1 Why we want a copy of kernel headers in tools?
2 ==============================================
3
4 There used to be no copies, with tools/ code u
5 directly. From time to time tools/perf/ broke
6 hacking. At some point Linus complained about
7 adopted the current model.
8
9 The way these headers are used in perf are not
10 including them to compile something.
11
12 There are sometimes used in scripts that conve
13 tables, etc, so some change may break one of t
14 may use some different #define pattern, etc.
15
16 E.g.:
17
18 $ ls -1 tools/perf/trace/beauty/*.sh | head
19 tools/perf/trace/beauty/arch_errno_names.sh
20 tools/perf/trace/beauty/drm_ioctl.sh
21 tools/perf/trace/beauty/fadvise.sh
22 tools/perf/trace/beauty/fsconfig.sh
23 tools/perf/trace/beauty/fsmount.sh
24 $
25 $ tools/perf/trace/beauty/fadvise.sh
26 static const char *fadvise_advices[] = {
27 [0] = "NORMAL",
28 [1] = "RANDOM",
29 [2] = "SEQUENTIAL",
30 [3] = "WILLNEED",
31 [4] = "DONTNEED",
32 [5] = "NOREUSE",
33 };
34 $
35
36 The tools/perf/check-headers.sh script, part o
37 process, points out changes in the original fi
38
39 So its important not to touch the copies in to
40 the original kernel headers, that will be done
41 check-headers.sh inform about the change to th
42
43 Another explanation from Ingo Molnar:
44 It's better than all the alternatives we tried
45
46 - Symbolic links and direct #includes: this w
47 was pushed back on from the kernel side, wh
48 headers and broke them accidentally for ker
49
50 - Duplicate self-defined ABI headers like gli
51 burden, double the chance for mistakes, plu
52 notification mechanism to look at new kerne
53
54 What we are doing now is a third option:
55
56 - A software-enforced copy-on-write mechanism
57 tooling, driven by non-fatal warnings on th
58 kernel headers get modified:
59
60 Warning: Kernel ABI header differences:
61 diff -u tools/include/uapi/drm/i915_drm.
62 diff -u tools/include/uapi/linux/fs.h in
63 diff -u tools/include/uapi/linux/kvm.h i
64 ...
65
66 The tooling policy is to always pick up the
67 and integate them into the tooling build. T
68 notification to tooling maintainers that th
69 side.
70
71 We've been using this for many years now, and
72 works surprisingly well.
73
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