[PATCH 2/2] ARM: prefer "bx reg" over "mov pc, reg" for all registers

[Ard Biesheuvel]

On 21 April 2015 at 12:07, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Tue, Apr 21, 2015 at 11:09:32AM +0200, Ard Biesheuvel wrote:
>> Commit 6ebbf2ce437b (ARM: convert all "mov.* pc, reg" to "bx reg"
>> for ARMv6+) replaced all occurrences of 'mov pc, <reg>' with the
>> 'ret' macro. However, this macro only emits the 'bx' instruction
>> when used with the 'lr' register, but still uses 'mov pc, <reg>'
>> for everything else.
>>
>> Since ARM/Thumb2 interworking is allowed in the static kernel
>> (i.e., inside vmlinux), this is potentially unsafe, since the mov
>> instruction will not switch modes based on the Thumb bit.
>>
>> So instead, emit the 'bx' instruction in all cases, and not just
>> for the 'lr' register.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>
> NAK.  This is very much intentional, but I'm unable to explain why
> publically.  Not every "mov pc, xx" should be a bx instruction.
>

Hhm, ok. How about only for CONFIG_THUMB2_KERNEL then?