Kbuild puts the objects listed in head-y at the head of vmlinux.
Conventionally, we do this for head*.S, which contains the kernel entry
point.

A counter approach is to control the section order by the linker script.
Actually, the code marked as __HEAD goes into the ".head.text" section,
which is placed before the normal ".text" section.

I do not know if both of them are needed. From the build system
perspective, head-y is not mandatory. If you can achieve the proper code
placement by the linker script only, it would be cleaner.

I collected the current head-y objects into head-object-list.txt. It is
a whitelist. My hope is it will be reduced in the long run.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Tested-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>

The objects placed at the head of vmlinux need special treatments:

 - arch/$(SRCARCH)/Makefile adds them to head-y in order to place
   them before other archives in the linker command line.

 - arch/$(SRCARCH)/kernel/Makefile adds them to extra-y instead of
   obj-y to avoid them going into built-in.a.

This commit gets rid of the latter.

Create vmlinux.a to collect all the objects that are unconditionally
linked to vmlinux. The objects listed in head-y are moved to the head
of vmlinux.a by using 'ar m'.

With this, arch/$(SRCARCH)/kernel/Makefile can consistently use obj-y
for builtin objects.

There is no *.o that is directly linked to vmlinux. Drop unneeded code
in scripts/clang-tools/gen_compile_commands.py.

$(AR) mPi needs 'T' to workaround the llvm-ar bug. The fix was suggested
by Nathan Chancellor [1].

[1]: https://lore.kernel.org/llvm/YyjjT5gQ2hGMH0ni@dev-arch.thelio-3990X/Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Tested-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>

V=1 (verbose build) shows commands executed by Make, but it may cause
misunderstanding.

For example, the following command shows the outstanding error message.

  $ make V=1 INSTALL_PATH=/tmp install
  test -e include/generated/autoconf.h -a -e include/config/auto.conf || (                \
  echo >&2;                                                       \
  echo >&2 "  ERROR: Kernel configuration is invalid.";           \
  echo >&2 "         include/generated/autoconf.h or include/config/auto.conf are missing.";\
  echo >&2 "         Run 'make oldconfig && make prepare' on kernel src to fix it.";      \
  echo >&2 ;                                                      \
  /bin/false)
    unset sub_make_done; ./scripts/install.sh

It is not an error. Make just showed the recipe lines it has executed,
but people may think that 'make install' has failed.

Likewise, the combination of V=1 and O= shows confusing
"*** The source tree is not clean, please run 'make mrproper'".

Suppress such misleading logs.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>

Currently, modpost is executed twice; first for vmlinux, second
for modules.

This commit merges them.

Current build flow
==================

  1) build obj-y and obj-m objects
    2) link vmlinux.o
      3) modpost for vmlinux
        4) link vmlinux
          5) modpost for modules
            6) link modules (*.ko)

The build steps 1) through 6) are serialized, that is, modules are
built after vmlinux. You do not get benefits of parallel builds when
scripts/link-vmlinux.sh is being run.

New build flow
==============

  1) build obj-y and obj-m objects
    2) link vmlinux.o
      3) modpost for vmlinux and modules
        4a) link vmlinux
        4b) link modules (*.ko)

In the new build flow, modpost is invoked just once.

vmlinux and modules are built in parallel. One exception is
CONFIG_DEBUG_INFO_BTF_MODULES=y, where modules depend on vmlinux.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Tested-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>

Move the build rules of vmlinux.o out of scripts/link-vmlinux.sh to
clearly separate 1) pre-modpost, 2) modpost, 3) post-modpost stages.
This will make further refactoring possible.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Tested-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>

.vmlinux.objs is used by modpost, so scripts/Makefile.modpost is
a better place to generate it.

It is used only when CONFIG_MODVERSIONS=y. It should be guarded
by "ifdef CONFIG_MODVERSIONS".
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Tested-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>

Use the ordinary obj-y syntax to list subdirectories.

Note1:
Previously, the link order of lib-y depended on CONFIG_MODULES; lib-y
was linked before drivers-y when CONFIG_MODULES=y, otherwise after
drivers-y. This was a bug of commit 7273ad2b ("kbuild: link lib-y
objects to vmlinux forcibly when CONFIG_MODULES=y"), but it was not a
big deal after all. Now, all objects listed in lib-y are linked last,
irrespective of CONFIG_MODULES.

Note2:
Finally, the single target build in arch/*/lib/ works correctly. There was
a bug report about this. [1]

  $ make ARCH=arm arch/arm/lib/findbit.o
    CALL    scripts/checksyscalls.sh
    AS      arch/arm/lib/findbit.o

[1]: https://lore.kernel.org/linux-kbuild/YvUQOwL6lD4%2F5%2FU6@shell.armlinux.org.uk/Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Tested-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>

cc-ifversion is GCC specific. Replace it with compiler specific
variants. Update the users of cc-ifversion to use these new macros.

