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

[Avi Kivity]

On 08/27/2012 11:17 AM, Jan Kiszka wrote:
> 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.

Using an Object * allows the simple case to be really simple (object ==
device) and the hard cases to be doable.

What would you suggest as a better interface?

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.