Re: [PATCH RESEND v5 1/3] x86/asm, x86/boot: expose inline memcmp

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 30, 2026 at 02:21:46PM -0300, Mauricio Faria de Oliveira wrote:
> Move the inline memcmp function currently only available in 'boot/string.c'
> into the shared string function header <asm/shared/string.h> to be reused.
> 
> This is not done through <asm/string.h> to avoid pulling unnecessary code
> in 'boot/string.c' that causes build errors in 'boot/compressed/string.c'
> and 'purgatory/purgatory.ro'.
> 
> Note that the inline memcmp() returns 0/1, not -1/0/1 as regular memcmp()
> (reported by David Laight <david.laight.linux@gmail.com>).
> 
> Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
> ---
>  arch/x86/boot/string.c               |  6 ++----
>  arch/x86/include/asm/shared/string.h | 16 ++++++++++++++++
>  arch/x86/include/asm/string.h        |  1 +
>  3 files changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
> index ac0f900ebc47efa81c92e1bb2010ea41677899c4..be454a6864225f3a972c3e81826b77ed4e8a57fe 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/shared/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/shared/string.h b/arch/x86/include/asm/shared/string.h
> new file mode 100644
> index 0000000000000000000000000000000000000000..422952152f533ad75b98f3873297b39c4f5e2477
> --- /dev/null
> +++ b/arch/x86/include/asm/shared/string.h
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_SHARED_STRING_H
> +#define _ASM_X86_SHARED_STRING_H
> +
> +/* Note: this memcmp() returns 0/1, not -1/0/1 as regular memcmp(). */
> +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_SHARED_STRING_H */
> diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
> index 9cb5aae7fba9ffcf0f5af8f939d30467750ccaa9..f0f4fd8227bf992e78c69209efb31f0a9a0cc3b1 100644
> --- a/arch/x86/include/asm/string.h
> +++ b/arch/x86/include/asm/string.h
> @@ -7,6 +7,7 @@
>  #else
>  # include <asm/string_64.h>
>  #endif
> +#include <asm/shared/string.h>
>  
>  static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
>  {

It seems weird to have __inline_mem{cpy,set}() in a different header than
__inline_memcmp(). I'm assuming this is because boot cannot include the
normal string thing?

Perhaps make inline_string.h or somesuch to carry all three of them and
include that from the relevant places?