_unlock(&monitor_lock);
-
-    return req_obj;
-}
-
-static void monitor_qmp_bh_dispatcher(void *data)
-{
-    QMPRequest *req_obj = monitor_qmp_requests_pop_any_with_lock();
-    QDict *rsp;
-    bool need_resume;
-    MonitorQMP *mon;
-
-    if (!req_obj) {
-        return;
-    }
-
-    mon = req_obj->mon;
-    /*  qmp_oob_enabled() might change after "qmp_capabilities" */
-    need_resume = !qmp_oob_enabled(mon) ||
-        mon->qmp_requests->length == QMP_REQ_QUEUE_LEN_MAX - 1;
-    qemu_mutex_unlock(&mon->qmp_queue_lock);
-    if (req_obj->req) {
-        QDict *qdict = qobject_to(QDict, req_obj->req);
-        QObject *id = qdict ? qdict_get(qdict, "id") : NULL;
-        trace_monitor_qmp_cmd_in_band(qobject_get_try_str(id) ?: "");
-        monitor_qmp_dispatch(mon, req_obj->req);
-    } else {
-        assert(req_obj->err);
-        rsp = qmp_error_response(req_obj->err);
-        req_obj->err = NULL;
-        monitor_qmp_respond(mon, rsp);
-        qobject_unref(rsp);
-    }
-
-    if (need_resume) {
-        /* Pairs with the monitor_suspend() in handle_qmp_command() */
-        monitor_resume(&mon->common);
-    }
-    qmp_request_free(req_obj);
-
-    /* Reschedule instead of looping so the main loop stays responsive */
-    qemu_bh_schedule(qmp_dispatcher_bh);
-}
-
-static void handle_qmp_command(void *opaque, QObject *req, Error *err)
-{
-    MonitorQMP *mon = opaque;
-    QObject *id = NULL;
-    QDict *qdict;
-    QMPRequest *req_obj;
-
-    assert(!req != !err);
-
-    qdict = qobject_to(QDict, req);
-    if (qdict) {
-        id = qdict_get(qdict, "id");
-    } /* else will fail qmp_dispatch() */
-
-    if (req && trace_event_get_state_backends(TRACE_HANDLE_QMP_COMMAND)) {
-        QString *req_json = qobject_to_json(req);
-        trace_handle_qmp_command(mon, qstring_get_str(req_json));
-        qobject_unref(req_json);
-    }
-
-    if (qdict && qmp_is_oob(qdict)) {
-        /* OOB commands are executed immediately */
-        trace_monitor_qmp_cmd_out_of_band(qobject_get_try_str(id) ?: "");
-        monitor_qmp_dispatch(mon, req);
-        qobject_unref(req);
-        return;
-    }
-
-    req_obj = g_new0(QMPRequest, 1);
-    req_obj->mon = mon;
-    req_obj->req = req;
-    req_obj->err = err;
-
-    /* Protect qmp_requests and fetching its length. */
-    qemu_mutex_lock(&mon->qmp_queue_lock);
-
-    /*
-     * Suspend the monitor when we can't queue more requests after
-     * this one.  Dequeuing in monitor_qmp_bh_dispatcher() will resume
-     * it.  Note that when OOB is disabled, we queue at most one
-     * command, for backward compatibility.
-     */
-    if (!qmp_oob_enabled(mon) ||
-        mon->qmp_requests->length == QMP_REQ_QUEUE_LEN_MAX - 1) {
-        monitor_suspend(&mon->common);
-    }
-
-    /*
-     * Put the request to the end of queue so that requests will be
-     * handled in time order.  Ownership for req_obj, req,
-     * etc. will be delivered to the handler side.
