From: Rong Xu <xur@google.com>

Add distributed ThinLTO build support for the Linux kernel.
This new mode offers several advantages: (1) Increased
flexibility in handling user-specified build options.
(2) Improved user-friendliness for developers. (3) Greater
convenience for integrating with objtool and livepatch.

Note that "distributed" in this context refers to a term
that differentiates in-process ThinLTO builds by invoking
backend compilation through the linker, not necessarily
building in distributed environments.

Distributed ThinLTO is enabled via the
`CONFIG_LTO_CLANG_THIN_DIST` Kconfig option. For example:
 > make LLVM=1 defconfig
 > scripts/config -e LTO_CLANG_THIN_DIST
 > make LLVM=1 oldconfig
 > make LLVM=1 vmlinux -j <..>

The build flow proceeds in four stages:
  1. Perform FE compilation, mirroring the in-process ThinLTO mode.
  2. Thin-link the generated IR files and object files.
  3. Find all IR files and perform BE compilation, using the flags
    stored in the .*.o.cmd files.
  4. Link the BE results to generate the final vmlinux.o.

NOTE: This patch currently implements the build for the main kernel
image (vmlinux) only. Kernel module support is planned for a
subsequent patch.

Tested on the following arch: x86, arm64, loongarch, and
riscv.

The earlier implementation details can be found here:
https://discourse.llvm.org/t/rfc-distributed-thinlto-build-for-kernel/85934

Signed-off-by: Rong Xu <xur@google.com>
Co-developed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---
 .gitignore                 |  2 ++
 Makefile                   | 11 +++++++----
 arch/Kconfig               | 19 ++++++++++++++++++
 scripts/Makefile.lib       |  7 +++++++
 scripts/Makefile.thinlto   | 40 ++++++++++++++++++++++++++++++++++++++
 scripts/Makefile.vmlinux_a | 37 +++++++++++++++++++++++++++++++++++
 scripts/mod/modpost.c      | 15 +++++++++++---
 7 files changed, 124 insertions(+), 7 deletions(-)
 create mode 100644 scripts/Makefile.thinlto

diff --git a/.gitignore b/.gitignore
index 3a7241c941f5..c12bdf5a97f9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -56,6 +56,7 @@
 *.zst
 Module.symvers
 dtbs-list
+builtin.order
 modules.order
 
 #
@@ -67,6 +68,7 @@ modules.order
 /vmlinux.32
 /vmlinux.map
 /vmlinux.symvers
+/vmlinux.thinlto-index
 /vmlinux.unstripped
 /vmlinux-gdb.py
 /vmlinuz
diff --git a/Makefile b/Makefile
index 69ccf9b8507d..26005c64016d 100644
--- a/Makefile
+++ b/Makefile
@@ -1047,11 +1047,13 @@ export CC_FLAGS_SCS
 endif
 
 ifdef CONFIG_LTO_CLANG
-ifdef CONFIG_LTO_CLANG_THIN
+ifdef CONFIG_LTO_CLANG_FULL
+CC_FLAGS_LTO	:= -flto
+else
 CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
+if CONFIG_LTO_CLANG_THIN
 KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
-else
-CC_FLAGS_LTO	:= -flto
+endif
 endif
 CC_FLAGS_LTO	+= -fvisibility=hidden
 
@@ -1657,6 +1659,7 @@ endif # CONFIG_MODULES
 CLEAN_FILES += vmlinux.symvers modules-only.symvers \
 	       modules.builtin modules.builtin.modinfo modules.nsdeps \
 	       modules.builtin.ranges vmlinux.o.map vmlinux.unstripped \
+	       vmlinux.thinlto-index builtin.order \
 	       compile_commands.json rust/test \
 	       rust-project.json .vmlinux.objs .vmlinux.export.c \
                .builtin-dtbs-list .builtin-dtb.S
@@ -2118,7 +2121,7 @@ clean: $(clean-dirs)
 	$(call cmd,rmfiles)
 	@find . $(RCS_FIND_IGNORE) \
 		\( -name '*.[aios]' -o -name '*.rsi' -o -name '*.ko' -o -name '.*.cmd' \
-		-o -name '*.ko.*' \
+		-o -name '*.ko.*' -o -name '*.o.thinlto.bc' \
 		-o -name '*.dtb' -o -name '*.dtbo' \
 		-o -name '*.dtb.S' -o -name '*.dtbo.S' \
 		-o -name '*.dt.yaml' -o -name 'dtbs-list' \
diff --git a/arch/Kconfig b/arch/Kconfig
index 102ddbd4298e..340550e44c50 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -861,6 +861,25 @@ config LTO_CLANG_THIN
 	    https://clang.llvm.org/docs/ThinLTO.html
 