Link: https://github.com/ClangBuiltLinux/linux/issues/350
Link: https://lore.kernel.org/llvm/CAGG=3QWSAUakO42kubrCap8fp-gm1ERJJAYXTnP1iHk_wrH=BQ@mail.gmail.com/Suggested-by: NBill Wendling <morbo@google.com>
Reviewed-by: NNathan Chancellor <nathan@kernel.org>
Signed-off-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

The single target build has a subtle bug for the combination for
an individual file and a subdirectory.

[1] 'make kernel/fork.i' builds only kernel/fork.i

  $ make kernel/fork.i
    CALL    scripts/checksyscalls.sh
    DESCEND objtool
    CPP     kernel/fork.i

[2] 'make kernel/' builds only under the kernel/ directory.

  $ make kernel/
    CALL    scripts/checksyscalls.sh
    DESCEND objtool
    CC      kernel/fork.o
    CC      kernel/exec_domain.o
       [snip]
    CC      kernel/rseq.o
    AR      kernel/built-in.a

But, if you try to do [1] and [2] in a single command, you will get
only [1] with a weird log:

  $ make kernel/fork.i kernel/
    CALL    scripts/checksyscalls.sh
    DESCEND objtool
    CPP     kernel/fork.i
  make[2]: Nothing to be done for 'kernel/'.

With 'make kernel/fork.i kernel/', you should get both [1] and [2].

Rewrite the single target build.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

Now that UTS_VERSION was separated out, this header can be generated
much earlier, and probably the top Makefile is a better place to do it
than init/Makefile.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

Unify the code between in-tree builds and external module builds.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

Remove the potentially invalid modules.order instead of using
the temporary file.

Also, KBUILD_MODULES is don't care for single builds. No need to
cancel it.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

The dependency, "modules: modules_check" is specified twice.
Commit 1a998be6 ("kbuild: check module name conflict for external
modules as well") missed to clean it up.

'PHONY += modules' also appears twice.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

The AWK code was added to deduplicate modules.order in case $(obj-m)
contains the same module multiple times, but it is actually unneeded
since commit b2c88554 ("kbuild: update modules.order only when
contained modules are updated").

The list is already deduplicated before being processed by AWK because
$^ is the deduplicated list of prerequisites.
(Please note the real-prereqs macro uses $^)

Yet, modules.order will contain duplication if two different Makefiles
build the same module:

  foo/Makefile:

      obj-m += bar/baz.o

  foo/bar/Makefile:

      obj-m += baz.o

However, the parallel builds cannot properly handle this case in the
first place. So, it is better to let it fail (as already done by
scripts/modules-check.sh).
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

It is unneeded to check the sha1sum every time.

Create the timestamp files to manage it.

Add '.' to clean-dirs because 'make clean' must visit ./Kbuild to
clean up the timestamp files.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

My future plan is to list subdirectories in ./Kbuild. When it occurs,
$(vmlinux-alldirs) will not contain all subdirectories.

Let's hard-code the directory list until I get around to implementing
a more sophisticated way for generating a source tarball.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Tested-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>

missing-syscalls and old-atomics are meant to be phony targets.
Adding them to always-y is odd. (always-y should generate something).

Add a new phony target 'prepare', which depends on all the other.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

Previously 'make ARCH=um headers' stopped because of missing
arch/um/include/uapi/asm/Kbuild.

The error is not shown since commit ed102bf2 ("um: Fix W=1
missing-include-dirs warnings") added arch/um/include/uapi/asm/Kbuild.

Hard-code the unsupported architecture, so it works like before.

Fixes: ed102bf2 ("um: Fix W=1 missing-include-dirs warnings")
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Acked-by: NRichard Weinberger <richard@nod.at>

Commit b2c88554 ("kbuild: update modules.order only when contained
modules are updated") accidentally changed the modules order.

Prior to that commit, the modules order was determined based on
vmlinux-dirs, which lists core-y/m, drivers-y/m, libs-y/m, in this order.

Now, subdir-modorder lists them in a different order: core-y/m, libs-y/m,
drivers-y/m.

Presumably, there was no practical issue because the modules in drivers
and libs are orthogonal, but there is no reason to have this distortion.

Get back to the original order.

Fixes: b2c88554 ("kbuild: update modules.order only when contained modules are updated")
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

Users of GNU ld (BFD) from binutils 2.39+ will observe multiple
instances of a new warning when linking kernels in the form:

  ld: warning: vmlinux: missing .note.GNU-stack section implies executable stack
  ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
  ld: warning: vmlinux has a LOAD segment with RWX permissions

Generally, we would like to avoid the stack being executable.  Because
there could be a need for the stack to be executable, assembler sources
have to opt-in to this security feature via explicit creation of the
.note.GNU-stack feature (which compilers create by default) or command
line flag --noexecstack.  Or we can simply tell the linker the
production of such sections is irrelevant and to link the stack as
--noexecstack.

LLVM's LLD linker defaults to -z noexecstack, so this flag isn't
strictly necessary when linking with LLD, only BFD, but it doesn't hurt
to be explicit here for all linkers IMO.  --no-warn-rwx-segments is
currently BFD specific and only available in the current latest release,
so it's wrapped in an ld-option check.