-     */
-    assert(mon->qmp_requests->length < QMP_REQ_QUEUE_LEN_MAX);
-    g_queue_push_tail(mon->qmp_requests, req_obj);
-    qemu_mutex_unlock(&mon->qmp_queue_lock);
-
-    /* Kick the dispatcher routine */
-    qemu_bh_schedule(qmp_dispatcher_bh);
-}
-
-static void monitor_qmp_read(void *opaque, const uint8_t *buf, int size)
-{
-    MonitorQMP *mon = opaque;
-
-    json_message_parser_feed(&mon->parser, (const char *) buf, size);
-}
-
 static void monitor_read(void *opaque, const uint8_t *buf, int size)
 {
     MonitorHMP *mon;
@@ -4312,56 +4033,6 @@ void monitor_resume(Monitor *mon)
     trace_monitor_suspend(mon, -1);
 }
 
-static QDict *qmp_greeting(MonitorQMP *mon)
-{
-    QList *cap_list = qlist_new();
-    QObject *ver = NULL;
-    QMPCapability cap;
-
-    qmp_marshal_query_version(NULL, &ver, NULL);
-
-    for (cap = 0; cap < QMP_CAPABILITY__MAX; cap++) {
-        if (mon->capab_offered[cap]) {
-            qlist_append_str(cap_list, QMPCapability_str(cap));
-        }
-    }
-
-    return qdict_from_jsonf_nofail(
-        "{'QMP': {'version': %p, 'capabilities': %p}}",
-        ver, cap_list);
-}
-
-static void monitor_qmp_event(void *opaque, int event)
-{
-    QDict *data;
-    MonitorQMP *mon = opaque;
-
-    switch (event) {
-    case CHR_EVENT_OPENED:
-        mon->commands = &qmp_cap_negotiation_commands;
-        monitor_qmp_caps_reset(mon);
-        data = qmp_greeting(mon);
-        qmp_send_response(mon, data);
-        qobject_unref(data);
-        mon_refcount++;
-        break;
-    case CHR_EVENT_CLOSED:
-        /*
-         * Note: this is only useful when the output of the chardev
-         * backend is still open.  For example, when the backend is
-         * stdio, it's possible that stdout is still open when stdin
-         * is closed.
-         */
-        monitor_qmp_cleanup_queues(mon);
-        json_message_parser_destroy(&mon->parser);
-        json_message_parser_init(&mon->parser, handle_qmp_command,
-                                 mon, NULL);
-        mon_refcount--;
-        monitor_fdsets_cleanup();
-        break;
-    }
-}
-
 static void monitor_event(void *opaque, int event)
 {
     Monitor *mon = opaque;
@@ -4495,7 +4166,7 @@ int error_vprintf_unless_qmp(const char *fmt, va_list ap)
     return -1;
 }
 
-static void monitor_list_append(Monitor *mon)
+void monitor_list_append(Monitor *mon)
 {
     qemu_mutex_lock(&monitor_lock);
     /*
@@ -4515,60 +4186,6 @@ static void monitor_list_append(Monitor *mon)
     }
 }
 
-static void monitor_qmp_setup_handlers_bh(void *opaque)
-{
-    MonitorQMP *mon = opaque;
-    GMainContext *context;
-
-    assert(mon->common.use_io_thread);
-    context = iothread_get_g_main_context(mon_iothread);
-    assert(context);
-    qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
-                             monitor_qmp_read, monitor_qmp_event,
-                             NULL, &mon->common, context, true);
-    monitor_list_append(&mon->common);
-}
-
-static void monitor_init_qmp(Chardev *chr, int flags)
-{
-    MonitorQMP *mon = g_new0(MonitorQMP, 1);
-
-    /* Only HMP supports readline */
-    assert(!(flags & MONITOR_USE_READLINE));
-
-    /* Note: we run QMP monitor in I/O thread when @chr supports that */
-    monitor_data_init(&mon->common, flags, false,
-                      qemu_chr_has_feature(chr, QEMU_CHAR_FEATURE_GCONTEXT));
-
-    qemu_mutex_init(&mon->qmp_queue_lock);
-    mon->qmp_requests = g_queue_new();
-
-    qemu_chr_fe_init(&mon->common.chr, chr, &error_abort);
-    qemu_chr_fe_set_echo(&mon->common.chr, true);
-
-    json_message_parser_init(&mon->parser, handle_qmp_command, mon, NULL);
-    if (mon->common.use_io_thread) {
-        /*
-         * Make sure the old iowatch is gone.  It's possible when
-         * e.g. the chardev is in client mode, with wait=on.
-         */
-        remove_fd_in_watch(chr);
-        /*
-         * We can't call qemu_chr_fe_set_handlers() directly here
-         * since chardev might be running in the monitor I/O
-         * thread.  Schedule a bottom half.
