linux-kernel - Re: [PATCH] MIPS: Add 'memory' clobber to csum_ipv6

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Date: Sun, 11 Feb 2024 19:53:06 -0800
From: Charlie Jenkins <charlie@...osinc.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
	Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: [PATCH] MIPS: Add 'memory' clobber to csum_ipv6_magic() inline
 assembler

On Sun, Feb 11, 2024 at 08:08:37AM -0800, Guenter Roeck wrote:
> After 'lib: checksum: Use aligned accesses for ip_fast_csum and
> csum_ipv6_magic tests' was applied, the test_csum_ipv6_magic unit test
> started failing for all mips platforms, both little and bit endian.
> Oddly enough, adding debug code into test_csum_ipv6_magic() made the
> problem disappear.
> 
> The gcc manual says:
> 
> "The "memory" clobber tells the compiler that the assembly code performs
>  memory reads or writes to items other than those listed in the input
>  and output operands (for example, accessing the memory pointed to by one
>  of the input parameters)
> "
> 
> This is definitely the case for csum_ipv6_magic(). Indeed, adding the
> 'memory' clobber fixes the problem.
> 
> Cc: Charlie Jenkins <charlie@...osinc.com>
> Cc: Palmer Dabbelt <palmer@...osinc.com>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
>  arch/mips/include/asm/checksum.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/include/asm/checksum.h b/arch/mips/include/asm/checksum.h
> index 4044eaf989ac..0921ddda11a4 100644
> --- a/arch/mips/include/asm/checksum.h
> +++ b/arch/mips/include/asm/checksum.h
> @@ -241,7 +241,8 @@ static __inline__ __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
>  	"	.set	pop"
>  	: "=&r" (sum), "=&r" (tmp)
>  	: "r" (saddr), "r" (daddr),
> -	  "0" (htonl(len)), "r" (htonl(proto)), "r" (sum));
> +	  "0" (htonl(len)), "r" (htonl(proto)), "r" (sum)
> +	: "memory");
>  
>  	return csum_fold(sum);
>  }
> -- 
> 2.39.2
> 

Thank you for looking into this. It fixed the failure on my mips64el
qemu setup.

Reviewed-by: Charlie Jenkins <charlie@...osinc.com>