On Tue, 5 Oct 2021 at 14:10, Arnd Bergmann [off-list ref] wrote:

On Tue, Oct 5, 2021 at 9:15 AM Ard Biesheuvel [off-list ref] wrote:

quoted

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@kernel.org>

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

quoted

+       .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?

No, it wouldn't. And the only reason for using '6' here is so that we
use blx in code that may run on both v6 and v7.

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