On 3/20/19 3:27 PM, Stanislav Fomichev wrote:
> On 03/20, Yonghong Song wrote:
>>
>>
>> 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@xxxxxxxxx>
>>>> ---
>>>>
>>>> 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.
> Hm, isn't it the opposite of what we want then? I think for llvm+bpf we always
> want to use the builtins to make it properly generate
> BPF_TO_BE/BPF_TO_LE instructions.
Okay, I see. Then this patch will not achieve that.
The following are two common ways to compile a bpf program:
- "clang -target bpf ...", maybe add macro __BPF__ somewhere
to indicate builtin_bswap16 always available?
- "clang <host target> ..." and then "llc -march=bpf ..."
in this case, __BPF__ macro is not available and
we will not be able to use builtin swap for bpf program.
Maybe use __clang__ macro (or gcc macro) to distinguish between clang
and gcc. If it is gcc we will check builtin availability, otherwise,
we assume builtin always available? This not pretty though.
>
>>>
>>>>
>>>> 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
>>>>
