Re: [PATCH 1/3] x86/asm, x86/boot: Expose inline memcmp

[David Laight <david.laight.linux@xxxxxxxxx>]

On Wed, 22 Apr 2026 17:07:45 -0300
Mauricio Faria de Oliveira <mfo@xxxxxxxxxx> wrote:

> Move the inline memcmp function, currently only available to boot code
> (boot/string.c), into the header with similar inline string functions
> (include/asm/string.h) so it may be reused.

This needs a comment/warning that it is incompatible with normal memcmp()
because it only returns 0/1 not -1/0/1.
Most callers don't care.

        David


> 
> Add a _SETUP guard in string.h so not to include the 32/64-bit specific
> string headers in boot/ code (16-bit, real mode) and avoid build errors.
> 
> Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
> ---
>  arch/x86/boot/string.c        |  6 ++----
>  arch/x86/include/asm/string.h | 11 +++++++++++
>  2 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
> index 
> b25c6a9303b7314d5caf5c9306239811705294fe..bbee78637b349e42e9281d8df50d89d48f4490b9
>  100644
> --- a/arch/x86/boot/string.c
> +++ b/arch/x86/boot/string.c
> @@ -15,6 +15,7 @@
>  #include <linux/errno.h>
>  #include <linux/limits.h>
>  #include <asm/asm.h>
> +#include <asm/string.h>
>  #include "ctype.h"
>  #include "string.h"
>  
> @@ -31,10 +32,7 @@
>  
>  int memcmp(const void *s1, const void *s2, size_t len)
>  {
> -     bool diff;
> -     asm("repe cmpsb"
> -         : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
> -     return diff;
> +     return __inline_memcmp(s1, s2, len);
>  }
>  
>  /*
> diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
> index 
> 9cb5aae7fba9ffcf0f5af8f939d30467750ccaa9..736a6f6a31f0a68281b4f17415aba0fcd95dc228
>  100644
> --- a/arch/x86/include/asm/string.h
> +++ b/arch/x86/include/asm/string.h
> @@ -2,11 +2,13 @@
>  #ifndef _ASM_X86_STRING_H
>  #define _ASM_X86_STRING_H
>  
> +#ifndef _SETUP
>  #ifdef CONFIG_X86_32
>  # include <asm/string_32.h>
>  #else
>  # include <asm/string_64.h>
>  #endif
> +#endif
>  
>  static __always_inline void *__inline_memcpy(void *to, const void *from, 
> size_t len)
>  {
> @@ -29,4 +31,13 @@ static __always_inline void *__inline_memset(void *s, int 
> v, size_t n)
>       return ret;
>  }
>  
> +static __always_inline int __inline_memcmp(const void *s1, const void *s2, 
> size_t len)
> +{
> +     bool diff;
> +
> +     asm("repe cmpsb"
> +         : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
> +     return diff;
> +}
> +
>  #endif /* _ASM_X86_STRING_H */
>