The default monitor is usually a long lived object that will exist for the entire lifetime of the VM. A monitor can only service a single client at a time though, and so it might be desirable to hotplug additional monitors at runtime for specific tasks. If doing that, however, there is a need to remove the monitor when it is no longer needed.
Allowing a client to run "object-del" against its own monitor adds complex edge cases, as it would be desirable to send the QMP response despite the monitor sending it being deleted. Doing "object-del" alone will also result in orphaning a character device backend instance, as there is no opportunity to run the companion "chardev-del" command.
A simpler way to ensure cleanup is to add the concept of auto-deleting monitor objects. Specifically when the "CHR_EVENT_CLOSED" event is emitted, the equivalent of "object-del" + "chardev-del" can be run internally. Since the transient client has already droppped its monitor connection, there is no synchronization to be concerned about.
This is implemented via a new "close-action=none|delete" property on the 'monitor-qmp' object. This concept could be extended with further actions in future, for example:
* close-action=shutdown - graceful guest shutdown * close-action=terminate - immediate guest poweroff * close-action=stop - pause guest CPUs while the monitor is not connected to any client
This is left as an exercise for future interested contributors.
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com> Message-ID: <20260623095551.2106712-33-berrange@redhat.com>
diff --git a/qapi/qom.json b/qapi/qom.json index 6ed510858e1..63335b8fd09 100644 --- a/qapi/qom.json +++ b/qapi/qom.json @@ -1213,18 +1213,37 @@ 'base': 'MonitorProperties', 'data': { '*readline': 'bool' } }
+ +## +# @MonitorQMPCloseAction: +# +# Action to take when the character device backend is +# closed. +# +# @none: take no action (the default) +# @delete: delete both the 'monitor-qmp' object and its associated +# character device backend object +# +# Since 11.1 +## +{ 'enum' : 'MonitorQMPCloseAction', + 'data': ['none', 'delete'] } + ## # @MonitorQMPProperties: # # Properties for the QMP monitor # # @pretty: whether to pretty print JSON responses (default: disabled) +# @close-action: action to take when the character device backend +# is closed (default: none) # # Since: 11.1 ## { 'struct': 'MonitorQMPProperties', 'base': 'MonitorProperties', - 'data': { '*pretty': 'bool' } } + 'data': { '*pretty': 'bool', + '*close-action': 'MonitorQMPCloseAction' } }
## # @ObjectType: diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h index 5522e05464b..23829f32f9a 100644 --- a/monitor/monitor-internal.h +++ b/monitor/monitor-internal.h @@ -28,6 +28,7 @@ #include "chardev/char-fe.h" #include "monitor/monitor.h" #include "qapi/qapi-types-control.h" +#include "qapi/qapi-types-qom.h" #include "qapi/qmp-registry.h" #include "qobject/json-parser.h" #include "qemu/readline.h" @@ -178,7 +179,9 @@ struct MonitorQMP { Monitor parent_obj; JSONMessageParser parser; bool pretty; + MonitorQMPCloseAction close_action; bool setup_pending; /* iothread BH has not yet set up chardev handlers */ + bool delete_pending; /* close_action has started 'delete' process */ /* * When a client connects, we're in capabilities negotiation mode. * @commands is &qmp_cap_negotiation_commands then. When command diff --git a/monitor/qmp.c b/monitor/qmp.c index 5301927f09f..f6b7fe65bbf 100644 --- a/monitor/qmp.c +++ b/monitor/qmp.c @@ -28,6 +28,7 @@ #include "monitor-internal.h" #include "qapi/error.h" #include "qapi/qapi-commands-control.h" +#include "qapi/qapi-commands-char.h" #include "qobject/qdict.h" #include "qobject/qjson.h" #include "qobject/qlist.h" @@ -103,6 +104,20 @@ static void monitor_qmp_set_pretty(Object *obj, bool val, Error **errp) mon->pretty = val; }
+static int monitor_qmp_get_close_action(Object *obj, Error **errp) +{ + MonitorQMP *mon = MONITOR_QMP(obj); + + return mon->close_action; +} + +static void monitor_qmp_set_close_action(Object *obj, int val, Error **errp) +{ + MonitorQMP *mon = MONITOR_QMP(obj); + + mon->close_action = val; +} + static void monitor_qmp_emit_event(Monitor *mon, QAPIEvent event, QDict *qdict); static bool monitor_qmp_requires_iothread(const Monitor *mon); static void monitor_qmp_complete(UserCreatable *uc, Error **errp); @@ -117,6 +132,11 @@ static void monitor_qmp_class_init(ObjectClass *cls, const void *data) object_class_property_add_bool(cls, "pretty", monitor_qmp_get_pretty, monitor_qmp_set_pretty); + object_class_property_add_enum(cls, "close-action", + "MonitorQMPCloseAction", + &MonitorQMPCloseAction_lookup, + monitor_qmp_get_close_action, + monitor_qmp_set_close_action);
moncls->emit_event = monitor_qmp_emit_event; moncls->requires_iothread = monitor_qmp_requires_iothread; @@ -550,11 +570,38 @@ static QDict *qmp_greeting(MonitorQMP *mon) ver, cap_list); }
+static void monitor_qmp_self_delete_bh(void *opaque) +{ + MonitorQMP *mon = opaque; + g_autofree char *mon_id = object_property_get_child_name( + object_get_objects_root(), OBJECT(mon)); + g_autofree char *chardev_id = g_strdup(mon->parent_obj.chardev_id); + Error *local_error = NULL; + + g_assert(mon_id);
If object-del beats this BH, mon_id might be NULL. Return silently?
+ + user_creatable_del(mon_id, &local_error); + object_unref(mon); + if (local_error != NULL) { + error_report_err(local_error); + } else { + qmp_chardev_remove(chardev_id, NULL); + } +} + static void monitor_qmp_event(void *opaque, QEMUChrEvent event) { QDict *data; MonitorQMP *mon = opaque;
+ /* Protect against race if a client drops & quickly + * reconnects - we'll have the delete BH scheduled + * so must not honour a new open request + */ + if (mon->delete_pending) { + return; + } + switch (event) { case CHR_EVENT_OPENED: WITH_QEMU_LOCK_GUARD(&mon->parent_obj.mon_lock) { @@ -566,6 +613,7 @@ static void monitor_qmp_event(void *opaque, QEMUChrEvent event) qobject_unref(data); break; case CHR_EVENT_CLOSED: + mon->delete_pending = true;
Shouldn't it be set in the MONITOR_QMP_CLOSE_ACTION_DELETE case only?
/* * Note: this is only useful when the output of the chardev * backend is still open. For example, when the backend is @@ -577,6 +625,26 @@ static void monitor_qmp_event(void *opaque, QEMUChrEvent event) json_message_parser_init(&mon->parser, handle_qmp_command, mon, NULL); monitor_fdsets_cleanup(); + switch (mon->close_action) { + case MONITOR_QMP_CLOSE_ACTION_NONE: + break; /* nada */ + case MONITOR_QMP_CLOSE_ACTION_DELETE: + /* + * Do NOT run in the AIO context associated with the + * monitor. We need to run in the default AIO context + * which is the same context in which 'qmp_object_del' + * will execute + * + * Hold an extra ref in case a separate monitor races + * with the BH by processing an expliciti 'object-del'
"expliciti" -> "explicit" -- Marc-André Lureau <marcandre.lureau@redhat.com>