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. > > > >> > >> 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 > >>
