Kernel/linux-next
1
0
Fork 0
mirror of https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git synced 2025-01-08 15:04:45 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity

bpf, docs: Better document the extended instruction format

Browse Source 
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@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220131183638.3934982-5-hch@lst.de
This commit is contained in:
Christoph Hellwig 2022-01-31 19:36:37 +01:00 committed by Alexei Starovoitov
parent 1517533627
commit 5ca15b8a93
1 changed files with 19 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

26
Documentation/bpf/instruction-set.rst
View File

@ -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
-------------------------------------