[PATCH v4 0/2] kbuild: distributed build support for Clang ThinLTO

[PATCH v4 0/2] kbuild: distributed build support for Clang ThinLTO

[xur]

From: Rong Xu <xur@google.com>

This V4 is based on the reimplementation provided by Masahiro Yamada
on May 26. The difference is that this version now saves the compiler
flags (_c_flags) from the Front-End (FE) compilation and re-uses them
for the subsequent Back-End (BE) compilation.

Rong Xu (2):
  kbuild: move vmlinux.a build rule to scripts/Makefile.vmlinux_a
  kbuild: distributed build support for Clang ThinLTO

 .gitignore                 |  2 +
 Makefile                   | 25 +++++-------
 arch/Kconfig               | 19 +++++++++
 scripts/Makefile.lib       |  7 ++++
 scripts/Makefile.thinlto   | 40 ++++++++++++++++++
 scripts/Makefile.vmlinux_a | 83 ++++++++++++++++++++++++++++++++++++++
 scripts/mod/modpost.c      | 15 +++++--
 7 files changed, 174 insertions(+), 17 deletions(-)
 create mode 100644 scripts/Makefile.thinlto
 create mode 100644 scripts/Makefile.vmlinux_a


base-commit: dcb6fa37fd7bc9c3d2b066329b0d27dedf8becaa
-- 
2.51.1.838.g19442a804e-goog

[PATCH v4 1/2] kbuild: move vmlinux.a build rule to scripts/Makefile.vmlinux_a

[xur]

From: Rong Xu <xur@google.com>

From: Masahiro Yamada <masahiroy@kernel.org>

Move the build rule for vmlinux.a to a separate file in preparation
for supporting distributed builds with Clang ThinLTO.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Rong Xu <xur@google.com>
---
 Makefile                   | 16 +++++--------
 scripts/Makefile.vmlinux_a | 46 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 10 deletions(-)
 create mode 100644 scripts/Makefile.vmlinux_a

diff --git a/Makefile b/Makefile
index b34a1f4c03967..89a25bac2bbab 100644
--- a/Makefile
+++ b/Makefile
@@ -1198,7 +1198,7 @@ export ARCH_DRIVERS	:= $(drivers-y) $(drivers-m)
 KBUILD_VMLINUX_OBJS := built-in.a $(patsubst %/, %/lib.a, $(filter %/, $(libs-y)))
 KBUILD_VMLINUX_LIBS := $(filter-out %/, $(libs-y))
 
-export KBUILD_VMLINUX_LIBS
+export KBUILD_VMLINUX_OBJS KBUILD_VMLINUX_LIBS
 export KBUILD_LDS          := arch/$(SRCARCH)/kernel/vmlinux.lds
 
 ifdef CONFIG_TRIM_UNUSED_KSYMS
@@ -1207,16 +1207,12 @@ ifdef CONFIG_TRIM_UNUSED_KSYMS
 KBUILD_MODULES := y
 endif
 
-# '$(AR) mPi' needs 'T' to workaround the bug of llvm-ar <= 14
-quiet_cmd_ar_vmlinux.a = AR      $@
-      cmd_ar_vmlinux.a = \
-	rm -f $@; \
-	$(AR) cDPrST $@ $(KBUILD_VMLINUX_OBJS); \
-	$(AR) mPiT $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
+PHONY += vmlinux_a
+vmlinux_a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux_a
 
-targets += vmlinux.a
-vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
-	$(call if_changed,ar_vmlinux.a)
+vmlinux.a: vmlinux_a
+	@:
 
