Re: [PATCH v1 2/5] board_init: Use memset_simple() in board_init_f_init_reserve()

[Rasmus Villemoes <rasmus.villemoes@prevas.dk>]

On 06/08/2021 15.38, Stefan Roese wrote:
> board_init_f_init_reserve() is called very early in the boot process,
> before the caches are enabled. Because of this, the optimized memset()
> version can't be used here on ARM64. With this patch, the simple memset
> version memset_simple() is used here instead.

I'm afraid this approach is pretty much whack-a-mole, and it's also a
pity that one arch's limitations/special needs affects common code. It
also means we lose the compiler's knowledge of "ah, here you're calling
a standard library function whose semantics I know".

The linux kernel has a lot of instances of self-modifying code
(alternatives, static keys, etc.). Could we do something like that in
U-Boot? That is, have the optimized asm version of memset start with an
unconditional jump to memset_simple, then when the arch is ready,
overwrite that jump instruction with a nop. Compared to the kernel, we
don't have any complications from multiple CPUs, so it shouldn't be too
hard.

That would allow all callers to just call memset(), which is better both
for the compiler and the human brain. Also, when somebody modifies the
common code later, they won't know that they need to spell memset
"memset_simple", as their modifications probably work just fine on their
platforms, but then it'll explode on ARM64. And finally, what happens if
somebody disables caches at run-time? Shouldn't we then also have a
mechanism to switch all mem* calls over to the simple versions?

Rasmus