Re: [PATCH v3 1/6] xen: implement byteswap

[Andrew Cooper <amc96@xxxxxxxx>]

On 10/05/2022 11:15, Lin Liu wrote:
> swab() is massively over complicated and can be simplified by builtins.
> The compilers provide builtin function to swap bytes.
> * gcc:   https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
> * clang: https://clang.llvm.org/docs/LanguageExtensions.html
> This patch simplify swab() with builtins and fallback for old compilers.

Arguably, this patch introduces a new byteswapping infrastructure in
terms of compiler builtins and bswapXX(), so the swab() infrastructure
can be retired.

> diff --git a/xen/arch/arm/include/asm/byteorder.h 
> b/xen/arch/arm/include/asm/byteorder.h
> index 9c712c4788..622eeaba07 100644
> --- a/xen/arch/arm/include/asm/byteorder.h
> +++ b/xen/arch/arm/include/asm/byteorder.h
> @@ -1,16 +1,10 @@
>  #ifndef __ASM_ARM_BYTEORDER_H__
>  #define __ASM_ARM_BYTEORDER_H__
>  
> -#define __BYTEORDER_HAS_U64__
> +#ifndef __BYTE_ORDER__
> +   #define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
> +#endif

This won't actually do what you want on GCC 4.5 or older.  You also want

#ifndef __ORDER_LITTLE_ENDIAN__
# define __ORDER_LITTLE_ENDIAN__ 1234
#endif

#ifndef __ORDER_BIG_ENDIAN__
# define __ORDER_BIG_ENDIAN__ 4321
#endif

in compiler.h to cope with older GCC.

Otherwise, LGTM.  Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

I can fix this on commit if its the only issue issue.  Otherwise, please
correct it when posting v4.

~Andrew