 PHONY += vmlinux_o
 vmlinux_o: vmlinux.a $(KBUILD_VMLINUX_LIBS)
diff --git a/scripts/Makefile.vmlinux_a b/scripts/Makefile.vmlinux_a
new file mode 100644
index 0000000000000..9774f02b43b2f
--- /dev/null
+++ b/scripts/Makefile.vmlinux_a
@@ -0,0 +1,46 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+PHONY := __default
+__default: vmlinux.a
+
+include include/config/auto.conf
+include $(srctree)/scripts/Kbuild.include
+include $(srctree)/scripts/Makefile.lib
+
+# Link of built-in-fixup.a
+# ---------------------------------------------------------------------------
+
+# '$(AR) mPi' needs --thin to workaround the bug of llvm-ar <= 14
+quiet_cmd_ar_builtin_fixup = AR      $@
+      cmd_ar_builtin_fixup = \
+	rm -f $@; \
+	$(AR) cDPrS --thin $@ $(KBUILD_VMLINUX_OBJS); \
+	$(AR) mPi --thin $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
+
+targets += built-in-fixup.a
+built-in-fixup.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
+	$(call if_changed,ar_builtin_fixup)
+
+# vmlinux.a
+# ---------------------------------------------------------------------------
+
+targets += vmlinux.a
+vmlinux.a: built-in-fixup.a FORCE
+	$(call if_changed,copy)
+
+# 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)
-- 
2.51.1.838.g19442a804e-goog

[PATCH v4 2/2] kbuild: distributed build support for Clang ThinLTO

[xur]

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                   |  9 +++++----
 arch/Kconfig               | 19 ++++++++++++++++++
 scripts/Makefile.lib       |  7 +++++++
 scripts/Makefile.thinlto   | 40 ++++++++++++++++++++++++++++++++++++++
 scripts/Makefile.vmlinux_a | 37 +++++++++++++++++++++++++++++++++++
 scripts/mod/modpost.c      | 15 +++++++++++---
 7 files changed, 122 insertions(+), 7 deletions(-)
 create mode 100644 scripts/Makefile.thinlto

diff --git a/.gitignore b/.gitignore
index 86a1ba0d90353..d34f28ca55400 100644
--- a/.gitignore
+++ b/.gitignore
@@ -55,6 +55,7 @@
 *.zst
 Module.symvers
 dtbs-list
+builtin.order
 modules.order
 
 #
@@ -66,6 +67,7 @@ modules.order
 /vmlinux.32
 /vmlinux.map
 /vmlinux.symvers
+/vmlinux.thinlto-index
 /vmlinux.unstripped
 /vmlinux-gdb.py
 /vmlinuz
diff --git a/Makefile b/Makefile
index 89a25bac2bbab..a608a082599d9 100644
--- a/Makefile
+++ b/Makefile
@@ -1000,10 +1000,10 @@ export CC_FLAGS_SCS
 endif
 
 ifdef CONFIG_LTO_CLANG
-ifdef CONFIG_LTO_CLANG_THIN
-CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
-else
+ifdef CONFIG_LTO_CLANG_FULL
 CC_FLAGS_LTO	:= -flto
+else
+CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
 endif
 CC_FLAGS_LTO	+= -fvisibility=hidden
 
@@ -1572,6 +1572,7 @@ 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
@@ -2014,7 +2015,7 @@ 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 74ff011335322..f323ff2f12f87 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -864,6 +864,25 @@ 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 1d581ba5df66f..65470adaa89ca 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -239,6 +239,12 @@ 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_$@ := $(_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) \
@@ -246,6 +252,7 @@ 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 0000000000000..03349ac69de5f
--- /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 9774f02b43b2f..382cebe1ee571 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 47c8aa2a69392..38f62eb4bd8ba 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1473,13 +1473,22 @@ 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;
-- 
2.51.1.838.g19442a804e-goog

Re: [PATCH v4 0/2] kbuild: distributed build support for Clang ThinLTO

[Piotr Gorski]

I tried using this on kernel 6.17.5, but unfortunately I get this error. 
I previously tested the version in CachyOS: 
https://github.com/CachyOS/linux/commit/68984063b5e27854272241549e47cb2930ef9710 
and https://github.com/CachyOS/linux/ 
commit/00a3ff73a566b7e27b4ced46af4ce27177d9fc37 and everything works fine.

