Re: Trying to understand QOM object creation and property linking

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Wed, 5 Jan 2022 at 21:05, Alex Bennée <alex.bennee@linaro.org> wrote:
>> Can't be added as a subregion to the container...
>>
>>   qemu-system-arm: ../../softmmu/memory.c:2538: memory_region_add_subregion_common: Assertion `!subregion->container' failed.
>
> This assert means you tried to add the same MemoryRegion
> as a subregion of more than one parent MR.

Right - that is probably something we should make (more?) explicitly
clear in the Memory API docs.

> You can either:
>  * pass all the CPUs the same container as their "memory" link,
>    if they all see the same view of the world

This should be the case - I don't think the different cores have any
particular different view of the world. The use of the two 4kb banks I
think is purely convention.

However trying for a single container shared between both cores fails
because armv7m_realize adds it's board_memory to another container:

    memory_region_add_subregion_overlap(&s->container, 0, s->board_memory, -1);

So I guess I just have to repeat the creation of the aliases for each
core. This seems needlessly messy...

>  * if they have different views of the world, you need to
>    create a container for each CPU to be the "memory" link,
>    and to populate that container you need to create N-1 alias MRs
>    of the board_memory MR (CPU 0's container can use the original
>    board_memory MR; CPU 1, ... use the aliases).
>
> Example of option 1: virt board
> Example of option 2: hw/arm/armsse.c (look at what it does with
> the s->cpu_container[] and s->container_alias[] arrays)
>
> -- PMM


-- 
Alex Bennée