linux-kernel - Re: [PATCH] powerpc/32: Remove memory clobber asm constraint on dcbX() functions

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Message-ID: <392ac8d7-c848-9661-706b-0fbb5b9e54bc@c-s.fr>
Date:   Fri, 3 May 2019 16:14:33 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/32: Remove memory clobber asm constraint on
 dcbX() functions

Segher,

A while ago I proposed the following patch, and didn't get any comment 
back on it.

Do you have any opinion on it ? Is it good and worth it ?

Thanks
Christophe

Le 09/01/2018 à 07:57, Christophe Leroy a écrit :
> Instead of just telling GCC that dcbz(), dcbi(), dcbf() and dcbst()
> clobber memory, tell it what it clobbers:
> * dcbz(), dcbi() and dcbf() clobbers one cacheline as output
> * dcbf() and dcbst() clobbers one cacheline as input
>
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
> arch/powerpc/include/asm/cache.h | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/cache.h 
> b/arch/powerpc/include/asm/cache.h
> index c1d257aa4c2d..fc8fe18acf8c 100644
> --- a/arch/powerpc/include/asm/cache.h
> +++ b/arch/powerpc/include/asm/cache.h
> @@ -82,22 +82,31 @@ extern void _set_L3CR(unsigned long);
> static inline void dcbz(void *addr)
> {
> - __asm__ __volatile__ ("dcbz 0, %0" : : "r"(addr) : "memory");
> + __asm__ __volatile__ ("dcbz 0, %1" :
> + "=m"(*(char (*)[L1_CACHE_BYTES])addr) :
> + "r"(addr) :);
> }
> static inline void dcbi(void *addr)
> {
> - __asm__ __volatile__ ("dcbi 0, %0" : : "r"(addr) : "memory");
> + __asm__ __volatile__ ("dcbi 0, %1" :
> + "=m"(*(char (*)[L1_CACHE_BYTES])addr) :
> + "r"(addr) :);
> }
> static inline void dcbf(void *addr)
> {
> - __asm__ __volatile__ ("dcbf 0, %0" : : "r"(addr) : "memory");
> + __asm__ __volatile__ ("dcbf 0, %1" :
> + "=m"(*(char (*)[L1_CACHE_BYTES])addr) :
> + "r"(addr), "m"(*(char (*)[L1_CACHE_BYTES])addr) :
> + );
> }
> static inline void dcbst(void *addr)
> {
> - __asm__ __volatile__ ("dcbst 0, %0" : : "r"(addr) : "memory");
> + __asm__ __volatile__ ("dcbst 0, %0" : :
> + "r"(addr), "m"(*(char (*)[L1_CACHE_BYTES])addr) :
> + );
> }
> #endif /* !__ASSEMBLY__ */
> #endif /* __KERNEL__ */
>