While the kernel makes extensive usage of ELF sections, it doesn't use
permissions from ELF segments.

Link: https://lore.kernel.org/linux-block/3af4127a-f453-4cf7-f133-a181cce06f73@kernel.dk/
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=ba951afb99912da01a6e8434126b8fac7aa75107
Link: https://github.com/llvm/llvm-project/issues/57009Reported-and-tested-by: NJens Axboe <axboe@kernel.dk>
Suggested-by: NFangrui Song <maskray@google.com>
Signed-off-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Factor out the common prerequisites for DT compilation into the new
target, dtbs_prepare.

Add comments to explain why include/config/kernel.release is the
prerequisite. Our policy is that installation targets must not rebuild
anything in the tree. If 'make modules_install' is executed as root,
include/config/kernel.release may be owned by root.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: NNicolas Schier <nicolas@fjasle.eu>
Reviewed-by: NDmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: NDmitry Baryshkov <dmitry.baryshkov@linaro.org>

If the directory path given to KBUILD_EXTMOD (or M=) contains % or :,
the module fails to build.

% is used in pattern rules, and : as the separator of dependencies.

Bail out with a clearer error message.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: NNicolas Schier <n.schier@avm.de>

The difference in most compilers between `-O3` and `-O2` is mostly down
to whether loops with statically determinable trip counts are fully
unrolled vs unrolled to a multiple of SIMD width.

This patch is effectively a revert of
commit 15f5db60 ("kbuild,arc: add
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 for ARC") without re-adding
ARCH_CFLAGS

Ever since
commit cfdbc2e1 ("ARC: Build system: Makefiles, Kconfig, Linker
script")
ARC has been built with -O3, though the reason for doing so was not
specified in inline comments or the commit message. This commit does not
re-add -O3 to arch/arc/Makefile.

Folks looking to experiment with `-O3` (or any compiler flag for that
matter) may pass them along to the command line invocation of make:

$ make KCFLAGS=-O3

Code that looks to re-add an explicit Kconfig option for `-O3` should
provide:
1. A rigorous and reproducible performance profile of a reasonable
   userspace workload that demonstrates a hot loop in the kernel that
   would benefit from `-O3` over `-O2`.
2. Disassembly of said loop body before and after.
3. Provides stats on terms of increase in file size.

Link: https://lore.kernel.org/linux-kbuild/CA+55aFz2sNBbZyg-_i8_Ldr2e8o9dfvdSfHHuRzVtP2VMAUWPg@mail.gmail.com/Signed-off-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

Replace headers_install with headers as kselftest uses the header
files from within the kernel tree rather than from a system-wide
installation.

We can still run this directly:

  $ make O=build kselftest-all

and when building from the selftests directory:

  $ make O=build headers
  $ make O=build -C tools/testing/selftests all
Signed-off-by: NGuillaume Tucker <guillaume.tucker@collabora.com>
Reported-by: NMasahiro Yamada <masahiroy@kernel.org>
Acked-by: NShuah Khan <skhan@linuxfoundation.org>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>

In preparation for splitting io_uring up a bit, move it into its own
top level directory. It didn't really belong in fs/ anyway, as it's
not a file system only API.

This adds io_uring/ and moves the core files in there, and updates the
MAINTAINERS file for the new location.
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Add headers_install as a dependency to kselftest targets so that they
can be run directly from the top of the tree.  The kselftest Makefile
used to try to call headers_install "backwards" but failed due to the
relative path not being consistent.

Now we can either run this directly:

  $ make O=build kselftest-all

or this:

  $ make O=build headers_install
  $ make O=build -C tools/testing/selftest all

The same commands work as well when building directly in the source
tree (no O=) or any arbitrary path (relative or absolute).
Signed-off-by: NGuillaume Tucker <guillaume.tucker@collabora.com>
Tested-by: NAnders Roxell <anders.roxell@linaro.org>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>

If CONFIG_TRIM_UNUSED_KSYMS is enabled and the kernel is built from
a pristine state, the vmlinux is linked twice.

Commit 3fdc7d3f ("kbuild: link vmlinux only once for
CONFIG_TRIM_UNUSED_KSYMS") explains why this happens, but it did not fix
the issue at all.

Now I realized I had applied a wrong patch.

In v1 patch [1], the autoksyms_recursive target correctly recurses to
"$(MAKE) -f $(srctree)/Makefile autoksyms_recursive".

In v2 patch [2], I accidentally dropped the diff line, and it recurses to
"$(MAKE) -f $(srctree)/Makefile vmlinux".

Restore the code I intended in v1.

[1]: https://lore.kernel.org/linux-kbuild/1521045861-22418-8-git-send-email-yamada.masahiro@socionext.com/
[2]: https://lore.kernel.org/linux-kbuild/1521166725-24157-8-git-send-email-yamada.masahiro@socionext.com/

Fixes: 3fdc7d3f ("kbuild: link vmlinux only once for CONFIG_TRIM_UNUSED_KSYMS")
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Tested-by: NSami Tolvanen <samitolvanen@google.com>
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>