/bin/sh: -c: line 1: syntax error near unexpected token `('
/bin/sh: -c: line 1: `set -e;    trap 'rm -f 
drivers/net/wireguard/main.o; trap - HUP; kill -s HUP $$' HUP; trap 'rm 
-f drivers/net/wireguard/main.o; trap - INT; kill -s INT $$' INT;  trap 
'rm -f drivers/net/wireguard/main.o
; trap - QUIT; kill -s QUIT $$' QUIT;  trap 'rm -f 
drivers/net/wireguard/main.o; trap - TERM; kill -s TERM $$' TERM; trap 
'rm -f drivers/net/wireguard/main.o; trap - PIPE; kill -s PIPE $$' PIPE; 
printf '\n%s\n' 'saved_c_flag
s_drivers/net/wireguard/main.o := -D__KERNEL__ --target=x86_64-linux-gnu 
-fintegrated-as -Werror=ignored-optimization-argument 
-Werror=option-ignored -std=gnu11 -fshort-wchar -funsigned-char 
-fno-common -fno-PIE -fno-strict-a
liasing -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx 
-fcf-protection=branch -fno-jump-tables -m64 -falign-loops=1 -mno-80387 
-mno-fp-ret-in-387 -mstack-alignment=8 -mskip-rax-setup -march=znver4 
-mno-red-zone -mcmodel=kern
el -mstack-protector-guard-reg=gs 
-mstack-protector-guard-symbol=__ref_stack_chk_guard -Wno-sign-compare 
-fno-asynchronous-unwind-tables -mretpoline-external-thunk 
-mindirect-branch-cs-prefix -mfunction-return=thunk-extern -m
harden-sls=all -fpatchable-function-entry=16,16 
-fno-delete-null-pointer-checks -O3 -fstack-protector-strong 
-ftrivial-auto-var-init=zero -fno-stack-clash-protection -pg -mfentry 
-DCC_USING_NOP_MCOUNT -DCC_USING_FENTRY -fno-l
to -flto=thin -fsplit-lto-unit -fvisibility=hidden -falign-functions=16 
-fstrict-flex-arrays=3 -fno-strict-overflow -fno-stack-check 
-fno-builtin-wcslen -Wall -Wextra -Wundef 
-Werror=implicit-function-declaration -Werror=impl
icit-int -Werror=return-type -Wno-format-security -Wno-trigraphs 
-Wno-frame-address -Wno-address-of-packed-member -Wmissing-declarations 
-Wmissing-prototypes -Wframe-larger-than=2048 -Wno-gnu 
-Wno-format-overflow-non-kprintf
-Wno-format-truncation-non-kprintf -Wno-default-const-init-unsafe 
-Wno-pointer-sign -Wcast-function-type 
-Wno-unterminated-string-initialization -Wimplicit-fallthrough 
-Werror=date-time -Wenum-conversion -Wunused -Wno-unused-
but-set-variable -Wno-unused-const-variable -Wno-format-overflow 
-Wno-override-init -Wno-pointer-to-enum-cast 
-Wno-tautological-constant-out-of-range-compare -Wno-unaligned-access 
-Wno-enum-compare-conditional -Wno-missing-fi
eld-initializers -Wno-type-limits -Wno-shift-negative-value 
-Wno-enum-enum-conversion -Wno-sign-compare -Wno-unused-parameter -g 
-gdwarf-5 -D'pr_fmt(fmt)=KBUILD_MODNAME ": " fmt'' >> 
drivers/net/wireguard/.main.o.cmd'
make[5]: *** [scripts/Makefile.build:287: drivers/net/wireguard/main.o] 
Error 2
make[5]: *** Deleting file 'drivers/net/wireguard/main.o'
make[4]: *** [scripts/Makefile.build:556: drivers/net/wireguard] Error 2

LLVM/CLANG: 21.1.4

If necessary, I can send more logs.

