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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH] secondary-vga: unregister vram on unplug.
Date: Sun, 12 Aug 2018 12:19:03 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 11/08/2018 21:07, Remy NOEL wrote:
> On 08/07/2018 05:09 PM, Dr. David Alan Gilbert wrote:
>> * Peter Maydell (address@hidden) wrote:
>>> 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.
>> 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.

I'll take a look...


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