[PATCH v2] Documentation/llvm: refresh docs
ndesaulniers at google.com
ndesaulniers at google.com
Thu Aug 24 13:48:43 PDT 2023
Recent fixes for an embargoed hardware security vulnerability failed to
link with ld.lld (LLVM's linker). [0] To be fair, our documentation
mentions ``CC=clang`` foremost with ``LLVM=1`` being buried "below the
fold."
We want to encourage the use of ``LLVM=1`` rather than just
``CC=clang``. Make that suggestion "above the fold" and "front and
center" in our docs.
While here, the following additional changes were made:
- remove the bit about CROSS_COMPILE setting --target=, that's no longer
true.
- Add ARCH=loongarch to the list of maintained targets (though we're
still working on getting defconfig building cleanly at the moment;
we're pretty close).
- Bump ARCH=powerpc from CC=clang to LLVM=1 status.
- Promote ARCH=riscv from being Maintained to being Supported. Android
is working towards supporting RISC-V, and we have excellent support
from multiple companies in this regard.
- Note that the toolchain distribution on kernel.org has been built with
profile data from kernel builds.
Link: https://github.com/ClangBuiltLinux/linux/issues/1907 [0]
Reviewed-by: Nathan Chancellor <nathan at kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers at google.com>
---
Changes in v2:
- Commit message changes:
- Put SOB tag above the fold.
- Mention PPC change in commit message.
- Fix typos in commit message.
- Add Nathan's RB tag to commit message.
- Fix kernel doc warnings
- Reword s390 example under `The LLVM= argument`.
- Reword and add examples to `The LLVM_IAS= argument`.
- Link to v1: https://lore.kernel.org/r/20230824-docs-v1-1-67e061278b8f@google.com
---
Documentation/kbuild/llvm.rst | 115 +++++++++++++++++++++++++-----------------
1 file changed, 68 insertions(+), 47 deletions(-)
diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index c3851fe1900d..1bfcadc41e7d 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -25,50 +25,38 @@ objects <https://www.aosabook.org/en/llvm.html>`_. Clang is a front-end to LLVM
that supports C and the GNU C extensions required by the kernel, and is
pronounced "klang," not "see-lang."
-Clang
------
-
-The compiler used can be swapped out via ``CC=`` command line argument to ``make``.
-``CC=`` should be set when selecting a config and during a build. ::
-
- make CC=clang defconfig
-
- make CC=clang
-
-Cross Compiling
----------------
+Building with LLVM
+------------------
-A single Clang compiler binary will typically contain all supported backends,
-which can help simplify cross compiling. ::
+Invoke ``make`` via::
- make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
+ make LLVM=1
-``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
-``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
-example: ::
+to compile for the host target. For cross compiling::
- clang --target=aarch64-linux-gnu foo.c
+ make LLVM=1 ARCH=arm64
-LLVM Utilities
---------------
+The LLVM= argument
+------------------
-LLVM has substitutes for GNU binutils utilities. They can be enabled individually.
-The full list of supported make variables::
+LLVM has substitutes for GNU binutils utilities. They can be enabled
+individually. The full list of supported make variables::
make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
-To simplify the above command, Kbuild supports the ``LLVM`` variable::
-
- make LLVM=1
+``LLVM=1`` expands to the above.
If your LLVM tools are not available in your PATH, you can supply their
location using the LLVM variable with a trailing slash::
make LLVM=/path/to/llvm/
-which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc.
+which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc. The
+following may also be used::
+
+ PATH=/path/to/llvm:$PATH make LLVM=1
If your LLVM tools have a version suffix and you want to test with that
explicit version rather than the unsuffixed executables like ``LLVM=1``, you
@@ -78,31 +66,59 @@ can pass the suffix using the ``LLVM`` variable::
which will use ``clang-14``, ``ld.lld-14``, etc.
+To support combinations of out of tree paths with version suffixes, we
+recommend::
+
+ PATH=/path/to/llvm/:$PATH make LLVM=-14
+
``LLVM=0`` is not the same as omitting ``LLVM`` altogether, it will behave like
-``LLVM=1``. If you only wish to use certain LLVM utilities, use their respective
-make variables.
