1Why we want a copy of kernel headers in tools? 2============================================== 3 4There used to be no copies, with tools/ code using kernel headers 5directly. From time to time tools/perf/ broke due to legitimate kernel 6hacking. At some point Linus complained about such direct usage. Then we 7adopted the current model. 8 9The way these headers are used in perf are not restricted to just 10including them to compile something. 11 12There are sometimes used in scripts that convert defines into string 13tables, etc, so some change may break one of these scripts, or new MSRs 14may use some different #define pattern, etc. 15 16E.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 36The tools/perf/check-headers.sh script, part of the tools/ build 37process, points out changes in the original files. 38 39So its important not to touch the copies in tools/ when doing changes in 40the original kernel headers, that will be done later, when 41check-headers.sh inform about the change to the perf tools hackers. 42 43Another explanation from Ingo Molnar: 44It'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 54What 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 71We've been using this for many years now, and it might seem hacky, but 72works surprisingly well. 73 74