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

[Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>]

On 2026-04-22 17:54, David Laight wrote:
> 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.

Thanks for catching and suggesting this.

I can also change it to -1/0/1 in a later patch, to keep v2 only a fix
and split a behavior change, with something like this (lightly tested):

static __always_inline int __inline_memcmp(const void *s1, const void
*s2, size_t len)
{
        bool above, below;

        asm("repe cmpsb"
            : "+S" (s1), "+D" (s2), "+c" (len),
              "=@cca" (above),
              "=@ccb" (below));

        return above - below;
}

cheers,

> 
>       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 */
>>

-- 
Mauricio