[PATCH v4 21/22] kbuild: build rust crates in parallel with the rest of the build

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

When CONFIG_RUST is specified make prepare builds all of the rust
components before descending into the tree.

This means nothing can be done until all of these are compiled, resulting
in a stall at the start of every clean CONFIG_RUST build.

This also implicitly slows down every rust-capable LLVM allmodconfig build
as this enables the CONFIG_RUST option.

Fix this by allowing rust crates to be built alongside the C code.

Remove the rust build from the make prepare step, then establish a
dependency between rust code located elsewhere in the tree upon rust
components contained in rust/.

Do this by establishing a top-level list of directories containing rust
code, KBUILD_RUST_DIRS, upon which the dependency is expressed.

Finally, ensure that no rust code is located elsewhere and fail the build
if any is found to ensure that nothing is missed in future.

Care is taken to ensure the rust/ dependency is also established for any
module being built to account for external rust modules which rely upon it,
and that only one rust/ build will be performed at any one time.

External rust modules builds are unaffected - make modules_prepare ...
(where ... are other goals) will build rust/ first.

The rustdoc and rusttest goals invoke rust/ directly rather than via
Kbuild, so build rust/ first there too.

Every Rust and C object was confirmed to be byte-for-byte identical after
this change.

Whole build, 128-thread Threadripper 9980X, best of N runs:

                                         before   after     delta
                                         -------------------------------
  x86 defconfig+RUST, clean                36.5s    32.6s     -3.9s (-11%)
  x86 allmodconfig, clean                 297.0s   278.3s    -18.7s (-6%)

Assisted-by: LLM
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 Kbuild                 |  5 +++++
 Makefile               | 17 ++++++++++++++++-
 scripts/Makefile.build |  5 ++++-
 3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/Kbuild b/Kbuild
index a6a0192dea08..9670556059dd 100644
--- a/Kbuild
+++ b/Kbuild
@@ -115,3 +115,8 @@ obj-$(CONFIG_NET)	+= net/
 obj-y			+= virt/
 obj-y			+= $(ARCH_DRIVERS)
 obj-$(CONFIG_DRM_HEADER_TEST)	+= include/
+
+# Rust code elsewhere in the tree depends upon rust/.
+ifdef CONFIG_RUST
+$(KBUILD_RUST_DIRS): | rust
+endif
diff --git a/Makefile b/Makefile
index 7745bb36e622..f0502b02408f 100644
--- a/Makefile
+++ b/Makefile
@@ -1429,11 +1429,14 @@ prepare0: archprepare
 	$(Q)$(MAKE) $(build)=. prepare
 	$(Q)$(MAKE) $(build)=scripts/mod
 
+ifdef CONFIG_RUST
+export KBUILD_RUST_DIRS := drivers lib mm samples
+endif
+
 # All the preparing..
 prepare: prepare0
 ifdef CONFIG_RUST
 	+$(Q)$(CONFIG_SHELL) $(srctree)/scripts/rust_is_available.sh
-	$(Q)$(MAKE) $(build)=rust
 endif
 
 PHONY += remove-stale-files
@@ -1760,6 +1763,14 @@ modules: modules_prepare
 # Target to prepare building external modules
 modules_prepare: prepare
 	$(Q)$(MAKE) $(build)=scripts scripts/module.lds
+ifdef CONFIG_RUST
+# Avoid two instances of rust/ being built at any one time.
+ifneq ($(filter modules_prepare,$(MAKECMDGOALS)),)
+ifeq ($(filter all vmlinux modules %Image% dtbs rustdoc rusttest,$(MAKECMDGOALS)),)
+	$(Q)$(MAKE) $(build)=rust
+endif
+endif
+endif
 
 endif # CONFIG_MODULES
 
@@ -2025,13 +2036,17 @@ rustavailable:
 # Documentation target
 #
 # Using the singular to avoid running afoul of `no-dot-config-targets`.
+#
+# rust/ is built alongside the rest of the tree, so must be built first here.
 PHONY += rustdoc
 rustdoc: prepare
+	$(Q)$(MAKE) $(build)=rust
 	$(Q)$(MAKE) $(build)=rust $@
 
 # Testing target
 PHONY += rusttest
 rusttest: prepare
+	$(Q)$(MAKE) $(build)=rust
 	$(Q)$(MAKE) $(build)=rust $@
 
 # Formatting targets
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 819cc1f750e8..a809aad060e7 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -360,8 +360,11 @@ quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
 		$(cmd_ld_single)) \
 	$(cmd_objtool)
 
+rust-dir-ok = $(or $(KBUILD_EXTMOD),$(filter rust $(KBUILD_RUST_DIRS),$(firstword $(subst /, ,$@))))
+
+# Ensure that any rust code located elsewhere from rust/ is listed in KBUILD_RUST_DIRS.
 define rule_rustc_o_rs
-	$(call cmd_and_fixdep,rustc_o_rs)
+	$(if $(rust-dir-ok),,$(error $@: Rust code in a directory not listed in KBUILD_RUST_DIRS, see the top Makefile))$(call cmd_and_fixdep,rustc_o_rs)
 	$(call cmd,gen_objtooldep)
 endef
 

-- 
2.55.0