monitor: only run coroutine commands in qemu_aio_context

monitor_qmp_dispatcher_co() runs in the iohandler AioContext that is not
polled during nested event loops. The coroutine currently reschedules
itself in the main loop's qemu_aio_context AioContext, which is polled
during nested event loops. One known problem is that QMP device-add
calls drain_call_rcu(), which temporarily drops the BQL, leading to all
sorts of havoc like other vCPU threads re-entering device emulation code
while another vCPU thread is waiting in device emulation code with
aio_poll().

Paolo Bonzini suggested running non-coroutine QMP handlers in the
iohandler AioContext. This avoids trouble with nested event loops. His
original idea was to move coroutine rescheduling to
monitor_qmp_dispatch(), but I resorted to moving it to qmp_dispatch()
because we don't know if the QMP handler needs to run in coroutine
context in monitor_qmp_dispatch(). monitor_qmp_dispatch() would have
been nicer since it's associated with the monitor implementation and not
as general as qmp_dispatch(), which is also used by qemu-ga.

A number of qemu-iotests need updated .out files because the order of
QMP events vs QMP responses has changed.

Solves Issue #1933.

Cc: qemu-stable@nongnu.org
Fixes: 7bed89958bfbf40df9ca681cefbdca63abdde39d ("device_core: use drain_call_rcu in in qmp_device_add")
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2215192
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2214985
Buglink: https://issues.redhat.com/browse/RHEL-17369
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240118144823.1497953-4-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 23 insertions, 1 deletions

diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c
index 555528b6bb..176b549473 100644
--- a/qapi/qmp-dispatch.c
+++ b/qapi/qmp-dispatch.c
@@ -206,9 +206,31 @@ QDict *coroutine_mixed_fn qmp_dispatch(const QmpCommandList *cmds, QObject *requ
     assert(!(oob && qemu_in_coroutine()));
     assert(monitor_cur() == NULL);
     if (!!(cmd->options & QCO_COROUTINE) == qemu_in_coroutine()) {
+        if (qemu_in_coroutine()) {
+            /*
+             * Move the coroutine from iohandler_ctx to qemu_aio_context for
+             * executing the command handler so that it can make progress if it
+             * involves an AIO_WAIT_WHILE().
+             */
+            aio_co_schedule(qemu_get_aio_context(), qemu_coroutine_self());
+            qemu_coroutine_yield();
+        }
+
         monitor_set_cur(qemu_coroutine_self(), cur_mon);
         cmd->fn(args, &ret, &err);
         monitor_set_cur(qemu_coroutine_self(), NULL);
+
+        if (qemu_in_coroutine()) {
+            /*
+             * Yield and reschedule so the main loop stays responsive.
+             *
+             * Move back to iohandler_ctx so that nested event loops for
+             * qemu_aio_context don't start new monitor commands.
+             */
+            aio_co_schedule(iohandler_get_aio_context(),
+                            qemu_coroutine_self());
+            qemu_coroutine_yield();
+        }
     } else {
        /*
         * Actual context doesn't match the one the command needs.
@@ -232,7 +254,7 @@ QDict *coroutine_mixed_fn qmp_dispatch(const QmpCommandList *cmds, QObject *requ
             .errp       = &err,
             .co         = qemu_coroutine_self(),
         };
-        aio_bh_schedule_oneshot(qemu_get_aio_context(), do_qmp_dispatch_bh,
+        aio_bh_schedule_oneshot(iohandler_get_aio_context(), do_qmp_dispatch_bh,
                                 &data);
         qemu_coroutine_yield();
     }