Re: [Qemu-devel] [PATCH memory v1 1/1] memory: remove may_overlap property

[Peter Maydell <peter.maydell@linaro.org>]

On 15 August 2014 08:17, Peter Crosthwaite <peter.crosthwaite@xilinx.com> wrote:
> In a5e1cbc80e88ed7d73b3fcb46053a3ba167293fc the enforcement of Memory
> collisions was disabled. This means that the MemoryRegion map_overlap
> state is unused. Remove it completely.
>
> The commit mentions that it should be fixed, but we have been living
> happily-every-after since removal of the check so it's probably
> unneeded complication.

Um. I think in general colliding memory regions imply a bug in the
machine model which is creating them. We have the collisions
disabled because nobody's got round to fixing this bug in the
PC model yet and it happens to be harmless there. I'm not
really convinced that yanking out the check code is better than
fixing the PC model...

> If we were to repair this, a simpler and more effective check would be
> to only assert collisions between same-priority regions. The fact that
> colliding memory regions may-overlap is then left as implicit by the
> fact that they have different priorities.

I'm not sure your suggestion here would work, because priorities
are only significant relative to other regions within the same
container, whereas collisions can occur between two regions
which don't have the same parent container and whose priorities are
therefore not comparable. (For instance, consider [ A [ B C ] ]
where A and B end up overlapping.)

thanks
-- PMM