Something went wrong. Try again.
Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Something went wrong. Try again.
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374Why we want a copy of kernel headers in tools?==============================================
There used to be no copies, with tools/ code using kernel headersdirectly. From time to time tools/perf/ broke due to legitimate kernelhacking. At some point Linus complained about such direct usage. Then weadopted the current model.
The way these headers are used in perf are not restricted to justincluding them to compile something.
There are sometimes used in scripts that convert defines into stringtables, etc, so some change may break one of these scripts, or new MSRsmay use some different #define pattern, etc.
E.g.:
$ ls -1 tools/perf/trace/beauty/*.sh | head -5 tools/perf/trace/beauty/arch_errno_names.sh tools/perf/trace/beauty/drm_ioctl.sh tools/perf/trace/beauty/fadvise.sh tools/perf/trace/beauty/fsconfig.sh tools/perf/trace/beauty/fsmount.sh $ $ tools/perf/trace/beauty/fadvise.sh static const char *fadvise_advices[] = { [0] = "NORMAL", [1] = "RANDOM", [2] = "SEQUENTIAL", [3] = "WILLNEED", [4] = "DONTNEED", [5] = "NOREUSE", }; $
The tools/perf/check-headers.sh script, part of the tools/ buildprocess, points out changes in the original files.
So its important not to touch the copies in tools/ when doing changes inthe original kernel headers, that will be done later, whencheck-headers.sh inform about the change to the perf tools hackers.
Another explanation from Ingo Molnar:It's better than all the alternatives we tried so far:
- Symbolic links and direct #includes: this was the original approach but was pushed back on from the kernel side, when tooling modified the headers and broke them accidentally for kernel builds.
- Duplicate self-defined ABI headers like glibc: double the maintenance burden, double the chance for mistakes, plus there's no tech-driven notification mechanism to look at new kernel side changes.
What we are doing now is a third option:
- A software-enforced copy-on-write mechanism of kernel headers to tooling, driven by non-fatal warnings on the tooling side build when kernel headers get modified:
Warning: Kernel ABI header differences: diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h diff -u tools/include/uapi/linux/fs.h include/uapi/linux/fs.h diff -u tools/include/uapi/linux/kvm.h include/uapi/linux/kvm.h ...
The tooling policy is to always pick up the kernel side headers as-is, and integate them into the tooling build. The warnings above serve as a notification to tooling maintainers that there's changes on the kernel side.
We've been using this for many years now, and it might seem hacky, butworks surprisingly well.