On 3/20/19 5:03 PM, Stanislav Fomichev wrote: > On 03/21, Daniel Borkmann wrote: >> On 03/20/2019 11:45 PM, Yonghong Song wrote: >>> 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. >> >> I think the way this should be fixed is the following: In case >> of LLVM (aka compiling BPF prog), we want the code to be as-is, >> in case if gcc is compiling the hostprog, we either want to keep >> using __builtin_bswap16() or fall-back to something else. Thus, >> I would suggest, we add a new feature test for tooling infra under >> tools/build/feature/ that compiles a dummy prog with __builtin_bswap16(). >> And in the bpf_endian.h we define __bpf_ntohs(x) to __bpf_swab16(x) >> which either resolves to __builtin_bswap16() or some fallback >> implementation if not available. I don't think there should be much >> of an issue and it would follow the standard way to do it. > It's not as easy as llvm vs gcc. We can compile userland tests with > llvm/clang as well. We really need to distinguish between the target: bfp vs > non-bpf: always use builtins in bpf case and fallback to swab.h for > userland (or use feature detection, but swab.h should be enough in > theory). > > Can we rely on __bpf__ define? > > $ cat tmp.c > #ifdef __bpf__ > #error a > #else > #error b > #endif > $ clang -c -target bpf tmp.c > tmp.c:2:2: error: a > #error a > ^ > 1 error generated. Yes, you can rely this, __bpf__, __bpf or __BPF__. These three are clang predefined macros for target 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 >>>>>>> >>
