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, 0 insertions, 17 deletions

diff --git a/monitor/qmp.c b/monitor/qmp.c
index 6eee450..a239945 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -321,14 +321,6 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
             qemu_coroutine_yield();
         }
 
-        /*
-         * 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(), qmp_dispatcher_co);
-        qemu_coroutine_yield();
-
         /* Process request */
         if (req_obj->req) {
             if (trace_event_get_state(TRACE_MONITOR_QMP_CMD_IN_BAND)) {
@@ -355,15 +347,6 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
         }
 
         qmp_request_free(req_obj);
-
-        /*
-         * 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(), qmp_dispatcher_co);
-        qemu_coroutine_yield();
     }
     qatomic_set(&qmp_dispatcher_co, NULL);
 }