On Wed, Apr 12, 2017 at 03:02:00PM +0200, Johannes Berg wrote: > On Tue, 2017-04-11 at 13:06 +0200, Daniel Borkmann wrote: > > > > There are instructions to convert endianess, see __bpf_prog_run(), > > the ALU_END_TO_BE, ALU_END_TO_LE labels for details. There's a > > BPF_ENDIAN() macro used in the test suite and other places. > > Are these hooked up to llvm intrinsics or so? If not, can I do that > through some kind of inline asm statement?
llvm doesn't support bpf inline asm yet. > In the samples, I only see people doing > > #define _htonl __builtin_bswap32 > > but I'm not even completely convinced that's correct, since it assumes > a little-endian host? oh well, time to face the music. In llvm backend I did: // bswap16, bswap32, bswap64 class BSWAP<bits<32> SizeOp, string OpcodeStr, list<dag> Pattern> ... let op = 0xd; // BPF_END let BPFSrc = 1; // BPF_TO_BE (TODO: use BPF_TO_LE for big-endian target) let BPFClass = 4; // BPF_ALU so __builtin_bswap32() is not a normal bswap. It's only doing bswap if the compiled program running on little endian arch. The plan was to fix it up for -march=bpfeb target (hence the comment above), but it turned out that such __builtin_bswap32 matches perfectly to _htonl() semantics, so I left it as-is even for -march=bpfeb. On little endian: ld_abs_W = *(u32*) + real bswap32 __builtin_bswap32() == bpf_to_be insn = real bswap32 On big endian: ld_abs_W = *(u32*) __builtin_bswap32() == bpf_to_be insn = nop so in samples/bpf/*.c: load_word() + _htonl()(__builtin_bswap32) has the same semantics for both little and big endian archs, hence all networking sample code in samples/bpf/*_kern.c works fine. imo the whole thing is crazy ugly. llvm doesn't have 'htonl' equivalent builtin, so builtin_bswap was the closest I could use to generate bpf_to_[bl]e insn. To solve this properly I think we need two things: . proper bswap32 insn in BPF . extend llvm with bpf_to_be/le builtins Both are not trivial amount of work.
