[PATCH bpf-next 0/2] bpf: Update doc about calling kernel function

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.

Martin KaFai Lau (2):
  bpf: Update bpf_design_QA.rst to clarify the kfunc call is not ABI
  bpf: selftests: Update clang requirement in README.rst for testing
    kfunc call

 Documentation/bpf/bpf_design_QA.rst    | 15 +++++++++++++++
 tools/testing/selftests/bpf/README.rst | 14 ++++++++++++++
 2 files changed, 29 insertions(+)

-- 
2.30.2

This patch updates bpf_design_QA.rst to clarify that the kernel
function callable by bpf program is not an ABI.

Signed-off-by: Martin KaFai Lau <redacted>
---
 Documentation/bpf/bpf_design_QA.rst | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst
index 0e15f9b05c9d..437de2a7a5de 100644
--- a/Documentation/bpf/bpf_design_QA.rst
+++ b/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.

-- 
2.30.2

This patch updates the README.rst to specify the clang requirement
to compile the bpf selftests that call kernel function.

Signed-off-by: Martin KaFai Lau <redacted>
---
 tools/testing/selftests/bpf/README.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/testing/selftests/bpf/README.rst b/tools/testing/selftests/bpf/README.rst
index 3464161c8eea..65fe318d1e71 100644
--- a/tools/testing/selftests/bpf/README.rst
+++ b/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

-- 
2.30.2

On Mar 29, 2021, at 10:41 PM, Martin KaFai Lau [off-list ref] wrote:

This patch updates bpf_design_QA.rst to clarify that the kernel
function callable by bpf program is not an ABI.

Signed-off-by: Martin KaFai Lau <redacted>

Acked-by: Song Liu <redacted>

On Mar 29, 2021, at 10:41 PM, Martin KaFai Lau [off-list ref] wrote:

This patch updates the README.rst to specify the clang requirement
to compile the bpf selftests that call kernel function.

Signed-off-by: Martin KaFai Lau <redacted>

Acked-by: Song Liu <redacted>

On Mon, Mar 29, 2021 at 10:41 PM Martin KaFai Lau [off-list ref] wrote:

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.

Applied.