On Sat, Mar 25, 2023 at 10:28:39PM -0500, David Vernet wrote: > On Sat, Mar 25, 2023 at 10:43:05PM +0000, Dave Thaler wrote: > > From: Dave Thaler <dthaler@xxxxxxxxxxxxx> > > > > Add docs on extended 64-bit immediate instructions, including six instructions > > previously undocumented. Include a brief description of map objects, and variables, > > as used by those instructions. > > > > --- > > V1 -> V2: rebased on top of latest master > > > > V2 -> V3: addressed comments from Alexei > > > > Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> > > --- > > Documentation/bpf/instruction-set.rst | 56 +++++++++++++++++++++++---- > > Documentation/bpf/linux-notes.rst | 13 +++++++ > > 2 files changed, 61 insertions(+), 8 deletions(-) > > > > diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst > > index db8789e6969..2c8347d63e7 100644 > > --- a/Documentation/bpf/instruction-set.rst > > +++ b/Documentation/bpf/instruction-set.rst > > @@ -385,14 +385,54 @@ and loaded back to ``R0``. > > ----------------------------- > > > > Instructions with the ``BPF_IMM`` 'mode' modifier use the wide instruction > > -encoding for an extra imm64 value. > > - > > -There is currently only one such instruction. > > - > > -``BPF_LD | BPF_DW | BPF_IMM`` means:: > > - > > - dst = imm64 > > - > > +encoding defined in `Instruction encoding`_, and use the 'src' field of the > > +basic instruction to hold an opcode subtype. > > + > > +The following instructions are defined, and use additional concepts defined below: > > nit: Perhaps this is a bit clearer? Wdyt? > > The following opcode subtypes are defined for `BPF_IMM | BPF_DQ | > BPF_LD` instructions, with new terms such as "map" defined further > below: Oops, meant to say `BPF_IMM | BPF_DW | BPF_LD` > > > + > > +========================= ====== === ========================================= =========== ============== > > +opcode construction opcode src pseudocode imm type dst type > > +========================= ====== === ========================================= =========== ============== > > +BPF_IMM | BPF_DW | BPF_LD 0x18 0x0 dst = imm64 integer integer > > +BPF_IMM | BPF_DW | BPF_LD 0x18 0x1 dst = map_by_fd(imm) map fd map > > +BPF_IMM | BPF_DW | BPF_LD 0x18 0x2 dst = map_val(map_by_fd(imm)) + next_imm map fd data pointer > > +BPF_IMM | BPF_DW | BPF_LD 0x18 0x3 dst = var_addr(imm) variable id data pointer > > +BPF_IMM | BPF_DW | BPF_LD 0x18 0x4 dst = code_addr(imm) integer code pointer > > +BPF_IMM | BPF_DW | BPF_LD 0x18 0x5 dst = map_by_idx(imm) map index map > > +BPF_IMM | BPF_DW | BPF_LD 0x18 0x6 dst = map_val(map_by_idx(imm)) + next_imm map index data pointer > > +========================= ====== === ========================================= =========== ============== > > + > > +where > > + > > +* map_by_fd(imm) means to convert a 32-bit POSIX file descriptor into an address of a map object (see `Map objects`_) > > +* map_by_idx(imm) means to convert a 32-bit index into an address of a map object > > +* map_val(map) gets the address of the first value in a given map object > > +* var_addr(imm) gets the address of a platform variable (see `Platform Variables`_) with a given id > > +* code_addr(imm) gets the address of the instruction at a specified relative offset in number of (64-bit) instructions > > +* the 'imm type' can be used by disassemblers for display > > +* the 'dst type' can be used for verification and JIT compilation purposes > > + > > +Map objects > > +~~~~~~~~~~~ > > + > > +Maps are shared memory regions accessible by eBPF programs on some platforms, where we use the term "map object" > > +to refer to an object containing the data and metadata (e.g., size) about the memory region. > > +A map can have various semantics as defined in a separate document, and may or may not have a single > > +contiguous memory region, but the 'map_val(map)' is currently only defined for maps that do have a single > > +contiguous memory region. > > + > > +Each map object can have a POSIX file descriptor (fd) if supported by the platform, > > +where 'map_by_fd(imm)' means to get the map with the specified file descriptor. > > +Each BPF program can also be defined to use a set of maps associated with the program > > +at load time, and 'map_by_idx(imm)' means to get the map with the given index in the set > > +associated with the BPF program containing the instruction. > > + > > +Platform Variables > > +~~~~~~~~~~~~~~~~~~ > > + > > +Platform variables are memory regions, identified by integer ids, exposed by the runtime and accessible by BPF programs on > > +some platforms. The 'var_addr(imm)' operation means to get the address of the memory region > > +identified by the given id. > > > > Legacy BPF Packet access instructions > > ------------------------------------- > > diff --git a/Documentation/bpf/linux-notes.rst b/Documentation/bpf/linux-notes.rst > > index 956b0c86699..2d161467105 100644 > > --- a/Documentation/bpf/linux-notes.rst > > +++ b/Documentation/bpf/linux-notes.rst > > @@ -12,6 +12,19 @@ Byte swap instructions > > > > ``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and ``BPF_TO_BE`` respectively. > > > > +Map objects > > +=========== > > + > > +Linux only supports the 'map_val(map)' operation on array maps with a single element. > > + > > +Linux uses an fd_array to store maps associated with a BPF program. Thus, > > +map_by_index(index) uses the fd at that index in the array. > > + > > +Variables > > +========= > > + > > +Linux uses BTF ids to identify variables. > > Not quite sure exactly what this means. Linux uses BTF ids to identify > _types_, right? This doesn't seem like something that needs to be > specified as Linux specific either, even if it's not yet supported > elsewhere. Certain legacy things such as Linux-specific helpers make > sense, but not sure about BTF ids. Ok, I read over this some more and I think I understand what the intention was here. You were specifying that for the following instruction encoding: ========================= ====== === =================== =========== ============ opcode construction opcode src pseudocode imm type dst type ========================= ====== === =================== =========== ============ BPF_IMM | BPF_DW | BPF_LD 0x18 0x3 dst = var_addr(imm) variable id data pointer imm is a BTF ID, and it is Linux-specific that var_addr(imm) happens to be mapping a BTF ID to an actual variable address that is stored in dst. Is that right? If so, I think this section needs to include a bit more context so that it makes sense on its own. What about something like this: The following 64-bit immediate instruction specifies that a variable address, which corresponds to some integer stored in the immediate field, should be loaded: ========================= ====== === ========================================= =========== ============== opcode construction opcode src pseudocode imm type dst type ========================= ====== === ========================================= =========== ============== BPF_IMM | BPF_DW | BPF_LD 0x18 0x3 dst = var_addr(imm) variable id data pointer On Linux, this integer is a BTF ID. > > + > > Legacy BPF Packet access instructions > > ===================================== > > > > -- > > 2.33.4 > > > > -- > > Bpf mailing list > > Bpf@xxxxxxxx > > https://www.ietf.org/mailman/listinfo/bpf