 	  If unsure, say Y.
+
+config LTO_CLANG_THIN_DIST
+	bool "Clang ThinLTO in distributed mode (EXPERIMENTAL)"
+	depends on HAS_LTO_CLANG && ARCH_SUPPORTS_LTO_CLANG_THIN
+	select LTO_CLANG
+	help
+	  This option enables Clang's ThinLTO in distributed build mode.
+	  In this mode, the linker performs the thin-link, generating
+	  ThinLTO index files. Subsequently, the build system explicitly
+	  invokes ThinLTO backend compilation using these index files
+	  and pre-linked IR objects. The resulting native object files
+	  are with the .thinlto-native.o suffix.
+
+	  This build mode offers improved visibility into the ThinLTO
+	  process through explicit subcommand exposure. It also makes
+	  final native object files directly available, benefiting
+	  tools like objtool and kpatch. Additionally, it provides
+	  crucial granular control over back-end options, enabling
+	  module-specific compiler options, and simplifies debugging.
 endchoice
 
 config ARCH_SUPPORTS_AUTOFDO_CLANG
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 0718e39cedda..b36c7c6817bd 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -249,6 +249,12 @@ ifdef CONFIG_LTO_CLANG
 cmd_ld_single = $(if $(objtool-enabled)$(is-single-obj-m), ; $(LD) $(ld_flags) -r -o $(tmp-target) $@; mv $(tmp-target) $@)
 endif
 
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+# Save the _c_flags, sliently.
+quiet_cmd_save_c_flags =
+      cmd_save_c_flags = printf '\n%s\n' 'saved_c_flags_$@ := $(call escsq,$(_c_flags))' >> $(dot-target).cmd
+endif
+
 quiet_cmd_cc_o_c = CC $(quiet_modtag)  $@
       cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $< \
 		$(cmd_ld_single) \
@@ -256,6 +262,7 @@ quiet_cmd_cc_o_c = CC $(quiet_modtag)  $@
 
 define rule_cc_o_c
 	$(call cmd_and_fixdep,cc_o_c)
+	$(call cmd,save_c_flags)
 	$(call cmd,checksrc)
 	$(call cmd,checkdoc)
 	$(call cmd,gen_objtooldep)
diff --git a/scripts/Makefile.thinlto b/scripts/Makefile.thinlto
new file mode 100644
index 000000000000..03349ac69de5
--- /dev/null
+++ b/scripts/Makefile.thinlto
@@ -0,0 +1,40 @@
+PHONY := __default
+__default:
+
+include include/config/auto.conf
+include $(srctree)/scripts/Kbuild.include
+include $(srctree)/scripts/Makefile.lib
+
+native-objs := $(patsubst %.o,%.thinlto-native.o,$(call read-file, vmlinux.thinlto-index))
+
+__default: $(native-objs)
+
+# Generate .thinlto-native.o (obj) from .o (bitcode) and .thinlto.bc (summary) files
+# ---------------------------------------------------------------------------
+quiet_cmd_cc_o_bc = CC $(quiet_modtag)  $@
+      be_flags = $(shell sed -n '/saved_c_flags_/s/.*:= //p' \
+		 $(dir $(<)).$(notdir $(<)).cmd)
+      cmd_cc_o_bc = \
+      $(CC) $(be_flags) -x ir -fno-lto -Wno-unused-command-line-argument \
+      -fthinlto-index=$(word 2, $^) -c -o $@ $<
+
+targets += $(native-objs)
+$(native-objs): %.thinlto-native.o: %.o %.o.thinlto.bc   FORCE
+	$(call if_changed,cc_o_bc)
+
+# Add FORCE to the prerequisites of a target to force it to be always rebuilt.
+# ---------------------------------------------------------------------------
+
+PHONY += FORCE
+FORCE:
+
+# Read all saved command lines and dependencies for the $(targets) we
+# may be building above, using $(if_changed{,_dep}). As an
+# optimization, we don't need to read them if the target does not
+# exist, we will rebuild anyway in that case.
+
+existing-targets := $(wildcard $(sort $(targets)))
+
+-include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
+
+.PHONY: $(PHONY)
diff --git a/scripts/Makefile.vmlinux_a b/scripts/Makefile.vmlinux_a
index 650d44330d1f..15d92ee82dee 100644
--- a/scripts/Makefile.vmlinux_a
+++ b/scripts/Makefile.vmlinux_a
@@ -21,6 +21,41 @@ targets += built-in-fixup.a
 built-in-fixup.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
 	$(call if_changed,ar_builtin_fixup)
 
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+
+quiet_cmd_builtin.order = GEN     $@
+      cmd_builtin.order = $(AR) t $< > $@
+
+targets += builtin.order
+builtin.order: built-in-fixup.a FORCE
+	$(call if_changed,builtin.order)
+
+quiet_cmd_ld_thinlto_index = LD      $@
+      cmd_ld_thinlto_index = \
+	$(LD) $(KBUILD_LDFLAGS) -r --thinlto-index-only=$@ @$<
+
+targets += vmlinux.thinlto-index
+vmlinux.thinlto-index: builtin.order FORCE
+	$(call if_changed,ld_thinlto_index)
+
+quiet_cmd_ar_vmlinux.a = GEN     $@
+      cmd_ar_vmlinux.a =					\
+	rm -f $@;						\
+	while read -r obj; do					\
+		if grep -q $${obj} $(word 2, $^); then		\
+			echo $${obj%.o}.thinlto-native.o;	\
+		else						\
+			echo $${obj};				\
+		fi;						\
+	done < $< | xargs $(AR) cDPrS --thin $@
+
+targets += vmlinux.a
+vmlinux.a: builtin.order vmlinux.thinlto-index FORCE
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.thinlto
+	$(call if_changed,ar_vmlinux.a)
+
+else
+
 # vmlinux.a
 # ---------------------------------------------------------------------------
 
