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

[Li Zhang <zhlcindy@gmail.com>]

On Sat, Apr 17, 2021 at 10:02 AM Markus Armbruster <armbru@redhat.com> wrote:
>
> Marc-André Lureau <marcandre.lureau@gmail.com> writes:
>
> > 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?
>
> Yes, it would.  Patches welcome.
>
> This patch is a somewhat ham-fisted and limited solution to the problem
> stated in the commit message.  However, it might *also* be a reasonable
> improvement to chardev-change on its own.  Not for me to judge.
>
Okay, Thanks.

> chardev-change comes with a number of restrictions.  Let's have a closer
> look.  It fails
>
> 1. when no such character device exists (d'oh)
>
> 2. for chardev-mux devices
>
> 3. in record/replay mode
>
> 4. when a backend is connected that doesn't implement the chr_be_change()
>    method
>
> 5. when chr_be_change() fails
>
> 6. when creating the new chardev fails[*]
>
> Items 2, 3, 4 are restrictions.  I figure 2 and 4 are simply not
> implemented, yet.  I'm not sure about 3.
>
For item 3,
source code mentions "chardev cannot  be changed in record/replay mode".
I never tried it yet. I am not quite sure why it couldn't be changed.

> Whether we want to accept patches lifting restrictions is up to the
> chardev maintainers.
>
> This patch lifts restriction 4 for QMP monitor backends.  Its monitor
> part looks acceptable to me, but I dislike its code duplication.  Before
> we spend time on cleaning that up (or on deciding to clean it up later),
> I'd like to hear the chardev mantainers' judgement, because that's about
> more serious matters than cleanliness.
>
> Do I make sense?
Yes, make sense.  Thanks.

>
> [...]
>
>
> [*] The code for creating the new chardev in the "no backend connected"
> case
>
>     be = chr->be;
>     if (!be) {
>         /* easy case */
>         object_unparent(OBJECT(chr));
>         return qmp_chardev_add(id, backend, errp);
>     }
>
> is problematic: when qmp_chardev_add() fails, we already destroyed the
> old chardev.  It should destroy the old chardev only when it can create
> its replacement.
>
You are right. It is a problem.


-- 

Best Regards
-Li