linux-kernel - Re: [PATCH 2/2] prefetch: Add ARCH_HAS_PREFETCH definition when the architecture is not defined

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Date: Thu, 30 May 2024 18:30:15 +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 2/2] prefetch: Add ARCH_HAS_PREFETCH definition when the
 architecture is not defined



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()"), the
> __builtin_prefetch implementation will be used by default, so add its definition
> to return to a state similar to before commit[1].

I find those references to the past state somewhat confusing and not 
really adding sustenance to the explanation.

> 
> Currently prefetch_range() will be empty implemented under the x86_64 architecture,
> there was a concrete implementation before "x86 & generic: change to
> __builtin_prefetch()", so fix it.

Actually prefetch_range will be empty for every architecture which 
doesn't defined ARCH_HAS_PREFETCH and since we have a working generic 
fallback this indeed seems backwards. So defining ARCH_HAS_PREFETCH 
makes sense.

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

> 
> No similar changes have been made to ARCH_HAS_PREFETCHW at this time.
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/include/linux/prefetch.h?id=8e4f2fd31bf737abb392e694898a1496157623b5
> 
> Fixes: ab483570a13b ("x86 & generic: change to __builtin_prefetch()")
> Signed-off-by: Youling Tang <tangyouling@...inos.cn>
> ---
>   include/linux/prefetch.h | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/prefetch.h b/include/linux/prefetch.h
> index b068e2e60939..162b7105e37c 100644
> --- a/include/linux/prefetch.h
> +++ b/include/linux/prefetch.h
> @@ -36,6 +36,7 @@ struct page;
>   */
>   
>   #ifndef ARCH_HAS_PREFETCH
> +#define ARCH_HAS_PREFETCH
>   #define prefetch(x) __builtin_prefetch(x)
>   #endif
>