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bpf, docs: Document the byte swapping instructions 

Add a section to document the byte swapping instructions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220131183638.3934982-2-hch@lst.de
This commit is contained in:
Christoph Hellwig 2022-01-31 19:36:34 +01:00 committed by Alexei Starovoitov
parent c3be3676de
commit dd33fb571f
1 changed files with 40 additions and 4 deletions
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44
Documentation/bpf/instruction-set.rst
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@ -82,9 +82,9 @@ BPF_ALU uses 32-bit wide operands while BPF_ALU64 uses 64-bit wide operands for
otherwise identical operations.
The code field encodes the operation as below:
======== ===== ==========================
======== ===== =================================================
code value description
======== ===== ==========================
======== ===== =================================================
BPF_ADD 0x00 dst += src
BPF_SUB 0x10 dst -= src
BPF_MUL 0x20 dst \*= src
@ -98,8 +98,8 @@ The code field encodes the operation as below:
BPF_XOR 0xa0 dst ^= src
BPF_MOV 0xb0 dst = src
BPF_ARSH 0xc0 sign extending shift right
BPF_END 0xd0 endianness conversion
======== ===== ==========================
BPF_END 0xd0 byte swap operations (see separate section below)
======== ===== =================================================
BPF_ADD | BPF_X | BPF_ALU means::
@ -118,6 +118,42 @@ BPF_XOR | BPF_K | BPF_ALU64 means::
src_reg = src_reg ^ imm32
Byte swap instructions
----------------------
The byte swap instructions use an instruction class of ``BFP_ALU`` and a 4-bit
code field of ``BPF_END``.
The byte swap instructions instructions operate on the destination register
only and do not use a separate source register or immediate value.
The 1-bit source operand field in the opcode is used to to select what byte
order the operation convert from or to:
========= ===== =================================================
source value description
========= ===== =================================================
BPF_TO_LE 0x00 convert between host byte order and little endian
BPF_TO_BE 0x08 convert between host byte order and big endian
========= ===== =================================================
The imm field encodes the width of the swap operations. The following widths
are supported: 16, 32 and 64.
Examples:
``BPF_ALU | BPF_TO_LE | BPF_END`` with imm = 16 means::
dst_reg = htole16(dst_reg)
``BPF_ALU | BPF_TO_BE | BPF_END`` with imm = 64 means::
dst_reg = htobe64(dst_reg)
``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and
``BPF_TO_LE`` respetively.
Jump instructions
-----------------