+``LLVM=1``. If you only wish to use certain LLVM utilities, use their
+respective make variables.
-The integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to
-disable it.
+The same value used for ``LLVM=`` should be set for each invocation of ``make``
+if configuring and building via distinct commands. ``LLVM=`` should also be set
+as an environment variable when running scripts that will eventually run
+``make``.
-Omitting CROSS_COMPILE
-----------------------
+Cross Compiling
+---------------
+
+A single Clang compiler binary (and corresponding LLVM utilities) will
+typically contain all supported back ends, which can help simplify cross
+compiling especially when ``LLVM=1`` is used. If you use only LLVM tools,
+``CROSS_COMPILE`` or target-triple-prefixes become unnecessary. Example::
+
+ make LLVM=1 ARCH=arm64
-As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
+As an example of mixing LLVM and GNU utilities, for a target like ``ARCH=s390``
+which does not yet have ``ld.lld`` or ``llvm-objcopy`` support, you could
+invoke ``make`` via::
-If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
-from ``ARCH``.
+ make LLVM=1 ARCH=s390 LD=s390x-linux-gnu-ld.bfd \
+ OBJCOPY=s390x-linux-gnu-objcopy
-That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
+This example will invoke ``s390x-linux-gnu-ld.bfd`` as the linker and
+``s390x-linux-gnu-objcopy``, so ensure those are reachable in your ``$PATH``.
+
+``CROSS_COMPILE`` is not used to prefix the Clang compiler binary (or
+corresponding LLVM utilities) as is the case for GNU utilities when ``LLVM=1``
+is not set.
+
+The LLVM_IAS= argument
+----------------------
-For example, to cross-compile the arm64 kernel::
+Clang can assemble assembler code. You can pass ``LLVM_IAS=0`` to disable this
+behavior and have Clang invoke the corresponding non-integrated assembler
+instead. Example::
- make ARCH=arm64 LLVM=1
+ make LLVM=1 LLVM_IAS=0
-If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive
-``--prefix=<path>`` to search for the GNU assembler and linker. ::
+``CROSS_COMPILE`` is necessary when cross compiling and ``LLVM_IAS=0``
+is used in order to set ``--prefix=`` for the compiler to find the
+corresponding non-integrated assembler (typically, you don't want to use the
+system assembler when targeting another architecture). Example::
- make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-
+ make LLVM=1 ARCH=arm LLVM_IAS=0 CROSS_COMPILE=arm-linux-gnueabi-
Supported Architectures
-----------------------
@@ -135,14 +151,17 @@ yet. Bug reports are always welcome at the issue tracker below!
* - hexagon
- Maintained
- ``LLVM=1``
+ * - loongarch
+ - Maintained
+ - ``LLVM=1``
* - mips
- Maintained
- ``LLVM=1``
* - powerpc
- Maintained
- - ``CC=clang``
+ - ``LLVM=1``
* - riscv
- - Maintained
+ - Supported
- ``LLVM=1``
* - s390
- Maintained
@@ -171,9 +190,11 @@ Getting Help
Getting LLVM
-------------
-We provide prebuilt stable versions of LLVM on `kernel.org <https://kernel.org/pub/tools/llvm/>`_.
-Below are links that may be useful for building LLVM from source or procuring
-it through a distribution's package manager.
+We provide prebuilt stable versions of LLVM on `kernel.org
+<https://kernel.org/pub/tools/llvm/>`_. These have been optimized with profile
+data for building Linux kernels. Below are links that may be useful for
+building LLVM from source or procuring it through a distribution's package
+manager.
- https://releases.llvm.org/download.html
- https://github.com/llvm/llvm-project
---
base-commit: 93f5de5f648d2b1ce3540a4ac71756d4a852dc23
change-id: 20230824-docs-c17a5de7f103
Best regards,
--
Nick Desaulniers <ndesaulniers at google.com>
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