Commit 5ca15b8a authored by Christoph Hellwig's avatar Christoph Hellwig Committed by Alexei Starovoitov
Browse files

bpf, docs: Better document the extended instruction format 



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: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220131183638.3934982-5-hch@lst.de
parent 15175336

Documentation/bpf/instruction-set.rst

+19 −7
Original line number Diff line number Diff line
@@ -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

-------------------------------------