@@ -28,6 +63,8 @@ targets += vmlinux.a
 vmlinux.a: built-in-fixup.a FORCE
 	$(call if_changed,copy)
 
+endif
+
 # Add FORCE to the prerequisites of a target to force it to be always rebuilt.
 # ---------------------------------------------------------------------------
 
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 0c25b5ad497b..b3c3cf42c9eb 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1482,13 +1482,22 @@ static void extract_crcs_for_object(const char *object, struct module *mod)
 	char cmd_file[PATH_MAX];
 	char *buf, *p;
 	const char *base;
-	int dirlen, ret;
+	int dirlen, baselen_without_suffix, ret;
 
 	base = get_basename(object);
 	dirlen = base - object;
 
-	ret = snprintf(cmd_file, sizeof(cmd_file), "%.*s.%s.cmd",
-		       dirlen, object, base);
+	baselen_without_suffix = strlen(object) - dirlen - strlen(".o");
+
+	/*
+	 * When CONFIG_LTO_CLANG_THIN_DIST=y, the ELF is *.thinlto-native.o
+	 * but the symbol CRCs are recorded in *.o.cmd file.
+	 */
+	if (strends(object, ".thinlto-native.o"))
+		baselen_without_suffix -= strlen(".thinlto-native");
+
+	ret = snprintf(cmd_file, sizeof(cmd_file), "%.*s.%.*s.o.cmd",
+		       dirlen, object, baselen_without_suffix, base);
 	if (ret >= sizeof(cmd_file)) {
 		error("%s: too long path was truncated\n", cmd_file);
 		return;
-- 
2.53.0.1018.g2bb0e51243-goog

Hi Rong,

On Tue, Mar 31, 2026 at 03:48:27PM +0000, xur@google.com wrote:
> diff --git a/Makefile b/Makefile
> index 69ccf9b8507d..26005c64016d 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1047,11 +1047,13 @@ export CC_FLAGS_SCS
>  endif
>  
>  ifdef CONFIG_LTO_CLANG
> -ifdef CONFIG_LTO_CLANG_THIN
> +ifdef CONFIG_LTO_CLANG_FULL
> +CC_FLAGS_LTO	:= -flto
> +else
>  CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
> +if CONFIG_LTO_CLANG_THIN

This should be an 'ifdef', not an 'if'. You copied Yonghong's mistake:

  https://lore.kernel.org/abgRRX3PH9IaExi8@sirena.org.uk/
  https://lore.kernel.org/6db3a2f6-d61c-42f1-9b9d-0aca021cc2d7@linux.dev/

Please slow down and test build your changes before sending them. Each
revision adds four new emails to everyone's inbox, which is just noise
when there are obvious, basic problems. 'b4 diff' shows no actual
difference from v8 and v9, which should have been caught by a simple
build test right before 'git send-email'.

>  KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
> -else
> -CC_FLAGS_LTO	:= -flto
> +endif
>  endif
>  CC_FLAGS_LTO	+= -fvisibility=hidden
>  
> @@ -1657,6 +1659,7 @@ endif # CONFIG_MODULES
>  CLEAN_FILES += vmlinux.symvers modules-only.symvers \
>  	       modules.builtin modules.builtin.modinfo modules.nsdeps \
>  	       modules.builtin.ranges vmlinux.o.map vmlinux.unstripped \
> +	       vmlinux.thinlto-index builtin.order \
>  	       compile_commands.json rust/test \
>  	       rust-project.json .vmlinux.objs .vmlinux.export.c \
>                 .builtin-dtbs-list .builtin-dtb.S
> @@ -2118,7 +2121,7 @@ clean: $(clean-dirs)
>  	$(call cmd,rmfiles)
>  	@find . $(RCS_FIND_IGNORE) \
>  		\( -name '*.[aios]' -o -name '*.rsi' -o -name '*.ko' -o -name '.*.cmd' \
> -		-o -name '*.ko.*' \
> +		-o -name '*.ko.*' -o -name '*.o.thinlto.bc' \
>  		-o -name '*.dtb' -o -name '*.dtbo' \
>  		-o -name '*.dtb.S' -o -name '*.dtbo.S' \
>  		-o -name '*.dt.yaml' -o -name 'dtbs-list' \

With that addressed above, the series survives my basic LLVM 22.1.2
build test with my distribution configuration. I'll provide formal tags
on a properly tested and fixed revision.

Cheers,
Nathan