linux-kernel - Re: [PATCH] powerpc/atomic: Use YZ constraints for DS-form instructions

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Message-ID: <20240917073750.GZ29862@gate.crashing.org>
Date: Tue, 17 Sep 2024 02:37:50 -0500
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: linuxppc-dev@...ts.ozlabs.org, christophe.leroy@...roup.eu,
        sfr@...b.auug.org.au, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, almasrymina@...gle.com, kuba@...nel.org
Subject: Re: [PATCH] powerpc/atomic: Use YZ constraints for DS-form instructions

Hi!

On Mon, Sep 16, 2024 at 10:05:10PM +1000, Michael Ellerman wrote:
> The 'ld' and 'std' instructions require a 4-byte aligned displacement
> because they are DS-form instructions. But the "m" asm constraint
> doesn't enforce that.
> 
> That can lead to build errors if the compiler chooses a non-aligned
> displacement, as seen with GCC 14:
> 
>   /tmp/ccuSzwiR.s: Assembler messages:
>   /tmp/ccuSzwiR.s:2579: Error: operand out of domain (39 is not a multiple of 4)
>   make[5]: *** [scripts/Makefile.build:229: net/core/page_pool.o] Error 1
> 
> Dumping the generated assembler shows:
> 
>   ld 8,39(8)       # MEM[(const struct atomic64_t *)_29].counter, t
> 
> Use the YZ constraints to tell the compiler either to generate a DS-form
> displacement, or use an X-form instruction, either of which prevents the
> build error.

Great explanation text, a perfect commit!  :-)

> See commit 2d43cc701b96 ("powerpc/uaccess: Fix build errors seen with
> GCC 13/14") for more details on the constraint letters.
> 
> Fixes: 9f0cbea0d8cc ("[POWERPC] Implement atomic{, 64}_{read, write}() without volatile")
> Cc: stable@...r.kernel.org # v2.6.24+
> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
> Closes: https://lore.kernel.org/all/20240913125302.0a06b4c7@canb.auug.org.au
> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>

Reviewed-By: Segher Boessenkool <segher@...nel.crashing.org>


Segher