rust: generate target specification files on the fly

Documentation/kbuild/makefiles.rst

@@ -1205,6 +1205,9 @@ When kbuild executes, the following steps are followed (roughly):
 
 	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
 

Makefile

@@ -551,7 +551,6 @@ KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 		   -Werror=return-type -Wno-format-security \
 		   -std=gnu89
 KBUILD_CPPFLAGS := -D__KERNEL__
-KBUILD_RUST_TARGET := $(srctree)/arch/$(SRCARCH)/rust/target.json
 KBUILD_RUSTFLAGS := $(rust_common_flags) \
 		    -Cpanic=abort -Cembed-bitcode=n -Clto=n -Crpath=n \
 		    -Cforce-unwind-tables=n -Ccodegen-units=1 \
@@ -592,7 +591,7 @@ export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
 export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
-export KBUILD_RUST_TARGET KBUILD_RUSTFLAGS RUSTFLAGS_KERNEL RUSTFLAGS_MODULE
+export KBUILD_RUSTFLAGS RUSTFLAGS_KERNEL RUSTFLAGS_MODULE
 export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
 export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_RUSTFLAGS_MODULE KBUILD_LDFLAGS_MODULE
 export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL KBUILD_RUSTFLAGS_KERNEL
@@ -1578,7 +1577,7 @@ MRPROPER_FILES += include/config include/generated          \
 		  certs/x509.genkey \
 		  vmlinux-gdb.py \
 		  *.spec \
-		  rust/libmacros.so
+		  rust/target.json rust/libmacros.so
 
 # clean - Delete most, but leave enough to build external modules
 #

arch/riscv/Makefile

@@ -58,7 +58,6 @@ riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
 
 KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
 KBUILD_AFLAGS += -march=$(riscv-march-y)
-KBUILD_RUST_TARGET := $(srctree)/arch/riscv/rust/$(subst fd,,$(riscv-march-y)).json
 
 KBUILD_CFLAGS += -mno-save-restore
 KBUILD_CFLAGS += -DCONFIG_PAGE_OFFSET=$(CONFIG_PAGE_OFFSET)

rust/.gitignore

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 
+target.json
 bindings_generated.rs
 bindings_helpers_generated.rs
 exports_*_generated.h

rust/Makefile

@@ -1,5 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0
 
+always-$(CONFIG_RUST) += target.json
+no-clean-files += target.json
+
 obj-$(CONFIG_RUST) += core.o compiler_builtins.o
 always-$(CONFIG_RUST) += exports_core_generated.h
 
@@ -206,6 +209,11 @@ rusttest-kernel: $(srctree)/rust/kernel/lib.rs rusttest-prepare \
 	$(call if_changed,rustc_test_library)
 	$(call if_changed,rustdoc_test)
 
+filechk_rust_target = $(objtree)/scripts/generate_rust_target < $<
+
+$(objtree)/rust/target.json: $(objtree)/include/config/auto.conf FORCE
+	$(call filechk,rust_target)
+
 ifdef CONFIG_CC_IS_CLANG
 bindgen_c_flags = $(c_flags)
 else
@@ -364,7 +372,8 @@ $(objtree)/rust/kernel.o: $(srctree)/rust/kernel/lib.rs $(objtree)/rust/alloc.o
 $(objtree)/rust/core.o: private skip_clippy = 1
 $(objtree)/rust/core.o: private skip_flags = -Dunreachable_pub --edition=2021
 $(objtree)/rust/core.o: private rustc_target_flags = $(core-cfgs) --edition=2018
-$(objtree)/rust/core.o: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
+$(objtree)/rust/core.o: $(RUST_LIB_SRC)/core/src/lib.rs \
+    $(objtree)/rust/target.json FORCE
 	$(call if_changed_dep,rustc_library)
 
 rustdoc-core: private rustc_target_flags = $(core-cfgs)

scripts/.gitignore

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 /asn1_compiler
 /bin2c
+/generate_rust_target
 /insert-sys-cert
 /kallsyms
 /module.lds

scripts/Makefile

@@ -10,6 +10,9 @@ hostprogs-always-$(CONFIG_BUILDTIME_TABLE_SORT)		+= sorttable
 hostprogs-always-$(CONFIG_ASN1)				+= asn1_compiler
 hostprogs-always-$(CONFIG_MODULE_SIG_FORMAT)		+= sign-file
 hostprogs-always-$(CONFIG_SYSTEM_EXTRA_CERTIFICATE)	+= insert-sys-cert
+hostprogs-always-$(CONFIG_RUST)				+= generate_rust_target
+
+generate_rust_target-rust := y
 
 HOSTCFLAGS_sorttable.o = -I$(srctree)/tools/include
 HOSTLDLIBS_sorttable = -lpthread

scripts/Makefile.build

@@ -328,7 +328,7 @@ $(obj)/%.lst: $(src)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_cross_flags := --target=$(KBUILD_RUST_TARGET)
+rust_cross_flags := --target=$(objtree)/rust/target.json
 rust_allowed_features := allocator_api,bench_black_box,concat_idents,generic_associated_types
 
 rust_common_cmd = \