Re: [PATCH v7 21/25] Kbuild: add Rust support

[Nick Desaulniers <ndesaulniers@google.com>]

On Sun, May 22, 2022 at 7:04 PM Miguel Ojeda <ojeda@kernel.org> wrote:
>
> --- a/Makefile
> +++ b/Makefile
> @@ -120,6 +120,15 @@ endif
>
>  export KBUILD_CHECKSRC
>
> +# Enable "clippy" (a linter) as part of the Rust compilation.
> +#
> +# Use 'make CLIPPY=1' to enable it.
> +ifeq ("$(origin CLIPPY)", "command line")
> +  KBUILD_CLIPPY := $(CLIPPY)
> +endif

Is there a reason to not just turn clippy on always? Might be nicer to
start off with good practices by default. :^)

> @@ -1494,7 +1588,8 @@ MRPROPER_FILES += include/config include/generated          \
>                   certs/signing_key.pem \
>                   certs/x509.genkey \
>                   vmlinux-gdb.py \
> -                 *.spec
> +                 *.spec \
> +                 rust/target.json rust/libmacros.so
>
>  # clean - Delete most, but leave enough to build external modules
>  #
> @@ -1519,6 +1614,9 @@ $(mrproper-dirs):
>
>  mrproper: clean $(mrproper-dirs)
>         $(call cmd,rmfiles)
> +       @find . $(RCS_FIND_IGNORE) \
> +               \( -name '*.rmeta' \) \
> +               -type f -print | xargs rm -f

Are there *.rmeta directories that we _don't_ want to remove via `make
mrproper`? I'm curious why *.rmeta isn't just part of MRPROPER_FILES?

>
>  # distclean
>  #
> @@ -1606,6 +1704,23 @@ help:
>         @echo  '  kselftest-merge   - Merge all the config dependencies of'
>         @echo  '                      kselftest to existing .config.'
>         @echo  ''
> +       @echo  'Rust targets:'
> +       @echo  '  rustavailable   - Checks whether the Rust toolchain is'
> +       @echo  '                    available and, if not, explains why.'
> +       @echo  '  rustfmt         - Reformat all the Rust code in the kernel'
> +       @echo  '  rustfmtcheck    - Checks if all the Rust code in the kernel'
> +       @echo  '                    is formatted, printing a diff otherwise.'
> +       @echo  '  rustdoc         - Generate Rust documentation'
> +       @echo  '                    (requires kernel .config)'
> +       @echo  '  rusttest        - Runs the Rust tests'
> +       @echo  '                    (requires kernel .config; downloads external repos)'
> +       @echo  '  rust-analyzer   - Generate rust-project.json rust-analyzer support file'
> +       @echo  '                    (requires kernel .config)'
> +       @echo  '  dir/file.[os]   - Build specified target only'
> +       @echo  '  dir/file.i      - Build macro expanded source, similar to C preprocessing'
> +       @echo  '                    (run with RUSTFMT=n to skip reformatting if needed)'
> +       @echo  '  dir/file.ll     - Build the LLVM assembly file'

I don't think we need to repeat dir/* here again for rust. The
existing targets listed above (outside this hunk) make sense in both
contexts.

Does rustc really use .i as a conventional suffix for macro expanded
sources? (The C compiler might use the -x flag to override the guess
it would make based on the file extension; I'm curious if rustc can
ingest .i files or will it warn?)

> diff --git a/init/Kconfig b/init/Kconfig
> index ddcbefe535e9..3457cf596588 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> +config RUSTC_VERSION_TEXT
> +       string
> +       depends on RUST
> +       default $(shell,command -v $(RUSTC) >/dev/null 2>&1 && $(RUSTC) --version || echo n)
> +
> +config BINDGEN_VERSION_TEXT
> +       string
> +       depends on RUST
> +       default $(shell,command -v $(BINDGEN) >/dev/null 2>&1 && $(BINDGEN) --version || echo n)

Are these two kconfigs used anywhere?

> diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
> index 0496efd6e117..83e850321eb6 100644
> --- a/scripts/Kconfig.include
> +++ b/scripts/Kconfig.include
> @@ -36,12 +36,12 @@ ld-option = $(success,$(LD) -v $(1))
>  as-instr = $(success,printf "%b\n" "$(1)" | $(CC) $(CLANG_FLAGS) -c -x assembler -o /dev/null -)
>
>  # check if $(CC) and $(LD) exist
> -$(error-if,$(failure,command -v $(CC)),compiler '$(CC)' not found)
> +$(error-if,$(failure,command -v $(CC)),C compiler '$(CC)' not found)

Not that it's important to do so, but a couple hunks s/compiler/C
compiler/. Those _could_ probably get submitted ahead of this, but not
important to do so.

> diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
> index 9f39b0130551..fe87389d52c0 100644
> --- a/scripts/Makefile.debug
> +++ b/scripts/Makefile.debug
> @@ -1,4 +1,5 @@
>  DEBUG_CFLAGS   :=
> +DEBUG_RUSTFLAGS        :=
>
>  ifdef CONFIG_DEBUG_INFO_SPLIT
>  DEBUG_CFLAGS   += -gsplit-dwarf
> @@ -10,6 +11,12 @@ ifndef CONFIG_AS_IS_LLVM
>  KBUILD_AFLAGS  += -Wa,-gdwarf-2
>  endif
>
> +ifdef CONFIG_DEBUG_INFO_REDUCED
> +DEBUG_RUSTFLAGS += -Cdebuginfo=1
> +else
> +DEBUG_RUSTFLAGS += -Cdebuginfo=2
> +endif
> +

How does enabling or disabling debug info work for rustc? I may have
missed it, but I was surprised to see no additional flags getting
passed to rustc based on CONFIG_DEBUG info.

> diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
> new file mode 100644
> index 000000000000..c146a3407183
> --- /dev/null
> +++ b/scripts/generate_rust_target.rs

Ah, that explains the host rust build infra.  Bravo! Hard coding the
target files was my major concern last I looked at the series. I'm
very happy to see it be generated properly from .config!

I haven't actually reviewed this yet, but it makes me significantly
more confident in the series to see this approach added. Good job
whoever wrote this.

> diff --git a/scripts/is_rust_module.sh b/scripts/is_rust_module.sh
> new file mode 100755
> index 000000000000..277a64d07f22
> --- /dev/null
> +++ b/scripts/is_rust_module.sh
> @@ -0,0 +1,13 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0
> +#
> +# is_rust_module.sh module.ko
> +#
> +# Returns `0` if `module.ko` is a Rust module, `1` otherwise.
> +
> +set -e
> +
> +# Using the `16_` prefix ensures other symbols with the same substring
> +# are not picked up (even if it would be unlikely). The last part is
> +# used just in case LLVM decides to use the `.` suffix.
> +${NM} "$*" | grep -qE '^[0-9a-fA-F]+ r _R[^[:space:]]+16___IS_RUST_MODULE[^[:space:]]*$'

Does `$(READELF) -p .comment foo.o` print anything about which
compiler was used? That seems less brittle IMO.

---

Modulo the RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C which I'm not a fan
of, this is looking good to me. Masahiro, what are your thoughts on
the build system support?
-- 
Thanks,
~Nick Desaulniers