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: > + > +========================= ====== === ========================================= =========== ============== > +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. > + > Legacy BPF Packet access instructions > ===================================== > > -- > 2.33.4 > > -- > Bpf mailing list > Bpf@xxxxxxxx > https://www.ietf.org/mailman/listinfo/bpf
