Commit be3193cd authored by Christoph Hellwig's avatar Christoph Hellwig Committed by Alexei Starovoitov
Browse files

bpf, docs: Add subsections for ALU and JMP instructions 



Add a little more stucture to the ALU/JMP documentation with sections and
improve the example text.

Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220103183556.41040-3-hch@lst.de
parent 62e46838

Documentation/bpf/instruction-set.rst

+32 −20
Original line number Diff line number Diff line
@@ -74,7 +74,13 @@ The 4th bit encodes the source operand: 


The four MSB bits store the operation code.



For class BPF_ALU or BPF_ALU64:



Arithmetic instructions

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



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
@@ -95,7 +101,29 @@ For class BPF_ALU or BPF_ALU64: 
  BPF_END   0xd0   endianness conversion

  ========  =====  =========================



For class BPF_JMP or BPF_JMP32:

BPF_ADD | BPF_X | BPF_ALU means::



  dst_reg = (u32) dst_reg + (u32) src_reg;



BPF_ADD | BPF_X | BPF_ALU64 means::



  dst_reg = dst_reg + src_reg



BPF_XOR | BPF_K | BPF_ALU means::



  src_reg = (u32) src_reg ^ (u32) imm32



BPF_XOR | BPF_K | BPF_ALU64 means::



  src_reg = src_reg ^ imm32





Jump instructions

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



BPF_JMP32 uses 32-bit wide operands while BPF_JMP uses 64-bit wide operands for

otherwise identical operations.

The code field encodes the operation as below:



  ========  =====  =========================

  code      value  description
@@ -116,24 +144,8 @@ For class BPF_JMP or BPF_JMP32: 
  BPF_JSLE  0xd0   signed '<='

  ========  =====  =========================



So BPF_ADD | BPF_X | BPF_ALU means::



  dst_reg = (u32) dst_reg + (u32) src_reg;



Similarly, BPF_XOR | BPF_K | BPF_ALU means::



  src_reg = (u32) src_reg ^ (u32) imm32



eBPF is using BPF_MOV | BPF_X | BPF_ALU to represent A = B moves.  BPF_ALU64

is used to mean exactly the same operations as BPF_ALU, but with 64-bit wide

operands instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.::



  dst_reg = dst_reg + src_reg



BPF_JMP | BPF_EXIT means function exit only. The eBPF program needs to store

the return value into register R0 before doing a BPF_EXIT. Class 6 is used as

BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide

operands for the comparisons instead.

The eBPF program needs to store the return value into register R0 before doing a

BPF_EXIT.





Load and store instructions