Re: [Qemu-devel] [PATCH] secondary-vga: unregister vram on unplug.

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On 7 August 2018 at 15:57, Dr. David Alan Gilbert <dgilbert@redhat.com> wrote:
> > * Gerd Hoffmann (kraxel@redhat.com) wrote:
> >> On Fri, Jul 20, 2018 at 10:19:48AM +0200, remy.noel@blade-group.com wrote:
> >> > From: "Remy Noel" <remy.noel@blade-group.com>
> >> >
> >> > When removing a secondary-vga device and then adding it back (or adding
> >> > an other one), qemu aborts with:
> >> >     "RAMBlock "0000:00:02.0/vga.vram" already registered, abort!".
> >> >
> >> > It is caused by the vram staying registered, preventing vga replugging.
> >>
> >> David?  Does that look ok?
> >>
> >> This balances the
> >>
> >>      vmstate_register_ram(&s->vram, s->global_vmstate ?  NULL : DEVICE(obj));
> >>
> >> call in vga_common_init().  I'm wondering whenever the manual cleanup is
> >> actually needed in case owner is not NULL?
> >
> > I can't see anyone who is calling unregister_ram or the functions it
> > calls as part of generic device cleanup, so I think it IS needed
> > to manually do it.
> >
> > Which is a bit worrying since we have vastly more register's than
> > unregister's.
> 
> Paolo suggested in an email last month that vmstate_unregister_ram()
> should simply not exist, because it doesn't actually do anything useful:
> https://lists.gnu.org/archive/html/qemu-devel/2018-07/msg01125.html
> 
> (ie it was added in the first place because we'd ended up with
> two identically named ramblocks, but that only happened because
> a reference-counting bug meant we hadn't deleted the first one
> properly before creating the second.)
> 
> So I think that the bug reported in this thread is similar:
> the problem is not that we're not calling vmstate_unregister_ram(),
> but that when the first instance of secondary-vga is removed
> it is not correctly destroying the ramblock.

Ah yes that makes more sense; I remember there was another similar bug
where a device screwed up and didn't delete it's RAM causing similar
problems.

Dave

> thanks
> -- PMM
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK