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bpf: Update bpf_design_QA.rst to clarify the kfunc call is not ABI 

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 <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210330054150.2933542-1-kafai@fb.com
This commit is contained in:
Martin KaFai Lau 2021-03-29 22:41:50 -07:00 committed by Alexei Starovoitov
parent 7aae231ac9
commit 5bdca94ff3
1 changed files with 15 additions and 0 deletions
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Documentation/bpf/bpf_design_QA.rst
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@ -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? helpers, etc be added out of kernel module code?
A: NO. 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.