Re: [PATCH 2/2] Support monitor chardev hotswap with QMP

[Li Zhang]

On Tue, Apr 13, 2021 at 8:41 AM Markus Armbruster <armbru@redhat.com> wrote:
>
> Li Zhang <zhlcindy@gmail.com> writes:
>
> > From: Li Zhang <li.zhang@cloud.ionos.com>
> >
> > For some scenarios, it needs to hot-add a monitor device.
> > But QEMU doesn't support hotplug yet. It also works by adding
> > a monitor with null backend by default and then change its
> > backend to socket by QMP command "chardev-change".
> >
> > So this patch is to support monitor chardev hotswap with QMP.
> >
> > Signed-off-by: Li Zhang <li.zhang@cloud.ionos.com>
>
> I think what what you're trying to say is that chardev-change does not
> work when the character device changes is used by a QMP monitor.
> Correct?
>
I mean that when the character device is a monitor device, it doesn't
work with a QMP monitor.
For example, I add 2 QMP monitors and change one of the monitor's
backends from socket to a null device.
It doesn't work because it needs the monitor device to support chardev-change.

> If yes, how exactly does it misbehave?
This command chardev-change needs specific device's change callback function.

>
> Does it work with an HMP monitor?
No, it doesn't work.

>
> > ---
> >  monitor/monitor-internal.h |  3 +++
> >  monitor/monitor.c          |  2 +-
> >  monitor/qmp.c              | 42 +++++++++++++++++++++++++++++++++++---
> >  3 files changed, 43 insertions(+), 4 deletions(-)
> >
> > diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
> > index 40903d6386..2df6dd21de 100644
> > --- a/monitor/monitor-internal.h
> > +++ b/monitor/monitor-internal.h
> > @@ -186,4 +186,7 @@ int hmp_compare_cmd(const char *name, const char *list);
> >  void qmp_query_qmp_schema(QDict *qdict, QObject **ret_data,
> >                                   Error **errp);
> >
> > +gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
> > +                               void *opaque);
> > +
> >  #endif
> > diff --git a/monitor/monitor.c b/monitor/monitor.c
> > index e94f532cf5..2d255bab18 100644
> > --- a/monitor/monitor.c
> > +++ b/monitor/monitor.c
> > @@ -157,7 +157,7 @@ static inline bool monitor_is_hmp_non_interactive(const 
> > Monitor *mon)
> >
> >  static void monitor_flush_locked(Monitor *mon);
> >
> > -static gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
> > +gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
> >                                    void *opaque)
> >  {
> >      Monitor *mon = opaque;
> > diff --git a/monitor/qmp.c b/monitor/qmp.c
> > index 2326bd7f9b..55cfb230d9 100644
> > --- a/monitor/qmp.c
> > +++ b/monitor/qmp.c
> > @@ -44,6 +44,7 @@ struct QMPRequest {
> >      Error *err;
> >  };
> >  typedef struct QMPRequest QMPRequest;
> > +static void monitor_qmp_set_handlers_bh(void *opaque);
> >
> >  QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
> >
> > @@ -480,7 +481,35 @@ void monitor_data_destroy_qmp(MonitorQMP *mon)
> >      g_queue_free(mon->qmp_requests);
> >  }
> >
> > -static void monitor_qmp_setup_handlers_bh(void *opaque)
> > +static int monitor_qmp_change(void *opaque)
> > +{
> > +    MonitorQMP *mon = opaque;
> > +
> > +    mon->common.use_io_thread =
> > +        qemu_chr_has_feature(mon->common.chr.chr, 
> > QEMU_CHAR_FEATURE_GCONTEXT);
> > +
> > +    if (mon->common.use_io_thread) {
> > +        aio_bh_schedule_oneshot(iothread_get_aio_context(mon_iothread),
> > +                                monitor_qmp_set_handlers_bh, mon);
> > +    } else {
> > +        qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
> > +                                 monitor_qmp_read, monitor_qmp_event,
> > +                                 monitor_qmp_change, &mon->common, NULL, 
> > true);
> > +    }
> > +
> > +    if (mon->common.out_watch) {
> > +        g_source_remove(mon->common.out_watch);
>
> All other updates of @out_watch are under @mon_lock.  Why not this one?

Sorry, I missed it. I will correct it.

>
> I have no idea whether g_source_remove() is the right function to call.
> Its documentation says "You must use g_source_destroy() for sources
> added to a non-default main context."  The qemu_chr_fe_set_handlers()
> contract is of no help.
>
> Documentation of g_source_destroy() confuses some more: "This does not
> unref the GSource: if you still hold a reference, use g_source_unref()
> to drop it.
>
> Marc-André, can you help?
>
> > +        qemu_mutex_lock(&mon->common.mon_lock);
> > +        mon->common.out_watch =
> > +        qemu_chr_fe_add_watch(&mon->common.chr, G_IO_OUT | G_IO_HUP,
> > +                               monitor_unblocked, &mon->common);
>
> Bad indentation.  Better:
>
>         mon->common.out_watch =
>             qemu_chr_fe_add_watch(&mon->common.chr, G_IO_OUT | G_IO_HUP,
>                                    monitor_unblocked, &mon->common);
>
> or
>
>         mon->common.out_watch = qemu_chr_fe_add_watch(&mon->common.chr,
>                                                       G_IO_OUT | G_IO_HUP,
>                                                       monitor_unblocked,
>                                                       &mon->common);
>
> or
>
>         mon->common.out_watch = qemu_chr_fe_add_watch(&mon->common.chr,
>                                         G_IO_OUT | G_IO_HUP,
>                                         monitor_unblocked, &mon->common);
>
> > +        qemu_mutex_unlock(&mon->common.mon_lock);
> > +    }
> > +
> > +    return 0;
> > +}
>

I will correct it. Thanks.

> This function copies from monitor_data_init(), monitor_init_qmp(), and
> monitor_flush_locked().  Feels like a refactoring would be in order.
> Possibly on top.
>
OK, I am considering it to avoid duplicated code.

> > +
> > +static void monitor_qmp_set_handlers_bh(void *opaque)
> >  {
> >      MonitorQMP *mon = opaque;
> >      GMainContext *context;
> > @@ -490,7 +519,14 @@ static void monitor_qmp_setup_handlers_bh(void *opaque)
> >      assert(context);
> >      qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
> >                               monitor_qmp_read, monitor_qmp_event,
> > -                             NULL, &mon->common, context, true);
> > +                             monitor_qmp_change, &mon->common, context, 
> > true);
> > +
> > +}
> > +
> > +static void monitor_qmp_setup_handlers_bh(void *opaque)
> > +{
> > +    MonitorQMP *mon = opaque;
> > +    monitor_qmp_set_handlers_bh(mon);
> >      monitor_list_append(&mon->common);
> >  }
> >
> > @@ -531,7 +567,7 @@ void monitor_init_qmp(Chardev *chr, bool pretty, Error 
> > **errp)
> >      } else {
> >          qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
> >                                   monitor_qmp_read, monitor_qmp_event,
> > -                                 NULL, &mon->common, NULL, true);
> > +                                 monitor_qmp_change, &mon->common, NULL, 
> > true);
> >          monitor_list_append(&mon->common);
> >      }
> >  }
>


--

Best Regards
-Li