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

["Marc-André Lureau" <marcandre.lureau@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 7800 bytes --]

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?


>
>> 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

[-- Attachment #2: Type: text/html, Size: 10395 bytes --]