docs: add Rust documentation

Most of the documentation for Rust is written within the source code
itself, as it is idiomatic for Rust projects. This applies to both
the shared infrastructure at `rust/` as well as any other Rust module
(e.g. drivers) written across the kernel.

However, these documents contain general information that does not
fit particularly well in the source code, like the Quick Start guide.

It also contains a few other small changes elsewhere in the
documentation folder.

Reviewed-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Co-developed-by: Finn Behrens <me@kloenk.de>
Signed-off-by: Finn Behrens <me@kloenk.de>
Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Co-developed-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Co-developed-by: Sven Van Asbroeck <thesven73@gmail.com>
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Co-developed-by: Wu XiangCheng <bobwxc@email.cn>
Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Co-developed-by: Boris-Chengbiao Zhou <bobo1239@web.de>
Signed-off-by: Boris-Chengbiao Zhou <bobo1239@web.de>
Co-developed-by: Yuki Okushi <jtitor@2k36.org>
Signed-off-by: Yuki Okushi <jtitor@2k36.org>
Co-developed-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Co-developed-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Co-developed-by: Julian Merkle <me@jvmerkle.de>
Signed-off-by: Julian Merkle <me@jvmerkle.de>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Documentation/doc-guide/kernel-doc.rst

@@ -14,6 +14,9 @@ when it is embedded in source files.
    reasons. The kernel source contains tens of thousands of kernel-doc
    comments. Please stick to the style described here.
 
+.. note:: kernel-doc does not cover Rust code: please see
+   Documentation/rust/general-information.rst instead.
+
 The kernel-doc structure is extracted from the comments, and proper
 `Sphinx C Domain`_ function and type descriptions with anchors are
 generated from them. The descriptions are filtered for special kernel-doc

Documentation/index.rst

@@ -82,6 +82,7 @@ merged much easier.
    maintainer/index
    fault-injection/index
    livepatch/index
+   rust/index
 
 
 Kernel API documentation

Documentation/kbuild/kbuild.rst

@@ -48,6 +48,10 @@ KCFLAGS
 -------
 Additional options to the C compiler (for built-in and modules).
 
+KRUSTFLAGS
+----------
+Additional options to the Rust compiler (for built-in and modules).
+
 CFLAGS_KERNEL
 -------------
 Additional options for $(CC) when used to compile
@@ -57,6 +61,15 @@ CFLAGS_MODULE
 -------------
 Additional module specific options to use for $(CC).
 
+RUSTFLAGS_KERNEL
+----------------
+Additional options for $(RUSTC) when used to compile
+code that is compiled as built-in.
+
+RUSTFLAGS_MODULE
+----------------
+Additional module specific options to use for $(RUSTC).
+
 LDFLAGS_MODULE
 --------------
 Additional options used for $(LD) when linking modules.
@@ -69,6 +82,10 @@ HOSTCXXFLAGS
 ------------
 Additional flags to be passed to $(HOSTCXX) when building host programs.
 
+HOSTRUSTFLAGS
+-------------
+Additional flags to be passed to $(HOSTRUSTC) when building host programs.
+
 HOSTLDFLAGS
 -----------
 Additional flags to be passed when linking host programs.

Documentation/kbuild/makefiles.rst

@@ -29,8 +29,9 @@ This document describes the Linux kernel Makefiles.
 	   --- 4.1 Simple Host Program
 	   --- 4.2 Composite Host Programs
 	   --- 4.3 Using C++ for host programs
-	   --- 4.4 Controlling compiler options for host programs
-	   --- 4.5 When host programs are actually built
+	   --- 4.4 Using Rust for host programs
+	   --- 4.5 Controlling compiler options for host programs
+	   --- 4.6 When host programs are actually built
 
 	=== 5 Userspace Program support
 	   --- 5.1 Simple Userspace Program
@@ -835,7 +836,24 @@ Both possibilities are described in the following.
 		qconf-cxxobjs := qconf.o
 		qconf-objs    := check.o
 
-4.4 Controlling compiler options for host programs
+4.4 Using Rust for host programs
+--------------------------------
+
+	Kbuild offers support for host programs written in Rust. However,
+	since a Rust toolchain is not mandatory for kernel compilation,
+	it may only be used in scenarios where Rust is required to be
+	available (e.g. when  ``CONFIG_RUST`` is enabled).
+
+	Example::
+
+		hostprogs     := target
+		target-rust   := y
+
+	Kbuild will compile ``target`` using ``target.rs`` as the crate root,
+	located in the same directory as the ``Makefile``. The crate may
+	consist of several source files (see ``samples/rust/hostprogs``).
+
+4.5 Controlling compiler options for host programs
 --------------------------------------------------
 
 	When compiling host programs, it is possible to set specific flags.
@@ -867,7 +885,7 @@ Both possibilities are described in the following.
 	When linking qconf, it will be passed the extra option
 	"-L$(QTDIR)/lib".
 
