Signed-off-by: Will Hawkins <hawkinsw@xxxxxx> --- .../bpf/standardization/instruction-set.rst | 31 ++++++++++++++++--- 1 file changed, 26 insertions(+), 5 deletions(-) diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst index fe296f35e5a7..6f3b34ef7b7c 100644 --- a/Documentation/bpf/standardization/instruction-set.rst +++ b/Documentation/bpf/standardization/instruction-set.rst @@ -73,6 +73,27 @@ Functions format and returns the equivalent number with the same bit width but opposite endianness. + +Definitions +----------- + +.. glossary:: + + Sign Extend + To `sign extend an` ``X`` `-bit number, A, to a` ``Y`` `-bit number, B ,` means to + + #. Copy all ``X`` bits from `A` to the lower ``X`` bits of `B`. + #. Set the value of the remaining ``Y`` - ``X`` bits of `B` to the value of + the most-significant bit of `A`. + +.. admonition:: Example + + Sign extend an 8-bit number ``A`` to a 16-bit number ``B`` on a big-endian platform: + :: + + A: 10000110 + B: 11111111 10000110 + Registers and calling convention ================================ @@ -263,17 +284,17 @@ where '(u32)' indicates that the upper 32 bits are zeroed. Note that most instructions have instruction offset of 0. Only three instructions (``BPF_SDIV``, ``BPF_SMOD``, ``BPF_MOVSX``) have a non-zero offset. -The devision and modulo operations support both unsigned and signed flavors. +The division and modulo operations support both unsigned and signed flavors. For unsigned operations (``BPF_DIV`` and ``BPF_MOD``), for ``BPF_ALU``, 'imm' is interpreted as a 32-bit unsigned value. For ``BPF_ALU64``, -'imm' is first sign extended from 32 to 64 bits, and then interpreted as -a 64-bit unsigned value. +'imm' is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then +interpreted as a 64-bit unsigned value. For signed operations (``BPF_SDIV`` and ``BPF_SMOD``), for ``BPF_ALU``, 'imm' is interpreted as a 32-bit signed value. For ``BPF_ALU64``, 'imm' -is first sign extended from 32 to 64 bits, and then interpreted as a -64-bit signed value. +is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then +interpreted as a 64-bit signed value. The ``BPF_MOVSX`` instruction does a move operation with sign extension. ``BPF_ALU | BPF_MOVSX`` sign extends 8-bit and 16-bit operands into 32 -- 2.41.0
