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

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: QEMU <qemu-devel@nongnu.org>,
	"Markus Armbruster" <armbru@redhat.com>,
	"Li Zhang" <li.zhang@cloud.ionos.com>,
	"Marc-André Lureau" <marcandre.lureau@gmail.com>,
	"Pankaj Gupta" <pankaj.gupta@ionos.com>,
	"Li Zhang" <zhlcindy@gmail.com>
Subject: Re: [PATCHv2 1/1] Support monitor chardev hotswap with QMP
Date: Tue, 4 May 2021 09:38:25 +0100	[thread overview]
Message-ID: <YJEIAfs4tNRN52K4@redhat.com> (raw)
In-Reply-To: <CAM9Jb+g0jUy5uEYmpu0nTYogRoDN1VZayLD_0xQ2ZZqB5tr21Q@mail.gmail.com>

On Tue, May 04, 2021 at 08:29:50AM +0200, Pankaj Gupta wrote:
> +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?

From the libvirt side I'd just like to see the real problem addressed.
ie actally implement monitor hotplug/unplug, instead of this hack
where you pre-create a monitor with null backend.

Regards,
Daniel
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