On Thu, Feb 9, 2023 at 2:55 PM Yonghong Song <yhs@xxxxxxxx> wrote: > > Over the past, there are some discussions to extend bpf > instruction ISA to accommodate some new use cases or > fix some potential issues. These new instructions will > be included in new cpu flavor -mcpu=v4. > > The following are the proposal > to add new instructions in 6 different categories. > The proposal is a little bit rough. You can find bpf insn > background information in Documentation/bpf/instruction-set.rst. > You comments or suggestions are welcome! > Great that we are trying to fix and complete the instruction set! Just one comment/question below for condition jumps. [...] > > 32-bit JA > ========= > > Currently, the whole range of operations with BPF_JMP32/BPF_JMP insn are > implemented like below > > ======== ===== ========================= ============ > code value description notes > ======== ===== ========================= ============ > BPF_JA 0x00 PC += off BPF_JMP only > BPF_JEQ 0x10 PC += off if dst == src > BPF_JGT 0x20 PC += off if dst > src unsigned > BPF_JGE 0x30 PC += off if dst >= src unsigned > BPF_JSET 0x40 PC += off if dst & src > BPF_JNE 0x50 PC += off if dst != src > BPF_JSGT 0x60 PC += off if dst > src signed > BPF_JSGE 0x70 PC += off if dst >= src signed > BPF_CALL 0x80 function call > BPF_EXIT 0x90 function / program return BPF_JMP only > BPF_JLT 0xa0 PC += off if dst < src unsigned > BPF_JLE 0xb0 PC += off if dst <= src unsigned > BPF_JSLT 0xc0 PC += off if dst < src signed > BPF_JSLE 0xd0 PC += off if dst <= src signed > ======== ===== ========================= ============ > > Here the 'off' is 16 bit so the range of jump is [-32768, 32767]. > In rare cases, people may have large programs or have loops fully unrolled. > This may cause some jump offset beyond the above range. In current > llvm implementation, wrong code (after truncation) will be generated. > > To fix this issue, the following new insn is proposed > > ======== ===== ========================= ============ > code value description notes > ======== ===== ========================= ============ > BPF_JA 0x00 PC += imm BPF_JMP32 only, src = 1 > > The way, the jump offset range become [-2^31, 2^31 - 1]. > > For other jump instructions, e.g., BPF_JEQ, with a jmp offset > beyond [-32768, 32767]. It can be simulated with a > 'BPF_JA (PC += imm)' followed by the original > BPF_JEQ with the range 'off', or BPF_JEQ with a short range followed > by a BPF_JA. Why not implement the same approach (using imm if src = 1) for all the conditional jumps? Just too much JIT work or some other reasons? [...]
