Re: [Qemu-devel] [PATCH 1/2] memory: Provide separate handling of unassigned io ports accesses

[Peter Maydell]

On 5 August 2013 11:30, Jan Kiszka <address@hidden> wrote:
> On 2013-08-05 11:59, Peter Maydell wrote:
>> Or do you mean that if we had:
>>
>>  [ system memory region, with its own default read/write ops ]
>
> I cannot imagine how this could work. The system memory region has no
> clue about what the regions below it can handle and what not. So it has
> to pass through the io window.

The system memory region's just a container, you can add a
background region to it at lowest-possible-priority, which
then takes accesses which are either (a) not in any subregion
or (b) in a subregion but that container doesn't specify
its own io ops and nothing in that container handles the
access. (Or you could create the system memory region with
its own IO ops, which would have the same effect.)

This works because the memory subsystem flattens the whole
tree, and at flattening time render_memory_region() effectively
knows if there are gaps in the subcontainer or not. At runtime
we never "pass through" anything (except for the special case
of an IOMMU) -- the tree of regions has been preflattened
and the target of the memory access is known in advance.

-- PMM