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

[Li Zhang]

On Tue, Apr 13, 2021 at 10:58 AM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Tue, Apr 13, 2021 at 08:40:59AM +0200, Markus Armbruster 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".
>
> If you need ability to hot-add monitor instances, why not just
> implement that feature directly, instead of pre-creating monitors
> with null backends and then later changing the backend ?
>

Hi Daniel,

I would like to use this solution first because we'd like to use it as
soon as possible.
It's the best solution to implement the hot-add feature and it may
need more effort to do it.
I will consider it.

> > >
> > > 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?
> >
> > If yes, how exactly does it misbehave?
> >
> > Does it work with an HMP monitor?
> >
> > > ---
> > >  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?
> >
> > 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;
> > > +}
> >
> > 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.
> >
> > > +
> > > +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);
> > >      }
> > >  }
> >
> >
>
> Regards,
> Daniel
> --
> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
>


-- 

Best Regards
-Li