qmp: Move dispatcher to a coroutine

This moves the QMP dispatcher to a coroutine and runs all QMP command
handlers that declare 'coroutine': true in coroutine context so they
can avoid blocking the main loop while doing I/O or waiting for other
events.

For commands that are not declared safe to run in a coroutine, the
dispatcher drops out of coroutine context by calling the QMP command
handler from a bottom half.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201005155855.256490-10-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>

2 files changed, 63 insertions, 5 deletions

diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c
index 5677ba92ca..9a2d7dd29a 100644
--- a/qapi/qmp-dispatch.c
+++ b/qapi/qmp-dispatch.c
@@ -12,12 +12,16 @@
  */
 
 #include "qemu/osdep.h"
+
+#include "block/aio.h"
 #include "qapi/error.h"
 #include "qapi/qmp/dispatch.h"
 #include "qapi/qmp/qdict.h"
 #include "qapi/qmp/qjson.h"
 #include "sysemu/runstate.h"
 #include "qapi/qmp/qbool.h"
+#include "qemu/coroutine.h"
+#include "qemu/main-loop.h"
 
 static QDict *qmp_dispatch_check_obj(QDict *dict, bool allow_oob,
                                      Error **errp)
@@ -88,6 +92,30 @@ bool qmp_is_oob(const QDict *dict)
         && !qdict_haskey(dict, "execute");
 }
 
+typedef struct QmpDispatchBH {
+    const QmpCommand *cmd;
+    Monitor *cur_mon;
+    QDict *args;
+    QObject **ret;
+    Error **errp;
+    Coroutine *co;
+} QmpDispatchBH;
+
+static void do_qmp_dispatch_bh(void *opaque)
+{
+    QmpDispatchBH *data = opaque;
+
+    assert(monitor_cur() == NULL);
+    monitor_set_cur(qemu_coroutine_self(), data->cur_mon);
+    data->cmd->fn(data->args, data->ret, data->errp);
+    monitor_set_cur(qemu_coroutine_self(), NULL);
+    aio_co_wake(data->co);
+}
+
+/*
+ * Runs outside of coroutine context for OOB commands, but in coroutine
+ * context for everything else.
+ */
 QDict *qmp_dispatch(const QmpCommandList *cmds, QObject *request,
                     bool allow_oob, Monitor *cur_mon)
 {
@@ -153,12 +181,39 @@ QDict *qmp_dispatch(const QmpCommandList *cmds, QObject *request,
         qobject_ref(args);
     }
 
+    assert(!(oob && qemu_in_coroutine()));
     assert(monitor_cur() == NULL);
-    monitor_set_cur(qemu_coroutine_self(), cur_mon);
-
-    cmd->fn(args, &ret, &err);
-
-    monitor_set_cur(qemu_coroutine_self(), NULL);
+    if (!!(cmd->options & QCO_COROUTINE) == qemu_in_coroutine()) {
+        monitor_set_cur(qemu_coroutine_self(), cur_mon);
+        cmd->fn(args, &ret, &err);
+        monitor_set_cur(qemu_coroutine_self(), NULL);
+    } else {
+       /*
+        * Actual context doesn't match the one the command needs.
+        *
+        * Case 1: we are in coroutine context, but command does not
+        * have QCO_COROUTINE.  We need to drop out of coroutine
+        * context for executing it.
+        *
+        * Case 2: we are outside coroutine context, but command has
+        * QCO_COROUTINE.  Can't actually happen, because we get here
+        * outside coroutine context only when executing a command
+        * out of band, and OOB commands never have QCO_COROUTINE.
+        */
+        assert(!oob && qemu_in_coroutine() && !(cmd->options & QCO_COROUTINE));
+
+        QmpDispatchBH data = {
+            .cur_mon    = cur_mon,
+            .cmd        = cmd,
+            .args       = args,
+            .ret        = &ret,
+            .errp       = &err,
+            .co         = qemu_coroutine_self(),
+        };
+        aio_bh_schedule_oneshot(qemu_get_aio_context(), do_qmp_dispatch_bh,
+                                &data);
+        qemu_coroutine_yield();
+    }
     qobject_unref(args);
     if (err) {
         /* or assert(!ret) after reviewing all handlers: */
diff --git a/qapi/qmp-registry.c b/qapi/qmp-registry.c
index d0f9a1d3e3..58c65b5052 100644
--- a/qapi/qmp-registry.c
+++ b/qapi/qmp-registry.c
@@ -20,6 +20,9 @@ void qmp_register_command(QmpCommandList *cmds, const char *name,
 {
     QmpCommand *cmd = g_malloc0(sizeof(*cmd));
 
+    /* QCO_COROUTINE and QCO_ALLOW_OOB are incompatible for now */
+    assert(!((options & QCO_COROUTINE) && (options & QCO_ALLOW_OOB)));
+
     cmd->name = name;
     cmd->fn = fn;
     cmd->enabled = true;