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

[Li Zhang]

Hi Marc-André,

On Fri, Apr 16, 2021 at 5:28 PM Marc-André Lureau
<marcandre.lureau@gmail.com> wrote:
>
> Hi
>
> On Fri, Apr 16, 2021 at 6:59 PM Marc-André Lureau 
> <marcandre.lureau@gmail.com> wrote:
>>
>> Hi
>>
>> On Fri, Apr 16, 2021 at 6:51 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().
>>
>>
>> My opinion isn't really worth much, my review would have a bit more value.
>>
>> GSource has indeed some peculiar lifetime management, that I got wrong in 
>> the past. So I would be extra careful.
>>
>> But before spending time on review, I would also clarify the motivation and 
>> ask for testing.
>>
>> Markus, hot-adding/removing monitors isn't supported?
>>
>
> I realize you answered my question below. That's surprising me. Wouldn't it 
> make more sense to support it rather than having a pre-opened null-based 
> monitor that can have its chardev swapped?

It's the best way to support hot-add/remove monitor devices. But we
would like to use it as soon as possible, so I use hotswap currently.
If possible, we can consider implementing hot-add/remove in the future.


>
>>>
>>>
>>> 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);
>>> >      }
>>> >  }
>>>
>>>
>>
>>
>> --
>> Marc-André Lureau
>
>
>
> --
> Marc-André Lureau



-- 

Best Regards
-Li