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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: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220103183556.41040-3-hch@lst.de
This commit is contained in:
Christoph Hellwig 2022-01-03 19:35:52 +01:00 committed by Alexei Starovoitov
parent 62e4683849
commit be3193cded
1 changed files with 32 additions and 20 deletions
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52
Documentation/bpf/instruction-set.rst
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@ -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