netdev - Re: [PATCH] bpf, tests: tweak endianness selection

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Date:   Tue, 19 Mar 2019 09:01:15 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Shuah Khan <shuah@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        linux-kselftest@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] bpf, tests: tweak endianness selection

On 03/19, Sergey Senozhatsky wrote:
> Not all compilers have __builtin_bswap16() and __builtin_bswap32(),
> thus not all compilers are able to compile the following code
> (bpf_htons):
> 
> 	(__builtin_constant_p(x) ? \
> 	 	___constant_swab16(x) : __builtin_bswap16(x))
> 
> That's why, for instance, bpf_htons() doesn't work on GCC < 4.8:
> 
> 	error: implicit declaration of function '__builtin_bswap16'
> 
> We can use __builtin_bswap16() only if compiler has this built-in,
> that is, only if __HAVE_BUILTIN_BSWAP16__ is defined. Standard UAPI
> __swab16()/__swab32() take care of that, and, additionally, handle
> __builtin_constant_p() cases as well (if compiler doesn't provide
> builtin bswap with constants folding):
> 
>  #ifdef __HAVE_BUILTIN_BSWAP16__
>  #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
>  #else
>  #define __swab16(x)                             \
>          (__builtin_constant_p((__u16)(x)) ?     \
>          ___constant_swab16(x) :                 \
>          __fswab16(x))
>  #endif
> 
> So we can tweak selftests/bpf/bpf_endian.h and use UAPI
> __swab16()/__swab32().
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
>  tools/testing/selftests/bpf/bpf_endian.h | 37 +++++-------------------
>  1 file changed, 8 insertions(+), 29 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/bpf_endian.h b/tools/testing/selftests/bpf/bpf_endian.h
> index b25595ea4a78..ba06222963d5 100644
> --- a/tools/testing/selftests/bpf/bpf_endian.h
> +++ b/tools/testing/selftests/bpf/bpf_endian.h
> @@ -20,38 +20,17 @@
>   * use different targets.
>   */
>  #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> -# define __bpf_ntohs(x)			__builtin_bswap16(x)
> -# define __bpf_htons(x)			__builtin_bswap16(x)
> -# define __bpf_constant_ntohs(x)	___constant_swab16(x)
> -# define __bpf_constant_htons(x)	___constant_swab16(x)
This breaks the build until your next patch is applied (in other
words, breaks bisection). Can we do it in three steps?
Convert to swab (without breaking existing tests), convert the tests,
remove unused __bpf_xyz defines?

Could you also send it as a series (git format-patch --thread)? Those
patches depend on each other. And pls use [PATCH bpf-next] ... subj.

> -# define __bpf_ntohl(x)			__builtin_bswap32(x)
> -# define __bpf_htonl(x)			__builtin_bswap32(x)
> -# define __bpf_constant_ntohl(x)	___constant_swab32(x)
> -# define __bpf_constant_htonl(x)	___constant_swab32(x)
> +# define bpf_ntohs(x)		__swab16(x)
> +# define bpf_htons(x)		__swab16(x)
> +# define bpf_ntohl(x)		__swab32(x)
> +# define bpf_htonl(x)		__swab32(x)
>  #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> -# define __bpf_ntohs(x)			(x)
> -# define __bpf_htons(x)			(x)
> -# define __bpf_constant_ntohs(x)	(x)
> -# define __bpf_constant_htons(x)	(x)
> -# define __bpf_ntohl(x)			(x)
> -# define __bpf_htonl(x)			(x)
> -# define __bpf_constant_ntohl(x)	(x)
> -# define __bpf_constant_htonl(x)	(x)
> +# define bpf_ntohs(x)		(x)
> +# define bpf_htons(x)		(x)
> +# define bpf_ntohl(x)		(x)
> +# define bpf_htonl(x)		(x)
>  #else
>  # error "Fix your compiler's __BYTE_ORDER__?!"
>  #endif
>  
> -#define bpf_htons(x)				\
> -	(__builtin_constant_p(x) ?		\
> -	 __bpf_constant_htons(x) : __bpf_htons(x))
> -#define bpf_ntohs(x)				\
> -	(__builtin_constant_p(x) ?		\
> -	 __bpf_constant_ntohs(x) : __bpf_ntohs(x))
> -#define bpf_htonl(x)				\
> -	(__builtin_constant_p(x) ?		\
> -	 __bpf_constant_htonl(x) : __bpf_htonl(x))
> -#define bpf_ntohl(x)				\
> -	(__builtin_constant_p(x) ?		\
> -	 __bpf_constant_ntohl(x) : __bpf_ntohl(x))
> -
>  #endif /* __BPF_ENDIAN__ */
> -- 
> 2.21.0
>