-         */
-        aio_bh_schedule_oneshot(iothread_get_aio_context(mon_iothread),
-                                monitor_qmp_setup_handlers_bh, mon);
-        /* The bottom half will add @mon to @mon_list */
-    } else {
-        qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
-                                 monitor_qmp_read, monitor_qmp_event,
-                                 NULL, &mon->common, NULL, true);
-        monitor_list_append(&mon->common);
-    }
-}
-
 static void monitor_init_hmp(Chardev *chr, int flags)
 {
     MonitorHMP *mon = g_new0(MonitorHMP, 1);
diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
index ebaf0201c8..d859bd3894 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -31,6 +31,7 @@
 #include "qapi/qmp/dispatch.h"
 #include "qapi/qmp/json-parser.h"
 #include "qemu/readline.h"
+#include "sysemu/iothread.h"
 
 /*
  * Supported types:
@@ -142,4 +143,33 @@ typedef struct {
     GQueue *qmp_requests;
 } MonitorQMP;
 
+/**
+ * Is @mon a QMP monitor?
+ */
+static inline bool monitor_is_qmp(const Monitor *mon)
+{
+    return mon->flags & MONITOR_USE_CONTROL;
+}
+
+typedef QTAILQ_HEAD(MonitorList, Monitor) MonitorList;
+extern IOThread *mon_iothread;
+extern QEMUBH *qmp_dispatcher_bh;
+extern QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
+extern QemuMutex monitor_lock;
+extern MonitorList mon_list;
+extern int mon_refcount;
+
+void monitor_init_qmp(Chardev *chr, int flags);
+
+int monitor_puts(Monitor *mon, const char *str);
+void monitor_data_init(Monitor *mon, int flags, bool skip_flush,
+                       bool use_io_thread);
+int monitor_can_read(void *opaque);
+void monitor_list_append(Monitor *mon);
+void monitor_fdsets_cleanup(void);
+
+void qmp_send_response(MonitorQMP *mon, const QDict *rsp);
+void monitor_data_destroy_qmp(MonitorQMP *mon);
+void monitor_qmp_bh_dispatcher(void *data);
+
 #endif
diff --git a/monitor/qmp.c b/monitor/qmp.c
new file mode 100644
index 0000000000..7c3d081a72
--- /dev/null
+++ b/monitor/qmp.c
@@ -0,0 +1,406 @@
+/*
+ * QEMU monitor
+ *
+ * Copyright (c) 2003-2004 Fabrice Bellard
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "qemu/osdep.h"
+
+#include "chardev/char-io.h"
+#include "monitor-internal.h"
+#include "qapi/error.h"
+#include "qapi/qapi-commands-misc.h"
+#include "qapi/qmp/qdict.h"
+#include "qapi/qmp/qjson.h"
+#include "qapi/qmp/qlist.h"
+#include "qapi/qmp/qstring.h"
+#include "trace.h"
+
+struct QMPRequest {
+    /* Owner of the request */
+    MonitorQMP *mon;
+    /*
+     * Request object to be handled or Error to be reported
+     * (exactly one of them is non-null)
+     */
+    QObject *req;
+    Error *err;
+};
+typedef struct QMPRequest QMPRequest;
+
+QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
+
+static bool qmp_oob_enabled(MonitorQMP *mon)
+{
+    return mon->capab[QMP_CAPABILITY_OOB];
+}
+
+static void monitor_qmp_caps_reset(MonitorQMP *mon)
+{
+    memset(mon->capab_offered, 0, sizeof(mon->capab_offered));
+    memset(mon->capab, 0, sizeof(mon->capab));
+    mon->capab_offered[QMP_CAPABILITY_OOB] = mon->common.use_io_thread;
+}
+
+static void qmp_request_free(QMPRequest *req)
+{
+    qobject_unref(req->req);
+    error_free(req->err);
+    g_free(req);
+}
+
+/* Caller must hold mon->qmp.qmp_queue_lock */
+static void monitor_qmp_cleanup_req_queue_locked(MonitorQMP *mon)
+{
+    while (!g_queue_is_empty(mon->qmp_requests)) {
+        qmp_request_free(g_queue_pop_head(mon->qmp_requests));
+    }
+}
+
+static void monitor_qmp_cleanup_queues(MonitorQMP *mon)
+{
+    qemu_mutex_lock(&mon->qmp_queue_lock);
+    monitor_qmp_cleanup_req_queue_locked(mon);
+    qemu_mutex_unlock(&mon->qmp_queue_lock);
+}
+
+void qmp_send_response(MonitorQMP *mon, const QDict *rsp)
+{
+    const QObject *data = QOBJECT(rsp);
+    QString *json;
+
+    json = mon->common.flags & MONITOR_USE_PRETTY ?
