Re: [PATCHv2 1/1] Support monitor chardev hotswap with QMP

[Li Zhang]

On Fri, Apr 16, 2021 at 4:50 PM Markus Armbruster <armbru@redhat.com> wrote:
>
> Marc-André, I'd like your opinion for this one, in particular the use of
> g_source_remove().
>
> Li Zhang <zhlcindy@gmail.com> writes:
>
> > 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>
> > Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
>
> Your commit message starts with a brief description of the problem.
> Appreciated!  But I think it could be a bit clearer.  What about this:
>
>     qmp: Support chardev-change
>
>     For some scenarios, we'd like to hot-add a monitor device.  But QEMU
>     doesn't support that, yet.  It does support hot-swapping character
>     backends with QMP command chardev-change.  This lets us pre-add a
>     monitor with a null character backend, then chardev-change to a
>     socket backend.  Except the chardev-change fails with "Chardev user
>     does not support chardev hotswap" because monitors don't provide the
>     required callback.  Implement it for QMP monitors.
>

It looks good to me.

> > ---
> >  v1 -> v2:
> >   - Change mutex lock mon_lock section
> >   - Fix indentation problems
> >
> >  monitor/monitor-internal.h |  3 +++
> >  monitor/monitor.c          |  2 +-
> >  monitor/qmp.c              | 43 +++++++++++++++++++++++++++++++++++---
> >  3 files changed, 44 insertions(+), 4 deletions(-)
> >
> > diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
> > index 9c3a09cb01..1b80c74883 100644
> > --- a/monitor/monitor-internal.h
> > +++ b/monitor/monitor-internal.h
> > @@ -183,4 +183,7 @@ void help_cmd(Monitor *mon, const char *name);
> >  void handle_hmp_command(MonitorHMP *mon, const char *cmdline);
> >  int hmp_compare_cmd(const char *name, const char *list);
> >
> > +gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
> > +                           void *opaque);
> > +
> >  #endif
> > diff --git a/monitor/monitor.c b/monitor/monitor.c
> > index 636bcc81c5..16a3620d02 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 2b0308f933..5fa65401ae 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;
> >
> > @@ -477,7 +478,36 @@ 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);
> > +    }
> > +
> > +    qemu_mutex_lock(&mon->common.mon_lock);
> > +    if (mon->common.out_watch) {
> > +        g_source_remove(mon->common.out_watch);
> > +        mon->common.out_watch = qemu_chr_fe_add_watch(&mon->common.chr,
> > +                                G_IO_OUT | G_IO_HUP,
> > +                                monitor_unblocked,
> > +                                &mon->common);
>
> Visually align the arguments:
>
>         mon->common.out_watch = qemu_chr_fe_add_watch(&mon->common.chr,
>                                                       G_IO_OUT | G_IO_HUP,
>                                                       monitor_unblocked,
>                                                       &mon->common);
>

I double check my patch, it is like the above style. But it looks
different in email. It's really strange.
I will correct it.

> You may reduce argument indentation some to gain extra space, but keep
> the arguments more indented than the function:
>
>         mon->common.out_watch = qemu_chr_fe_add_watch(&mon->common.chr,
>                                         G_IO_OUT | G_IO_HUP,
>                                         monitor_unblocked, &mon->common);
>
> Do this only when you actually use the extra space for readability.
>
OK, thanks for explanation.

> > +    }
> > +    qemu_mutex_unlock(&mon->common.mon_lock);
> > +
> > +    return 0;
> > +}
>
> As I wrote in my review of v1, this function copies from
> monitor_data_init(), monitor_init_qmp(), and monitor_flush_locked().
> Feels like a refactoring would be in order.  Doing it on top might be
> easier.

 I may misunderstand what you mean. What's the meaning of "doing it on
top" exactly?

>
> > +
> > +static void monitor_qmp_set_handlers_bh(void *opaque)
> >  {
> >      MonitorQMP *mon = opaque;
> >      GMainContext *context;
> > @@ -487,7 +517,14 @@ 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);
> >  }
> >
> > @@ -528,7 +565,7 @@ 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