On 08/07/2018 05:09 PM, Dr. David Alan Gilbert wrote:

* Peter Maydell (peter.mayd...@linaro.org) wrote:
On 7 August 2018 at 15:57, Dr. David Alan Gilbert <dgilb...@redhat.com> wrote:
* Gerd Hoffmann (kra...@redhat.com) wrote:
On Fri, Jul 20, 2018 at 10:19:48AM +0200, remy.n...@blade-group.com wrote:
From: "Remy Noel" <remy.n...@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 for the feedback, after closer inspection, the secondary-vga refcount does, indeed, never reach 0.

I noticed the bug was not present in v2.12.0 and had been visible since 93abfc88bd649de1933588bfc7175605331b3ea9 (https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg07547.html).

This patch causes the secondary-vga object to be referenced by its subregions (mrs) which are themselves referenced by its mmio region which is referenced by the device causing a reference loop. We should probably break this loop upon exit, however, i am not sure whether we should deletes the subregions or delete the mmio properly.

Thanks

Remy

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