From: Dave Thaler <dthaler@xxxxxxxxxxxxx> Add docs on extended 64-bit immediate instructions, including six instructions previously undocumented. Include a brief description of maps and variables, as used by those instructions. V1 -> V2: rebased on top of latest master V2 -> V3: addressed comments from Alexei V3 -> V4: addressed comments from David Vernet Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> --- Documentation/bpf/instruction-set.rst | 57 +++++++++++++++++++++++---- Documentation/bpf/linux-notes.rst | 22 +++++++++++ 2 files changed, 71 insertions(+), 8 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index b4464058905..b3efa4b74ec 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -401,14 +401,55 @@ 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 table defines a set of ``BPF_IMM | BPF_DW | BPF_LD`` instructions +with opcode subtypes in the 'src' field, using 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 (see `Maps`_) +* map_by_idx(imm) means to convert a 32-bit index into an address of a map +* map_val(map) gets the address of the first value in a given map +* 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 + +Maps +~~~~ + +Maps are shared memory regions accessible by eBPF programs on some platforms. +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 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 f43b9c797bc..508d009d3be 100644 --- a/Documentation/bpf/linux-notes.rst +++ b/Documentation/bpf/linux-notes.rst @@ -20,6 +20,28 @@ integer would be read from a specified register, is not currently supported by the verifier. Any programs with this instruction will fail to load until such support is added. +Maps +==== + +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_idx(imm) uses the fd at that index in the array. + +Variables +========= + +The following 64-bit immediate instruction specifies that a variable address, +which corresponds to some integer stored in the 'imm' 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
