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

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Message-ID: <CAHS8izM-3DSw+hvFasu=xge5st9cE9MrwJ3FOOHpYHsj5r0Ydg@mail.gmail.com>
Date: Mon, 16 Sep 2024 07:49:09 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: linuxppc-dev@...ts.ozlabs.org, christophe.leroy@...roup.eu, 
	segher@...nel.crashing.org, sfr@...b.auug.org.au, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, kuba@...nel.org
Subject: Re: [PATCH] powerpc/atomic: Use YZ constraints for DS-form instructions

On Mon, Sep 16, 2024 at 5:05 AM Michael Ellerman <mpe@...erman.id.au> 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.
>
> 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>

I'm not familiar enough with the code around the changes, but I have
been able to confirm cherry-picking this resolves the build issue I'm
seeing on net-next, so, FWIW,

Tested-by: Mina Almasry <almasrymina@...gle.com>

-- 
Thanks,
Mina