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@gmail.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.
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