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

[Markus Armbruster]

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.

> ---
>  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);

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.

> +    }
> +    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.

> +
> +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);
>      }
>  }