> Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > [...] > > > Although the Linux verifier doesn't support them, the fact that gcc > > > does support them tells me that it's probably safest to list the DW > > > and LDX variants as deprecated as well, which is what the draft > > > already did in the appendix so that's good (nothing to change there, > > > I think). > > > > DW never existed in classic bpf, so abs/ind never had DW flavor. > > If some assembler/compiler decided to "support" them it's on them. > > The standard must not list such things as deprecated. They never > > existed. So nothing is deprecated. > > Ack, I will remove the ABS/IND + DW lines from the appendix. > > > Same with MSH. BPF_LDX | BPF_MSH | BPF_B is the only insn ever existed. > > It's a legacy insn. Just like abs/ind. > > Should it be listed in the legacy conformance group then? > > Currently it's not mentioned in instruction-set.rst at all, so the opcode is > available to use by any new instruction. If we do list it in instruction-set.rst > then, like abs/ind, it will be avoided by anyone proposing new instructions. Here's my understanding of this thread so far: * (IND/ABS) | (W/H/B) | LD : these are accepted by the Linux verifier and are supported by clang and gcc. They should be in the legacy conformance group of deprecated instructions. * (IND/ABS) | DW | (LD/LDX) : these are not accepted by the Linux verifier and were never used. Clang doesn't generate them but gcc did which is now removed based on this discussion. They should NOT be in the legacy conformance group of deprecated instructions because they were never defined in the first place, and instruction-set.rst should be updated to clarify this. * (IND/ABS) | (W/H/B) | LDX : these are not accepted by the Linux verifier and were never used. Clang doesn't generate them but gcc does. They should NOT be in the legacy conformance group of deprecated instructions because they were never defined in the first place, and instruction-set.rst should be updated to clarify this. * (IND/ABS) | (W/H/B/DW) | (ST/STX): these are not accepted by the Linux verifier and were never used. I don't know whether clang or gcc generates them. They should NOT be in the legacy conformance group of deprecated instructions because they were never defined in the first place, and instruction-set.rst should be updated to clarify this. * MSH | B | LDX: this existed in classic BPF but does not exist in (e)BPF since it is not accepted by the Linux verifier. I don't know whether clang ever generated them, but gcc never did. The "Legacy BPF Packet access instructions" section of instruction-set.rst says > BPF previously introduced special instructions for access to packet data that were carried > over from classic BPF. However, these instructions are deprecated and should no longer be used. I read Alexei's comment "It's a legacy insn. Just like abs/ind" as a possible argument that MSH|B|LDX should be mentioned in instruction-set.rst, pointing to the above section, like IND/ABS do. But Yonghong argued that it was never accepted by the verifier, so need not be mentioned. * MSH | (W/H/DW) | (LD/ST/STX): These are not accepted by the Linux verifier and were never used. They should NOT be in the legacy conformance group of deprecated instructions because they were never defined in the first place. Let me know if any of the above is incorrect and I can submit a doc patch. Dave
