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. >>>>> 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 >>>>>
