Subject: Re: [PATCH] selftests/powerpc: Only test lwm/stmw on big endian

From: Segher Boessenkool <[email-masked]>
Date: Tue, 19 Jan 2021 19:12:21 -0600

Hi!

On Tue, Jan 19, 2021 at 03:18:00PM +1100, Michael Ellerman wrote:
> Newer binutils (>= 2.36) refuse to assemble lmw/stmw when building in
> little endian mode. That breaks compilation of our alignment handler
> test:
> 
>   /tmp/cco4l14N.s: Assembler messages:
>   /tmp/cco4l14N.s:1440: Error: `lmw' invalid when little-endian
>   /tmp/cco4l14N.s:1814: Error: `stmw' invalid when little-endian
>   make[2]: *** [../../lib.mk:139: /output/kselftest/powerpc/alignment/alignment_handler] Error 1
> 
> These tests do pass on little endian machines, as the kernel will
> still emulate those instructions even when running little
> endian (which is arguably a kernel bug).

The opposite: in older ISAs it is *required* to.  On all very old ISA
versions, and when not on the Server Environment on everything before
ISA 2.07.

Many older implementations did an alignment interrupt, but that was an
implementation detail (they could still be compliant with proper system
software support, e.g. kernel emulation handlers).  Nowadays that
interrupt is required, so you can still support it like that.

(The patch is fine of course.)

Segher