Documentation/llvm: refresh docs

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.
- Note how to use ccache with clang.

Link: ClangBuiltLinux#1907 [0]
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: nickdesaulniers

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
+Building with LLVM
+------------------
 
-Cross Compiling
----------------
+Invoke ``make`` via::
 
-A single Clang compiler binary will typically contain all supported backends,
-which can help simplify cross compiling. ::
-
-	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 +66,72 @@ 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 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``.
 
-The integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to
-disable it.
+Cross Compiling
+---------------
 
-Omitting CROSS_COMPILE
+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 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::
+
+	make LLVM=1 ARCH=s390 LD=s390x-linux-gnu-ld.bfd \
+	  OBJCOPY=s390x-linux-gnu-objcopy
+
+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
 ----------------------
 
-As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
+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 LLVM=1 LLVM_IAS=0
+
+``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::
 
-If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
-from ``ARCH``.
+	make LLVM=1 ARCH=arm LLVM_IAS=0 CROSS_COMPILE=arm-linux-gnueabi-
 
-That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
 
-For example, to cross-compile the arm64 kernel::
+Ccache
+------
 
-	make ARCH=arm64 LLVM=1
+``ccache`` can be used with ``clang`` to improve subsequent builds, (though
+KBUILD_BUILD_TIMESTAMP_ should be set to a deterministic value between builds
+in order to avoid 100% cache misses, see Reproducible_builds_ for more info):
 
-If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive
-``--prefix=<path>`` to search for the GNU assembler and linker. ::
+	KBUILD_BUILD_TIMESTAMP='' make LLVM=1 CC="ccache clang"
 
-	make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-
+.. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp
+.. _Reproducible_builds: reproducible-builds.html#timestamps
 
 Supported Architectures
 -----------------------
@@ -135,14 +164,17 @@ 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
@@ -171,7 +203,11 @@ Getting Help
 Getting LLVM
 -------------
 
-We provide prebuilt stable versions of LLVM on `kernel.org <https://kernel.org/pub/tools/llvm/>`_.
+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, which should improve kernel build times
+relative to other distributions of LLVM.
+
 Below are links that may be useful for building LLVM from source or procuring
 it through a distribution's package manager.