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