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

[Peter Maydell]

On 7 August 2018 at 15:57, Dr. David Alan Gilbert <address@hidden> wrote:
> * Gerd Hoffmann (address@hidden) wrote:
>> On Fri, Jul 20, 2018 at 10:19:48AM +0200, address@hidden wrote:
>> > From: "Remy Noel" <address@hidden>
>> >
>> > 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.

thanks
-- PMM