Best regards,

Piotr

W dniu 27.10.2025 o 23:09, xur@google.com pisze:
> From: Rong Xu <xur@google.com>
>
> This V4 is based on the reimplementation provided by Masahiro Yamada
> on May 26. The difference is that this version now saves the compiler
> flags (_c_flags) from the Front-End (FE) compilation and re-uses them
> for the subsequent Back-End (BE) compilation.
>
> Rong Xu (2):
>    kbuild: move vmlinux.a build rule to scripts/Makefile.vmlinux_a
>    kbuild: distributed build support for Clang ThinLTO
>
>   .gitignore                 |  2 +
>   Makefile                   | 25 +++++-------
>   arch/Kconfig               | 19 +++++++++
>   scripts/Makefile.lib       |  7 ++++
>   scripts/Makefile.thinlto   | 40 ++++++++++++++++++
>   scripts/Makefile.vmlinux_a | 83 ++++++++++++++++++++++++++++++++++++++
>   scripts/mod/modpost.c      | 15 +++++--
>   7 files changed, 174 insertions(+), 17 deletions(-)
>   create mode 100644 scripts/Makefile.thinlto
>   create mode 100644 scripts/Makefile.vmlinux_a
>
>
> base-commit: dcb6fa37fd7bc9c3d2b066329b0d27dedf8becaa

Re: [PATCH v4 0/2] kbuild: distributed build support for Clang ThinLTO

[Rong Xu]

Thanks for testing this!

The culprit was the string -D'pr_fmt(fmt)=KBUILD_MODNAME ": " fmt'.

The previous patch parsed c_flags and removed -D% and a bunch of other
FE flags. Masahiro suggested using _c_flags instead.
I erroneously assumed there were no -D% in _c_flags. But this is
wrong. It seems that we do need to filter out -D% even using _c_flags.

I'll send an updated patch shortly.

