[PATCH 4/4] score: Ensure stack alignment requirement

[Sebastian Huber]

On 03/03/2021 17:04, Gedare Bloom wrote:
>> +#if CPU_STACK_ALIGNMENT > CPU_HEAP_ALIGNMENT
>> +  stack_align = CPU_STACK_ALIGNMENT;
>> +  stack_begin = (char *) RTEMS_ALIGN_UP( (uintptr_t) stack_begin, stack_align );
>> +  stack_end = (char *) RTEMS_ALIGN_DOWN( (uintptr_t) stack_end, stack_align );
> Why align down here? Why align the end at all?
Most stacks grow down, so the alignment of the end address is important.
> Is there a requirement
> for the stack area to be a multiple of the stack alignment? I guess we
> can lose some bytes and return less than config->stack_size as a
> result of this?

The CPU_STACK_ALIGNMENT should be set to the stack alignment required by 
the ABI (in SMP configurations, it should be probably at least cache 
aligned, but this is another topic). In this case all stack frames will 
be on such a boundary and you can't use these extra bytes. Independent 
of this, you should probably not calculate your stack size so that you 
actually use the last byte of your stack.

Also the changed _Stack_Extend_size() includes the alignment overhead.

>
> Shouldn't we better do...
> #if CPU_STACK_ALIGNMENT > CPU_HEAP_ALIGNMENT
>> +  stack_align = CPU_STACK_ALIGNMENT;
>> +  stack_begin = (char *) RTEMS_ALIGN_UP( (uintptr_t) config->stack_area, stack_align );
> #else
> stack_begin = config->stack_area;
No, I think the code in the patch is fine, however, I will add an assert 
after the block to ensure that begin and end are aligned.
> #endif
> stack_end = stack_begin + config->stack_size;
>
> So we always return the requested stack size?
>
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