Re: [Qemu-devel] [PATCH 3/4] vfio: Move container list to iommu MemoryRegion

[Paolo Bonzini]

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Il 30/04/2013 04:23, David Gibson ha scritto:
>>> I think this is a different problem.  Basically the question is
>>> "what happens if a MemoryRegion 'disappears' while an
>>> AddressSpace is still referring to it", and the answer right
>>> now is "badness".
>> 
>> Well.. no.  The same problem may well exist for AddressSpace
>> objects, but in this case it's for the VFIO private
>> per-address-space object.

Well, once we clarify the lifetime of AddressSpaces, we can clarify
the lifetime of the VFIO private object.

>>> We should look at generic fixes before dropping hooks in the
>>> code.  At the very least an "assert(mr->parent == NULL);" is
>>> missing in memory_region_destroy.
>> 
>> Well, sure that's probably also a good idea.  But the whole point
>> here is you're insisting that the MemoryRegion code doesn't know
>> about the vfio private data, even as an opaque handle, and so
>> there's no possible assert we can put there to check it has been
>> destroyed.
> 
> Oh, yes, forgot to ask.  I'm still unclear on what the conceptual 
> difference is supposed to between a MemoryRegion and an
> AddressSpace. AFAICT AddressSpace seems to be roughly just a
> wrapper on a MemoryRegion that gives it some more features.

Yes, an AddressSpace is a wrapper for a MemoryRegion that has a NULL
- ->parent, with two extra features:

- - QEMU computes the "flat" view of the MemoryRegion and a compressed
radix tree representation of that view

- - MemoryListeners can ask to be called only for some AddressSpaces

Paolo

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