Re: [PATCH v3 32/35] monitor: add support for auto-deleting monitors upon close

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Jun 18, 2026 at 03:54:32PM +0400, marcandre.lureau@redhat.com 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 <berrange@redhat.com>
> > Message-ID: <20260618105836.991609-33-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..7257edc19d2 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 +570,33 @@ static QDict *qmp_greeting(MonitorQMP *mon)
> >          ver, cap_list);
> >  }
> >  
> > +static void monitor_qmp_self_delete_bh(void *opaque)
> > +{
> > +    MonitorQMP *mon = opaque;
> 
> There is a potential race if object-del is called on the same MonitorQMP before the BH fires.
> 
> > +    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);
> > +
> > +    user_creatable_del(mon_id, &local_error);
> > +    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;
> >  
> > +    if (mon->delete_pending) {
> > +        return;
> > +    }
> > +
> >      switch (event) {
> >      case CHR_EVENT_OPENED:
> >          WITH_QEMU_LOCK_GUARD(&mon->parent_obj.mon_lock) {
> > @@ -577,6 +619,23 @@ 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:
> > +            mon->delete_pending = true;
> > +            /*
> > +             * 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
> > +             */
> > +            aio_bh_schedule_oneshot(qemu_get_aio_context(),
> > +                                    monitor_qmp_self_delete_bh, mon);
> 
> ref it?

We should have checked the "delete_pending" flag in the
prepare_delete callback too, to block this situation.

With regards,
Daniel
-- 
|: https://berrange.com       ~~        https://hachyderm.io/@berrange :|
|: https://libvirt.org          ~~          https://entangle-photo.org :|
|: https://pixelfed.art/berrange   ~~    https://fstop138.berrange.com :|