-4.5 When host programs are actually built
+4.6 When host programs are actually built
 -----------------------------------------
 
 	Kbuild will only build host-programs when they are referenced
@@ -1181,6 +1199,17 @@ When kbuild executes, the following steps are followed (roughly):
 	The first example utilises the trick that a config option expands
 	to 'y' when selected.
 
+    KBUILD_RUSTFLAGS
+	$(RUSTC) compiler flags
+
+	Default value - see top level Makefile
+	Append or modify as required per architecture.
+
+	Often, the KBUILD_RUSTFLAGS variable depends on the configuration.
+
+	Note that target specification file generation (for ``--target``)
+	is handled in ``scripts/generate_rust_target.rs``.
+
     KBUILD_AFLAGS_KERNEL
 	Assembler options specific for built-in
 
@@ -1208,6 +1237,19 @@ When kbuild executes, the following steps are followed (roughly):
 	are used for $(CC).
 	From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
 
+    KBUILD_RUSTFLAGS_KERNEL
+	$(RUSTC) options specific for built-in
+
+	$(KBUILD_RUSTFLAGS_KERNEL) contains extra Rust compiler flags used to
+	compile resident kernel code.
+
+    KBUILD_RUSTFLAGS_MODULE
+	Options for $(RUSTC) when building modules
+
+	$(KBUILD_RUSTFLAGS_MODULE) is used to add arch-specific options that
+	are used for $(RUSTC).
+	From commandline RUSTFLAGS_MODULE shall be used (see kbuild.rst).
+
     KBUILD_LDFLAGS_MODULE
 	Options for $(LD) when linking modules
 

Documentation/process/changes.rst

@@ -31,6 +31,8 @@ you probably needn't concern yourself with pcmciautils.
 ====================== ===============  ========================================
 GNU C                  5.1              gcc --version
 Clang/LLVM (optional)  11.0.0           clang --version
+Rust (optional)        1.62.0           rustc --version
+bindgen (optional)     0.56.0           bindgen --version
 GNU make               3.81             make --version
 bash                   4.2              bash --version
 binutils               2.23             ld -v
@@ -80,6 +82,29 @@ kernels. Older releases aren't guaranteed to work, and we may drop workarounds
 from the kernel that were used to support older versions. Please see additional
 docs on :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
 
+Rust (optional)
+---------------
+
+A particular version of the Rust toolchain is required. Newer versions may or
+may not work because the kernel depends on some unstable Rust features, for
+the moment.
+
+Each Rust toolchain comes with several "components", some of which are required
+(like ``rustc``) and some that are optional. The ``rust-src`` component (which
+is optional) needs to be installed to build the kernel. Other components are
+useful for developing.
+
+Please see Documentation/rust/quick-start.rst for instructions on how to
+satisfy the build requirements of Rust support. In particular, the ``Makefile``
+target ``rustavailable`` is useful to check why the Rust toolchain may not
+be detected.
+
+bindgen (optional)
+------------------
+
+``bindgen`` is used to generate the Rust bindings to the C side of the kernel.
+It depends on ``libclang``.
+
 Make
 ----
 
@@ -348,6 +373,12 @@ Sphinx
 Please see :ref:`sphinx_install` in :ref:`Documentation/doc-guide/sphinx.rst <sphinxdoc>`
 for details about Sphinx requirements.
 
+rustdoc
+-------
+
+``rustdoc`` is used to generate the documentation for Rust code. Please see
+Documentation/rust/general-information.rst for more information.
+
 Getting updated software
 ========================
 
@@ -364,6 +395,16 @@ Clang/LLVM
 
 - :ref:`Getting LLVM <getting_llvm>`.
 
+Rust
+----
+
+- Documentation/rust/quick-start.rst.
+
+bindgen
+-------
+
+- Documentation/rust/quick-start.rst.
+
 Make
 ----
 

Documentation/rust/arch-support.rst

@@ -0,0 +1,18 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Arch Support
+============
+
+Currently, the Rust compiler (``rustc``) uses LLVM for code generation,
+which limits the supported architectures that can be targeted. In addition,
+support for building the kernel with LLVM/Clang varies (please see
+Documentation/kbuild/llvm.rst). This support is needed for ``bindgen``
+which uses ``libclang``.
+
+Below is a general summary of architectures that currently work. Level of
+support corresponds to ``S`` values in the ``MAINTAINERS`` file.
+
+============  ================  ==============================================
+Architecture  Level of support  Constraints
+============  ================  ==============================================
+============  ================  ==============================================