From: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: "Daniel P. Berrange" <berrange@redhat.com>,
QEMU <qemu-devel@nongnu.org>,
"Pankaj Gupta" <pankaj.gupta@ionos.com>,
"Li Zhang" <li.zhang@cloud.ionos.com>,
"Marc-André Lureau" <marcandre.lureau@gmail.com>,
"Li Zhang" <zhlcindy@gmail.com>
Subject: Re: [PATCHv2 1/1] Support monitor chardev hotswap with QMP
Date: Tue, 4 May 2021 08:29:50 +0200 [thread overview]
Message-ID: <CAM9Jb+g0jUy5uEYmpu0nTYogRoDN1VZayLD_0xQ2ZZqB5tr21Q@mail.gmail.com> (raw)
In-Reply-To: <87blad2v9x.fsf@dusky.pond.sub.org>
+CC Danpb
> >>> 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.
>
> 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.
>
> Whether we want to accept patches lifting restrictions is up to the
> chardev maintainers.
Maybe we can handle or already handle the restrictions at libvirt side?
>
> 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.
Sure. But I also feel allowing to change monitor device is a useful feature
independent of monitor hotplug/unplug feature .
>
> Do I make sense?
>
> [...]
>
>
> [*] 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.
Good point. I agree. We should fix this.
Thanks,
Pankaj
next prev parent reply other threads:[~2021-05-04 6:55 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-13 21:34 [PATCHv2 1/1] Support monitor chardev hotswap with QMP Li Zhang
2021-04-15 15:07 ` Li Zhang
2021-04-16 14:50 ` Markus Armbruster
2021-04-16 14:59 ` Marc-André Lureau
2021-04-16 15:28 ` Marc-André Lureau
2021-04-16 15:46 ` Li Zhang
2021-04-17 8:02 ` Markus Armbruster
2021-04-19 11:56 ` Li Zhang
2021-05-04 6:29 ` Pankaj Gupta [this message]
2021-05-04 8:38 ` Daniel P. Berrangé
2021-04-16 15:20 ` Li Zhang
2021-04-17 8:05 ` Markus Armbruster
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