linux-kernel - Re: [PATCH 1/2] x86: Remove the prefetch() specific implementation on x86

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Date: Thu, 30 May 2024 18:26:19 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Youling Tang <youling.tang@...ux.dev>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
 Mateusz Guzik <mjguzik@...il.com>, Youling Tang <tangyouling@...inos.cn>
Subject: Re: [PATCH 1/2] x86: Remove the prefetch() specific implementation on
 x86_64



On 29.05.24 г. 6:20 ч., Youling Tang wrote:
> From: Youling Tang <tangyouling@...inos.cn>
> 
> After commit ab483570a13b ("x86 & generic: change to __builtin_prefetch()"),
> x86_64 directly uses __builtin_prefetch() without the specific implementation
> of prefetch(). Also, x86_64 use a generic definition until commit ae2e15eb3b6c
> ("x86: unify prefetch operations"). So remove it.


So this patch just ensures the x86-specific prefetch() implementation is 
defined only for 32bit case, otherwise we have it defined for the 64bit 
case as well but effectively it's not used since ARCH_HAS_PREFETCH is 
not defined for 64bit, meaning in the 64bit case prefetch() is still 
defined to __builtint_prefetch in include/linux/prefetch.h.


In essence this is a purely cosmetic cleanup , am I right?


I compiled a file that utilizes prefetch with and without your patch and 
the generated assembly is identical.


Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>


> 
> Signed-off-by: Youling Tang <tangyouling@...inos.cn>
> ---
>   arch/x86/include/asm/processor.h | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index cb4f6c513c48..44371bdcc59d 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -599,9 +599,6 @@ extern char			ignore_fpu_irq;
>   #ifdef CONFIG_X86_32
>   # define BASE_PREFETCH		""
>   # define ARCH_HAS_PREFETCH
> -#else
> -# define BASE_PREFETCH		"prefetcht0 %1"
> -#endif
>   
>   /*
>    * Prefetch instructions for Pentium III (+) and AMD Athlon (+)
> @@ -616,6 +613,10 @@ static inline void prefetch(const void *x)
>   			  "m" (*(const char *)x));
>   }
>   
> +#else
> +# define BASE_PREFETCH		"prefetcht0 %1"
> +#endif
> +
>   /*
>    * 3dnow prefetch to get an exclusive cache line.
>    * Useful for spinlocks to avoid one state transition in the