Re: [Qemu-devel] [PATCH 10/10] qdev: fix create in place obj's life cycle problem

[Jan Kiszka]

On 2012-08-27 20:09, Avi Kivity wrote:
> On 08/27/2012 10:14 AM, Jan Kiszka wrote:
>>>
>>> Deregistration is fine, the problem is destruction.
>>>
>>
>> It isn't as you access memory region states that can change after
>> deregistration. Devices can remove memory regions from the mapping,
>> alter and then reinsert them. The last to steps must not happen while
>> anyone is still using a reference to that region.
>>
> 
> Why not?  If the guest is accessing an mmio region while reconfiguring
> it in a way that changes its meaning, either the previous or the next
> meaning is valid.

If the memory region owner sets the content to zero or even releases it
(nothing states a memory region can only live inside a device
structure), we will crash. Restricting how a memory region can be
created and handled after it was once registered somewhere is an
unnatural interface, waiting to cause subtle bugs.

Jan

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