Commit c1f9e14e authored by Dave Thaler's avatar Dave Thaler Committed by Alexei Starovoitov
Browse files

bpf, docs: Explain helper functions 



Add brief text about existence of helper functions, with details to go in
separate psABI text.

Note that text about runtime functions (kfuncs) is part of a separate patch,
not this one.

Signed-off-by: default avatarDave Thaler <dthaler@microsoft.com>
Link: https://lore.kernel.org/r/20230308205303.1308-1-dthaler1968@googlemail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 74843b57

Documentation/bpf/clang-notes.rst

+6 −0
Original line number Diff line number Diff line
@@ -20,6 +20,12 @@ Arithmetic instructions 
For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with

``-Xclang -target-feature -Xclang +alu32``.  In CPU version 3, support is automatically included.



Jump instructions

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



If ``-O0`` is used, Clang will generate the ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d)

instruction, which is not supported by the Linux kernel verifier.



Atomic operations

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

Documentation/bpf/instruction-set.rst

+8 −1
Original line number Diff line number Diff line
@@ -253,7 +253,7 @@ BPF_JSET 0x40 PC += off if dst & src 
BPF_JNE   0x50   PC += off if dst != src

BPF_JSGT  0x60   PC += off if dst > src     signed

BPF_JSGE  0x70   PC += off if dst >= src    signed

BPF_CALL  0x80   function call

BPF_CALL  0x80   function call              see `Helper functions`_

BPF_EXIT  0x90   function / program return  BPF_JMP only

BPF_JLT   0xa0   PC += off if dst < src     unsigned

BPF_JLE   0xb0   PC += off if dst <= src    unsigned
@@ -264,6 +264,13 @@ BPF_JSLE 0xd0 PC += off if dst <= src signed 
The eBPF program needs to store the return value into register R0 before doing a

BPF_EXIT.



Helper functions

~~~~~~~~~~~~~~~~



Helper functions are a concept whereby BPF programs can call into a

set of function calls exposed by the runtime.  Each helper

function is identified by an integer used in a ``BPF_CALL`` instruction.

The available helper functions may differ for each program type.



Load and store instructions

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

Documentation/bpf/linux-notes.rst

+8 −0
Original line number Diff line number Diff line
@@ -12,6 +12,14 @@ Byte swap instructions 


``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and ``BPF_TO_BE`` respectively.



Jump instructions

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



``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function

integer would be read from a specified register, is not currently supported

by the verifier.  Any programs with this instruction will fail to load

until such support is added.



Legacy BPF Packet access instructions

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