Re: [Qemu-devel] [PATCH v4 1/3] memory: allow MemoryRegion's priority field to accept negative values

[Peter Maydell]

On 15 September 2013 17:16, Marcel Apfelbaum <address@hidden> wrote:
> Priority is used to make visible some subregions by obscuring
> the parent MemoryRegion addresses overlapping with the subregion.
>
> By allowing the priority to be negative the opposite can be done:
> Allow a subregion to be visible on all the addresses not covered
> by the parent MemoryRegion or other subregions.
>
> Signed-off-by: Marcel Apfelbaum <address@hidden>

Codewise this looks good, so:
Reviewed-by: Peter Maydell <address@hidden>

If you have to do a v5 for some reason it would be nice to:
1. update docs/memory.txt to say that priority is a signed value.
Here's my suggested text from my comments on v2 of your patch
(for adding to the 'Overlapping regions and priority' section):
====begin====
Priority values are signed, and the default value is zero. This means
that you can use memory_region_add_subregion_overlap() both to
specify a region that must sit 'above' any others (with a positive priority)
and also a background region that sits 'below' others (with a negative
priority).
====endit====

2. improve the commit message so it doesn't imply that this
  is allowing us to do something previously impossible.
(There is no difference between having a region at priority
-1 and five at priority 0, versus one region at priority 0 and
five at priority 1 -- it's merely more convenient to be able
to create the five regions at default priority (0) and have one
special case for the background region, rather than having
to specify the priority explicitly for the five others.)

I would suggest as a commit message:

===begin===
memory: Change MemoryRegion priorities from unsigned to signed

When memory regions overlap, priority can be used to specify
which of them takes priority. By making the priority values signed
rather than unsigned, we make it more convenient to implement
a situation where one "background" region should appear only
where no other region exists: rather than having to explicitly
specify a high priority for all the other regions, we can let them take
the default (zero) priority and specify a negative priority for the
background region.
===endit===

(I have read MST's followup suggested improved commit message
and borrowed bits of it for this.)

-- PMM