Merge branch 'bpf: Update doc about calling kernel function'

Martin KaFai says:

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

This set updates the document about the bpf program calling kernel
function.  In particular, updates are regarding to the clang
requirement in selftests and kfunc-call not an ABI.
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/bpf_design_QA.rst

@@ -258,3 +258,18 @@ Q: Can BPF functionality such as new program or map types, new
 helpers, etc be added out of kernel module code?
 
 A: NO.
+
+Q: Directly calling kernel function is an ABI?
+----------------------------------------------
+Q: Some kernel functions (e.g. tcp_slow_start) can be called
+by BPF programs.  Do these kernel functions become an ABI?
+
+A: NO.
+
+The kernel function protos will change and the bpf programs will be
+rejected by the verifier.  Also, for example, some of the bpf-callable
+kernel functions have already been used by other kernel tcp
+cc (congestion-control) implementations.  If any of these kernel
+functions has changed, both the in-tree and out-of-tree kernel tcp cc
+implementations have to be changed.  The same goes for the bpf
+programs and they have to be adjusted accordingly.

tools/testing/selftests/bpf/README.rst

@@ -179,3 +179,17 @@ types, which was introduced in `Clang 13`__. The older Clang versions will
 either crash when compiling these tests, or generate an incorrect BTF.
 
 __  https://reviews.llvm.org/D83289
+
+Kernel function call test and Clang version
+===========================================
+
+Some selftests (e.g. kfunc_call and bpf_tcp_ca) require a LLVM support
+to generate extern function in BTF.  It was introduced in `Clang 13`__.
+
+Without it, the error from compiling bpf selftests looks like:
+
+.. code-block:: console
+
+  libbpf: failed to find BTF for extern 'tcp_slow_start' [25] section: -2
+
+__ https://reviews.llvm.org/D93563