Re: [PATCH V2 3/3] riscv: xchg: Prefetch the destination word for sc.w

[Andrew Jones]

On Sun, Dec 31, 2023 at 03:29:53AM -0500, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> 
> The cost of changing a cacheline from shared to exclusive state can be
> significant, especially when this is triggered by an exclusive store,
> since it may result in having to retry the transaction.
> 
> This patch makes use of prefetch.w to prefetch cachelines for write
> prior to lr/sc loops when using the xchg_small atomic routine.
> 
> This patch is inspired by commit: 0ea366f5e1b6 ("arm64: atomics:
> prefetch the destination word for write prior to stxr").
> 
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> ---
>  arch/riscv/include/asm/cmpxchg.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index 26cea2395aae..d7b9d7951f08 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -10,6 +10,7 @@
>  
>  #include <asm/barrier.h>
>  #include <asm/fence.h>
> +#include <asm/processor.h>
>  
>  #define __arch_xchg_masked(prepend, append, r, p, n)			\

Are you sure this is based on v6.7-rc7? Because I don't see this macro.

>  ({									\
> @@ -23,6 +24,7 @@
>  									\
>  	__asm__ __volatile__ (						\
>  	       prepend							\
> +	       PREFETCHW_ASM(%5)					\
>  	       "0:	lr.w %0, %2\n"					\
>  	       "	and  %1, %0, %z4\n"				\
>  	       "	or   %1, %1, %z3\n"				\
> @@ -30,7 +32,7 @@
>  	       "	bnez %1, 0b\n"					\
>  	       append							\
>  	       : "=&r" (__retx), "=&r" (__rc), "+A" (*(__ptr32b))	\
> -	       : "rJ" (__newx), "rJ" (~__mask)				\
> +	       : "rJ" (__newx), "rJ" (~__mask), "rJ" (__ptr32b)		\

I'm pretty sure we don't want to allow the J constraint for __ptr32b.

>  	       : "memory");						\
>  									\
>  	r = (__typeof__(*(p)))((__retx & __mask) >> __s);		\
> -- 
> 2.40.1
> 

Thanks,
drew