perf tools: Add tools/include/uapi/README

Why we want a copy of kernel headers in tools?

==============================================

There used to be no copies, with tools/ code using kernel headers

directly. From time to time tools/perf/ broke due to legitimate kernel

hacking. At some point Linus complained about such direct usage. Then we

adopted the current model.

The way these headers are used in perf are not restricted to just

including them to compile something.

There are sometimes used in scripts that convert defines into string

tables, etc, so some change may break one of these scripts, or new MSRs

may 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/ build

process, points out changes in the original files.

So its important not to touch the copies in tools/ when doing changes in

the original kernel headers, that will be done later, when

check-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, but

works surprisingly well.