Best regards,

-Rong


On Tue, Oct 28, 2025 at 3:43 AM Piotr Gorski <piotrgorski@cachyos.org> wrote:
>
> I tried using this on kernel 6.17.5, but unfortunately I get this error.
> I previously tested the version in CachyOS:
> https://github.com/CachyOS/linux/commit/68984063b5e27854272241549e47cb2930ef9710
> and https://github.com/CachyOS/linux/
> commit/00a3ff73a566b7e27b4ced46af4ce27177d9fc37 and everything works fine.
>
> /bin/sh: -c: line 1: syntax error near unexpected token `('
> /bin/sh: -c: line 1: `set -e;    trap 'rm -f
> drivers/net/wireguard/main.o; trap - HUP; kill -s HUP $$' HUP; trap 'rm
> -f drivers/net/wireguard/main.o; trap - INT; kill -s INT $$' INT;  trap
> 'rm -f drivers/net/wireguard/main.o
> ; trap - QUIT; kill -s QUIT $$' QUIT;  trap 'rm -f
> drivers/net/wireguard/main.o; trap - TERM; kill -s TERM $$' TERM; trap
> 'rm -f drivers/net/wireguard/main.o; trap - PIPE; kill -s PIPE $$' PIPE;
> printf '\n%s\n' 'saved_c_flag
> s_drivers/net/wireguard/main.o := -D__KERNEL__ --target=x86_64-linux-gnu
> -fintegrated-as -Werror=ignored-optimization-argument
> -Werror=option-ignored -std=gnu11 -fshort-wchar -funsigned-char
> -fno-common -fno-PIE -fno-strict-a
> liasing -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
> -fcf-protection=branch -fno-jump-tables -m64 -falign-loops=1 -mno-80387
> -mno-fp-ret-in-387 -mstack-alignment=8 -mskip-rax-setup -march=znver4
> -mno-red-zone -mcmodel=kern
> el -mstack-protector-guard-reg=gs
> -mstack-protector-guard-symbol=__ref_stack_chk_guard -Wno-sign-compare
> -fno-asynchronous-unwind-tables -mretpoline-external-thunk
> -mindirect-branch-cs-prefix -mfunction-return=thunk-extern -m
> harden-sls=all -fpatchable-function-entry=16,16
> -fno-delete-null-pointer-checks -O3 -fstack-protector-strong
> -ftrivial-auto-var-init=zero -fno-stack-clash-protection -pg -mfentry
> -DCC_USING_NOP_MCOUNT -DCC_USING_FENTRY -fno-l
> to -flto=thin -fsplit-lto-unit -fvisibility=hidden -falign-functions=16
> -fstrict-flex-arrays=3 -fno-strict-overflow -fno-stack-check
> -fno-builtin-wcslen -Wall -Wextra -Wundef
> -Werror=implicit-function-declaration -Werror=impl
> icit-int -Werror=return-type -Wno-format-security -Wno-trigraphs
> -Wno-frame-address -Wno-address-of-packed-member -Wmissing-declarations
> -Wmissing-prototypes -Wframe-larger-than=2048 -Wno-gnu
> -Wno-format-overflow-non-kprintf
> -Wno-format-truncation-non-kprintf -Wno-default-const-init-unsafe
> -Wno-pointer-sign -Wcast-function-type
> -Wno-unterminated-string-initialization -Wimplicit-fallthrough
> -Werror=date-time -Wenum-conversion -Wunused -Wno-unused-
> but-set-variable -Wno-unused-const-variable -Wno-format-overflow
> -Wno-override-init -Wno-pointer-to-enum-cast
> -Wno-tautological-constant-out-of-range-compare -Wno-unaligned-access
> -Wno-enum-compare-conditional -Wno-missing-fi
> eld-initializers -Wno-type-limits -Wno-shift-negative-value
> -Wno-enum-enum-conversion -Wno-sign-compare -Wno-unused-parameter -g
> -gdwarf-5 -D'pr_fmt(fmt)=KBUILD_MODNAME ": " fmt'' >>
> drivers/net/wireguard/.main.o.cmd'
> make[5]: *** [scripts/Makefile.build:287: drivers/net/wireguard/main.o]
> Error 2
> make[5]: *** Deleting file 'drivers/net/wireguard/main.o'
> make[4]: *** [scripts/Makefile.build:556: drivers/net/wireguard] Error 2
>
> LLVM/CLANG: 21.1.4
>
> If necessary, I can send more logs.
>
> Best regards,
>
> Piotr
>
> W dniu 27.10.2025 o 23:09, xur@google.com pisze:
> > From: Rong Xu <xur@google.com>
> >
> > This V4 is based on the reimplementation provided by Masahiro Yamada
> > on May 26. The difference is that this version now saves the compiler
> > flags (_c_flags) from the Front-End (FE) compilation and re-uses them
> > for the subsequent Back-End (BE) compilation.
> >
> > Rong Xu (2):
> >    kbuild: move vmlinux.a build rule to scripts/Makefile.vmlinux_a
> >    kbuild: distributed build support for Clang ThinLTO
> >
> >   .gitignore                 |  2 +
> >   Makefile                   | 25 +++++-------
> >   arch/Kconfig               | 19 +++++++++
> >   scripts/Makefile.lib       |  7 ++++
> >   scripts/Makefile.thinlto   | 40 ++++++++++++++++++
> >   scripts/Makefile.vmlinux_a | 83 ++++++++++++++++++++++++++++++++++++++
> >   scripts/mod/modpost.c      | 15 +++++--
> >   7 files changed, 174 insertions(+), 17 deletions(-)
> >   create mode 100644 scripts/Makefile.thinlto
> >   create mode 100644 scripts/Makefile.vmlinux_a
> >
> >
> > base-commit: dcb6fa37fd7bc9c3d2b066329b0d27dedf8becaa