In addition to the normal 64-bit instruction encoding, eBPF also has a single instruction that uses a second 64-bit bits for a second immediate value. Instead of only documenting this format deep down in the document mention it in the instruction encoding section. Signed-off-by: Christoph Hellwig <hch@xxxxxx> --- Documentation/bpf/instruction-set.rst | 26 +++++++++++++++++++------- 1 file changed, 19 insertions(+), 7 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index b3c2621216c97..048aea0952a2e 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -22,7 +22,13 @@ necessary across calls. Instruction encoding ==================== -eBPF uses 64-bit instructions with the following encoding: +eBPF has two instruction encodings: + + * the basic instruction encoding, which uses 64 bits to encode an instruction + * the wide instruction encoding, which appends a second 64-bit immediate value + (imm64) after the basic instruction for a total of 128 bits. + +The basic instruction encoding looks as follows: ============= ======= =============== ==================== ============ 32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB) @@ -212,7 +218,7 @@ The mode modifier is one of: ============= ===== ==================================== mode modifier value description ============= ===== ==================================== - BPF_IMM 0x00 used for 64-bit mov + BPF_IMM 0x00 64-bit immediate instructions BPF_ABS 0x20 legacy BPF packet access (absolute) BPF_IND 0x40 legacy BPF packet access (indirect) BPF_MEM 0x60 regular load and store operations @@ -287,12 +293,18 @@ You may encounter ``BPF_XADD`` - this is a legacy name for ``BPF_ATOMIC``, referring to the exclusive-add operation encoded when the immediate field is zero. -16-byte instructions --------------------- +64-bit immediate instructions +----------------------------- + +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_reg = imm64 -eBPF has one 16-byte instruction: ``BPF_LD | BPF_DW | BPF_IMM`` which consists -of two consecutive ``struct bpf_insn`` 8-byte blocks and interpreted as single -instruction that loads 64-bit immediate value into a dst_reg. Legacy BPF Packet access instructions ------------------------------------- -- 2.30.2
