Re: [PATCH v2] ARM: Prevent the compiler from using NEON registers

[Andre Przywara <andre.przywara@arm.com>]

On Sat, 21 Aug 2021 19:54:02 -0500
Samuel Holland <samuel@sholland.org> wrote:

Hi Samuel,

thanks for the update.

> For ARMv8-A, NEON is standard, so the compiler can use it even when no
> special target flags are provided.

That is not entirely true, NEON is architecturally an optional feature,
it's just so prevalent that GCC decided to include it in the default
"armv8-a" arch definition:
https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html -march=name

> For example, it can use stores from
> NEON registers to zero-initialize large structures. GCC 11 decides to
> do this inside the DRAM init code for the Allwinner H6.
> 
> However, GCC 11 has a bug where it generates misaligned NEON register
> stores even with -mstrict-align. Since the MMU is not enabled this early
> in SPL, the misaligned store causes an exception and breaks booting.

For the records: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101934
 
> Work around this issue by restricting the compiler to using GPRs only,
> not vector registers. This prevents any future surprises relating to
> NEON use as well.

While one could argue that this is papering over a compiler bug, we
actually have no guarantee that *every* ARMv8 target core we compile
for has NEON support. Cortex-A53 (and even A55) for instance allow to
not include NEON during the integration process, so any access to NEON
registers would crash there already.

> Signed-off-by: Samuel Holland <samuel@sholland.org>

Acked-by: Andre Przywara <andre.przywara@arm.com>

Cheers,
Andre

> ---
> 
> Changes in v2:
>  - Updated commit message to describe problem more precisely
> 
>  arch/arm/config.mk | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm/config.mk b/arch/arm/config.mk
> index 16c63e12667..964c6b026ec 100644
> --- a/arch/arm/config.mk
> +++ b/arch/arm/config.mk
> @@ -25,6 +25,7 @@ endif
>  
>  PLATFORM_RELFLAGS += -fno-common -ffixed-r9
>  PLATFORM_RELFLAGS += $(call cc-option, -msoft-float) \
> +		     $(call cc-option,-mgeneral-regs-only) \
>        $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
>  
>  # LLVM support