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Name Size Last modified (GMT) Description
Back Parent directory 2024-10-29 14:06:25
Folder asm/ 2024-10-29 14:06:25
Folder asm-generic/ 2024-10-29 14:06:25
Folder drm/ 2024-10-29 14:06:25
Folder linux/ 2024-10-29 14:06:25
File README 2763 bytes 2024-10-29 14:06:25

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

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