hmp: Add support for coroutine command handlers

Often, QMP command handlers are not only called to handle QMP commands,
but also from a corresponding HMP command handler. In order to give them
a consistent environment, optionally run HMP command handlers in a
coroutine, too.

The implementation is a lot simpler than in QMP because for HMP, we
still block the VM while the coroutine is running.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20201005155855.256490-11-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/devel/qapi-code-gen.txt

@@ -617,8 +617,8 @@ pitfalls are:
 
 Since the command handler may assume coroutine context, any callers
 other than the QMP dispatcher must also call it in coroutine context.
-In particular, HMP commands calling such a QMP command handler must
-enter coroutine context before calling the handler.
+In particular, HMP commands calling such a QMP command handler must be
+marked .coroutine = true in hmp-commands.hx.
 
 It is an error to specify both 'coroutine': true and 'allow-oob': true
 for a command.  We don't currently have a use case for both together and

monitor/hmp.c

@@ -1056,12 +1056,26 @@ fail:
     return NULL;
 }
 
+typedef struct HandleHmpCommandCo {
+    Monitor *mon;
+    const HMPCommand *cmd;
+    QDict *qdict;
+    bool done;
+} HandleHmpCommandCo;
+
+static void handle_hmp_command_co(void *opaque)
+{
+    HandleHmpCommandCo *data = opaque;
+    data->cmd->cmd(data->mon, data->qdict);
+    monitor_set_cur(qemu_coroutine_self(), NULL);
+    data->done = true;
+}
+
 void handle_hmp_command(MonitorHMP *mon, const char *cmdline)
 {
     QDict *qdict;
     const HMPCommand *cmd;
     const char *cmd_start = cmdline;
-    Monitor *old_mon;
 
     trace_handle_hmp_command(mon, cmdline);
 
@@ -1080,10 +1094,23 @@ void handle_hmp_command(MonitorHMP *mon, const char *cmdline)
         return;
     }
 
-    /* old_mon is non-NULL when called from qmp_human_monitor_command() */
-    old_mon = monitor_set_cur(qemu_coroutine_self(), &mon->common);
-    cmd->cmd(&mon->common, qdict);
-    monitor_set_cur(qemu_coroutine_self(), old_mon);
+    if (!cmd->coroutine) {
+        /* old_mon is non-NULL when called from qmp_human_monitor_command() */
+        Monitor *old_mon = monitor_set_cur(qemu_coroutine_self(), &mon->common);
+        cmd->cmd(&mon->common, qdict);
+        monitor_set_cur(qemu_coroutine_self(), old_mon);
+    } else {
+        HandleHmpCommandCo data = {
+            .mon = &mon->common,
+            .cmd = cmd,
+            .qdict = qdict,
+            .done = false,
+        };
+        Coroutine *co = qemu_coroutine_create(handle_hmp_command_co, &data);
+        monitor_set_cur(co, &mon->common);
+        aio_co_enter(qemu_get_aio_context(), co);
+        AIO_WAIT_WHILE(qemu_get_aio_context(), !data.done);
+    }
 
     qobject_unref(qdict);
 }

monitor/monitor-internal.h

@@ -74,6 +74,7 @@ typedef struct HMPCommand {
     const char *help;
     const char *flags; /* p=preconfig */
     void (*cmd)(Monitor *mon, const QDict *qdict);
+    bool coroutine;
     /*
      * @sub_table is a list of 2nd level of commands. If it does not exist,
      * cmd should be used. If it exists, sub_table[?].cmd should be