[PATCH v2 2/9] ARM: assembler: introduce bl_r and bl_m macros

[Arnd Bergmann]

On Tue, Oct 5, 2021 at 9:15 AM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> Add some macros that abstract the difference between the ways indirect
> calls are performed on older and newer ARM architecture revisions.
>
> The main difference is to prefer blx instructions over explicit LR
> assignments when possible, as these tend to confuse the prediction logic
> in out-of-order cores when speculating across a function return.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

Reviewed-by: Arnd Bergmann <arnd at arndb.de>

> +       .macro          bl_r, dst:req, c
> +       .if             __LINUX_ARM_ARCH__ < 6
> +       mov\c           lr, pc
> +       mov\c           pc, \dst
> +       .else
> +       blx\c           \dst
> +       .endif
> +       .endm

I suppose we could use blx on ARMv5 as well, but it wouldn't make a difference
there, right?

        Arnd