+           qobject_to_json_pretty(data) : qobject_to_json(data);
+    assert(json != NULL);
+
+    qstring_append_chr(json, '\n');
+    monitor_puts(&mon->common, qstring_get_str(json));
+
+    qobject_unref(json);
+}
+
+/*
+ * Emit QMP response @rsp with ID @id to @mon.
+ * Null @rsp can only happen for commands with QCO_NO_SUCCESS_RESP.
+ * Nothing is emitted then.
+ */
+static void monitor_qmp_respond(MonitorQMP *mon, QDict *rsp)
+{
+    if (rsp) {
+        qmp_send_response(mon, rsp);
+    }
+}
+
+static void monitor_qmp_dispatch(MonitorQMP *mon, QObject *req)
+{
+    Monitor *old_mon;
+    QDict *rsp;
+    QDict *error;
+
+    old_mon = cur_mon;
+    cur_mon = &mon->common;
+
+    rsp = qmp_dispatch(mon->commands, req, qmp_oob_enabled(mon));
+
+    cur_mon = old_mon;
+
+    if (mon->commands == &qmp_cap_negotiation_commands) {
+        error = qdict_get_qdict(rsp, "error");
+        if (error
+            && !g_strcmp0(qdict_get_try_str(error, "class"),
+                    QapiErrorClass_str(ERROR_CLASS_COMMAND_NOT_FOUND))) {
+            /* Provide a more useful error message */
+            qdict_del(error, "desc");
+            qdict_put_str(error, "desc", "Expecting capabilities negotiation"
+                          " with 'qmp_capabilities'");
+        }
+    }
+
+    monitor_qmp_respond(mon, rsp);
+    qobject_unref(rsp);
+}
+
+/*
+ * Pop a QMP request from a monitor request queue.
+ * Return the request, or NULL all request queues are empty.
+ * We are using round-robin fashion to pop the request, to avoid
+ * processing commands only on a very busy monitor.  To achieve that,
+ * when we process one request on a specific monitor, we put that
+ * monitor to the end of mon_list queue.
+ *
+ * Note: if the function returned with non-NULL, then the caller will
+ * be with qmp_mon->qmp_queue_lock held, and the caller is responsible
+ * to release it.
+ */
+static QMPRequest *monitor_qmp_requests_pop_any_with_lock(void)
+{
+    QMPRequest *req_obj = NULL;
+    Monitor *mon;
+    MonitorQMP *qmp_mon;
+
+    qemu_mutex_lock(&monitor_lock);
+
+    QTAILQ_FOREACH(mon, &mon_list, entry) {
+        if (!monitor_is_qmp(mon)) {
+            continue;
+        }
+
+        qmp_mon = container_of(mon, MonitorQMP, common);
+        qemu_mutex_lock(&qmp_mon->qmp_queue_lock);
+        req_obj = g_queue_pop_head(qmp_mon->qmp_requests);
+        if (req_obj) {
+            /* With the lock of corresponding queue held */
+            break;
+        }
+        qemu_mutex_unlock(&qmp_mon->qmp_queue_lock);
+    }
+
+    if (req_obj) {
+        /*
+         * We found one request on the monitor. Degrade this monitor's
+         * priority to lowest by re-inserting it to end of queue.
