On Tue, Jun 23, 2026 at 03:53:37PM +0400, [email protected] wrote:
> > 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 <[email protected]>
> > Message-ID: <[email protected]>
> >
> > 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?
Yeah, will do.
> > @@ -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?
Yes, this is the cause of the bug that Peter reported with libvirt
hanging.
>
> > /*
> > * 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"
With regards,
Daniel
--
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