[RFC PATCH 0/6] kbuild: build the userspace tools in tools/ with the kernel

[Sasha Levin <sashal@kernel.org>]

The userspace programs under tools/ are disconnected from the rest of the
kernel's build system. Each has its own makefiles and build framework, and
"make" never builds them: apart from the few host tools the build itself needs,
such as objtool and resolve_btfids, nothing under tools/ is compiled as part of
a kernel build.

That makes them easy to break. Most people, and most of the build bots, check a
change with "make allmodconfig && make", which doesn't cover tools/ at all. A
change that breaks a tool passes every build check and is only noticed when
someone happens to build that tool by hand, often much later.

We keep seeing this in tools/testing/: selftests that stop building after a
change elsewhere in the tree, or after a backport to a stable tree, and stay
broken until someone tries to build them. Others brought up the same concern at
LPC.

This series lets the kernel build also build the tools it is configured for:

- tools/Kconfig adds a "Build userspace tools from tools/" menu
  (CONFIG_TOOLS_BUILD) with one entry per tool and per selftests
  directory, so .config records what gets built, and a tool that breaks
  can be turned off on its own.

- "make tools_build" builds the selected tools for the target, each with
  its own build system as it is built standalone, into tools-build/ in
  the object tree.

- With CONFIG_TOOLS_BUILD=y, "make all" builds them too. allmodconfig
  and allyesconfig enable every entry when the compiler can link
  userspace programs, so the usual build test catches tools breakage.
  Toolchains without a C library are unaffected, as are "make vmlinux",
  "make modules" and external module builds.

This is useful beyond build testing: the tools are built for the same
target, from the same tree, as the kernel they go with.

Sasha Levin (6):
  kbuild: factor the userspace ABI flags out into KBUILD_USERABI
  kbuild: add "make tools_build" to build userspace tools from tools/
  tools: build perf in tools_build
  tools: build kernel selftests in tools_build
  kbuild: build the tools selected in tools/Kconfig as part of "make
    all"
  Documentation: describe building tools with the kernel

 .gitignore                                 |   5 +
 Documentation/dev-tools/kselftest.rst      |   4 +
 Documentation/kbuild/kbuild.rst            |   5 +-
 Documentation/process/submit-checklist.rst |   5 +
 Makefile                                   |  24 +-
 lib/Kconfig.debug                          |   2 +
 tools/Kconfig                              | 326 +++++++++++++++++++++
 tools/Makefile.kbuild                      | 198 +++++++++++++
 8 files changed, 562 insertions(+), 7 deletions(-)
 create mode 100644 tools/Kconfig
 create mode 100644 tools/Makefile.kbuild


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prerequisite-patch-id: a71f47de3fd339d680c9f18e80f5bcde7eae0abf
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2.53.0