netdev - Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection

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Date:   Wed, 20 Mar 2019 22:20:45 +0000
From:   Yonghong Song <yhs@...com>
To:     Stanislav Fomichev <sdf@...ichev.me>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
CC:     Shuah Khan <shuah@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection



On 3/20/19 10:13 AM, Stanislav Fomichev wrote:
> On 03/20, Sergey Senozhatsky wrote:
>> Not all compilers have __builtin_bswap16() and __builtin_bswap32(),
>> thus not all compilers are able to compile the following code:
>>
>>          (__builtin_constant_p(x) ? \
>>                  ___constant_swab16(x) : __builtin_bswap16(x))
>>
>> That's the reason why bpf_ntohl() doesn't work on GCC < 4.8, for
>> instance:
>>
>>          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:
>>
>>   #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>
>> ---
>>
>> v2: fixed build error, reshuffled patches (Stanislav Fomichev)
> Tested them locally with the compiler I saw the initial issues with - all
> fine, I don't see any errors with the older gcc.
> 
> One last question I have is: what happens in the llvm+bpf case? Have
> you tested that? I think LLVM has all the builtins required, but since
> we are relying on the swab.h now (and it relies on
> __HAVE_BUILTIN_BSWAP16__), I wonder whether this detection works
> correctly on the llvm when targeting bpf. (sidenote: bpf_endian.h can be
> used from both userspace and bpf programs).

Inside kernel clang compiler header (linux/compiler-clang.h) does not 
define __HAVE_BUILTIN_BSWAP16__. So it will go to the "else" branch in 
the above. So I think it should work with clang + bpf.

> 
>>
>>   tools/testing/selftests/bpf/bpf_endian.h | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/testing/selftests/bpf/bpf_endian.h b/tools/testing/selftests/bpf/bpf_endian.h
>> index b25595ea4a78..1ed268b2002b 100644
>> --- a/tools/testing/selftests/bpf/bpf_endian.h
>> +++ b/tools/testing/selftests/bpf/bpf_endian.h
>> @@ -20,12 +20,12 @@
>>    * 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_ntohs(x)			__swab16(x)
>> +# define __bpf_htons(x)			__swab16(x)
>>   # define __bpf_constant_ntohs(x)	___constant_swab16(x)
>>   # define __bpf_constant_htons(x)	___constant_swab16(x)
>> -# define __bpf_ntohl(x)			__builtin_bswap32(x)
>> -# define __bpf_htonl(x)			__builtin_bswap32(x)
>> +# define __bpf_ntohl(x)			__swab32(x)
>> +# define __bpf_htonl(x)			__swab32(x)
>>   # define __bpf_constant_ntohl(x)	___constant_swab32(x)
>>   # define __bpf_constant_htonl(x)	___constant_swab32(x)
>>   #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
>> -- 
>> 2.21.0
>>