linux-kernel - Re: [PATCH 3/4] x86/asm: delete dummy variables in movdir64b()

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Message-ID: <Z8rdRljzdRdh9hu6@gmail.com>
Date: Fri, 7 Mar 2025 12:49:26 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Alexey Dobriyan <adobriyan@...il.com>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] x86/asm: delete dummy variables in movdir64b()


* Alexey Dobriyan <adobriyan@...il.com> wrote:

> Cast to pointer-to-array instead.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
>  arch/x86/include/asm/special_insns.h | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index d349aa0f0a83..b24c6c945c38 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -215,13 +215,10 @@ static __always_inline void serialize(void)
>  /* The dst parameter must be 64-bytes aligned */
>  static inline void movdir64b(void *dst, const void *src)
>  {
> -	const struct { char _[64]; } *__src = src;
> -	struct { char _[64]; } *__dst = dst;
> -
>  	/*
>  	 * MOVDIR64B %(rdx), rax.
>  	 *
> -	 * Both __src and __dst must be memory constraints in order to tell the
> +	 * Both src and dst must be memory constraints in order to tell the
>  	 * compiler that no other memory accesses should be reordered around
>  	 * this one.
>  	 *
> @@ -230,8 +227,8 @@ static inline void movdir64b(void *dst, const void *src)
>  	 * I.e., not the pointers but what they point to, thus the deref'ing '*'.
>  	 */
>  	asm volatile(".byte 0x66, 0x0f, 0x38, 0xf8, 0x02"
> -		     : "+m" (*__dst)
> -		     :  "m" (*__src), "a" (__dst), "d" (__src));
> +		     : "+m" (*(char(*)[64])dst)
> +		     :  "m" (*(const char(*)[64])src), "a" (dst), "d" (src));

In what world is putting type casts inside asm() statements an 
improvement to the code?

Thanks,

	Ingo