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

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Message-ID: <878qvqrj7e.fsf@mail.lhotse>
Date: Tue, 17 Sep 2024 14:35:17 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Mina Almasry <almasrymina@...gle.com>
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

Mina Almasry <almasrymina@...gle.com> writes:
> 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.

cheers