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: Dave Thaler <dthaler@microsoft.com>
Link: https://lore.kernel.org/r/20230308205303.1308-1-dthaler1968@googlemail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/clang-notes.rst

@@ -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

@@ -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

@@ -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
 =====================================