+         */
+        QTAILQ_REMOVE(&mon_list, mon, entry);
+        QTAILQ_INSERT_TAIL(&mon_list, mon, entry);
+    }
+
+    qemu_mutex_unlock(&monitor_lock);
+
+    return req_obj;
+}
+
+void monitor_qmp_bh_dispatcher(void *data)
+{
+    QMPRequest *req_obj = monitor_qmp_requests_pop_any_with_lock();
+    QDict *rsp;
+    bool need_resume;
+    MonitorQMP *mon;
+
+    if (!req_obj) {
+        return;
+    }
+
+    mon = req_obj->mon;
+    /*  qmp_oob_enabled() might change after "qmp_capabilities" */
+    need_resume = !qmp_oob_enabled(mon) ||
+        mon->qmp_requests->length == QMP_REQ_QUEUE_LEN_MAX - 1;
+    qemu_mutex_unlock(&mon->qmp_queue_lock);
+    if (req_obj->req) {
+        QDict *qdict = qobject_to(QDict, req_obj->req);
+        QObject *id = qdict ? qdict_get(qdict, "id") : NULL;
+        trace_monitor_qmp_cmd_in_band(qobject_get_try_str(id) ?: "");
+        monitor_qmp_dispatch(mon, req_obj->req);
+    } else {
+        assert(req_obj->err);
+        rsp = qmp_error_response(req_obj->err);
+        req_obj->err = NULL;
+        monitor_qmp_respond(mon, rsp);
+        qobject_unref(rsp);
+    }
+
+    if (need_resume) {
+        /* Pairs with the monitor_suspend() in handle_qmp_command() */
+        monitor_resume(&mon->common);
+    }
+    qmp_request_free(req_obj);
+
+    /* Reschedule instead of looping so the main loop stays responsive */
+    qemu_bh_schedule(qmp_dispatcher_bh);
+}
+
+static void handle_qmp_command(void *opaque, QObject *req, Error *err)
+{
+    MonitorQMP *mon = opaque;
+    QObject *id = NULL;
+    QDict *qdict;
+    QMPRequest *req_obj;
+
+    assert(!req != !err);
+
+    qdict = qobject_to(QDict, req);
+    if (qdict) {
+        id = qdict_get(qdict, "id");
+    } /* else will fail qmp_dispatch() */
+
+    if (req && trace_event_get_state_backends(TRACE_HANDLE_QMP_COMMAND)) {
+        QString *req_json = qobject_to_json(req);
+        trace_handle_qmp_command(mon, qstring_get_str(req_json));
+        qobject_unref(req_json);
+    }
+
+    if (qdict && qmp_is_oob(qdict)) {
+        /* OOB commands are executed immediately */
+        trace_monitor_qmp_cmd_out_of_band(qobject_get_try_str(id) ?: "");
+        monitor_qmp_dispatch(mon, req);
+        qobject_unref(req);
+        return;
+    }
+
+    req_obj = g_new0(QMPRequest, 1);
+    req_obj->mon = mon;
+    req_obj->req = req;
+    req_obj->err = err;
+
+    /* Protect qmp_requests and fetching its length. */
+    qemu_mutex_lock(&mon->qmp_queue_lock);
+
+    /*
+     * Suspend the monitor when we can't queue more requests after
+     * this one.  Dequeuing in monitor_qmp_bh_dispatcher() will resume
+     * it.  Note that when OOB is disabled, we queue at most one
+     * command, for backward compatibility.
+     */
+    if (!qmp_oob_enabled(mon) ||
+        mon->qmp_requests->length == QMP_REQ_QUEUE_LEN_MAX - 1) {
+        monitor_suspend(&mon->common);
+    }
+
+    /*
+     * Put the request to the end of queue so that requests will be
+     * handled in time order.  Ownership for req_obj, req,
+     * etc. will be delivered to the handler side.
+     */
+    assert(mon->qmp_requests->length < QMP_REQ_QUEUE_LEN_MAX);
+    g_queue_push_tail(mon->qmp_requests, req_obj);
+    qemu_mutex_unlock(&mon->qmp_queue_lock);
+
+    /* Kick the dispatcher routine */
+    qemu_bh_schedule(qmp_dispatcher_bh);
+}
+
+static void monitor_qmp_read(void *opaque, const uint8_t *buf, int size)
+{
+    MonitorQMP *mon = opaque;
+
+    json_message_parser_feed(&mon->parser, (const char *) buf, size);
+}
+
+static QDict *qmp_greeting(MonitorQMP *mon)
+{
+    QList *cap_list = qlist_new();
+    QObject *ver = NULL;
+    QMPCapability cap;
+
+    qmp_marshal_query_version(NULL, &ver, NULL);
+
+    for (cap = 0; cap < QMP_CAPABILITY__MAX; cap++) {
+        if (mon->capab_offered[cap]) {
+            qlist_append_str(cap_list, QMPCapability_str(cap));
+        }
+    }
+
+    return qdict_from_jsonf_nofail(
+        "{'QMP': {'version': %p, 'capabilities': %p}}",
+        ver, cap_list);
+}
+
+static void monitor_qmp_event(void *opaque, int event)
+{
+    QDict *data;
+    MonitorQMP *mon = opaque;
+
+    switch (event) {
+    case CHR_EVENT_OPENED:
+        mon->commands = &qmp_cap_negotiation_commands;
+        monitor_qmp_caps_reset(mon);
+        data = qmp_greeting(mon);
+        qmp_send_response(mon, data);
+        qobject_unref(data);
+        mon_refcount++;
+        break;
+    case CHR_EVENT_CLOSED:
+        /*
+         * Note: this is only useful when the output of the chardev
+         * backend is still open.  For example, when the backend is
+         * stdio, it's possible that stdout is still open when stdin
+         * is closed.
+         */
+        monitor_qmp_cleanup_queues(mon);
+        json_message_parser_destroy(&mon->parser);
+        json_message_parser_init(&mon->parser, handle_qmp_command,
+                                 mon, NULL);
+        mon_refcount--;
+        monitor_fdsets_cleanup();
+        break;
+    }
+}
+
+void monitor_data_destroy_qmp(MonitorQMP *mon)
+{
+    json_message_parser_destroy(&mon->parser);
+    qemu_mutex_destroy(&mon->qmp_queue_lock);
+    monitor_qmp_cleanup_req_queue_locked(mon);
+    g_queue_free(mon->qmp_requests);
+}
+
+static void monitor_qmp_setup_handlers_bh(void *opaque)
+{
+    MonitorQMP *mon = opaque;
+    GMainContext *context;
+
+    assert(mon->common.use_io_thread);
+    context = iothread_get_g_main_context(mon_iothread);
+    assert(context);
+    qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
+                             monitor_qmp_read, monitor_qmp_event,
+                             NULL, &mon->common, context, true);
+    monitor_list_append(&mon->common);
+}
+
+void monitor_init_qmp(Chardev *chr, int flags)
+{
+    MonitorQMP *mon = g_new0(MonitorQMP, 1);
+
+    /* Only HMP supports readline */
+    assert(!(flags & MONITOR_USE_READLINE));
+
+    /* Note: we run QMP monitor in I/O thread when @chr supports that */
+    monitor_data_init(&mon->common, flags, false,
+                      qemu_chr_has_feature(chr, QEMU_CHAR_FEATURE_GCONTEXT));
+
+    qemu_mutex_init(&mon->qmp_queue_lock);
+    mon->qmp_requests = g_queue_new();
+
+    qemu_chr_fe_init(&mon->common.chr, chr, &error_abort);
+    qemu_chr_fe_set_echo(&mon->common.chr, true);
+
+    json_message_parser_init(&mon->parser, handle_qmp_command, mon, NULL);
+    if (mon->common.use_io_thread) {
+        /*
+         * Make sure the old iowatch is gone.  It's possible when
+         * e.g. the chardev is in client mode, with wait=on.
+         */
+        remove_fd_in_watch(chr);
+        /*
+         * We can't call qemu_chr_fe_set_handlers() directly here
+         * since chardev might be running in the monitor I/O
+         * thread.  Schedule a bottom half.
+         */
+        aio_bh_schedule_oneshot(iothread_get_aio_context(mon_iothread),
+                                monitor_qmp_setup_handlers_bh, mon);
+        /* The bottom half will add @mon to @mon_list */
+    } else {
+        qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
+                                 monitor_qmp_read, monitor_qmp_event,
+                                 NULL, &mon->common, NULL, true);
+        monitor_list_append(&mon->common);
+    }
+}
diff --git a/monitor/trace-events b/monitor/trace-events
index abfdf20b14..a06dde3fd3 100644
--- a/monitor/trace-events
+++ b/monitor/trace-events
@@ -5,7 +5,9 @@ monitor_protocol_event_handler(uint32_t event, void *qdict) "event=%d data=%p"
 monitor_protocol_event_emit(uint32_t event, void *data) "event=%d data=%p"
 monitor_protocol_event_queue(uint32_t event, void *qdict, uint64_t rate) "event=%d data=%p rate=%" PRId64
 handle_hmp_command(void *mon, const char *cmdline) "mon %p cmdline: %s"
-handle_qmp_command(void *mon, const char *req) "mon %p req: %s"
 monitor_suspend(void *ptr, int cnt) "mon %p: %d"
+
+# qmp.c
 monitor_qmp_cmd_in_band(const char *id) "%s"
 monitor_qmp_cmd_out_of_band(const char *id) "%s"
+handle_qmp_command(void *mon, const char *req) "mon %p req: %s"