.gitignore | 2 ++ MAINTAINERS | 5 +++ Makefile | 40 ++++++++++++++++++++--- arch/Kconfig | 19 +++++++++++ scripts/Makefile.build | 52 +++++++++++++++++++++++++++--- scripts/Makefile.lib | 7 +++- scripts/Makefile.vmlinux_o | 16 +++++++--- scripts/Makefile.vmlinux_thinlink | 53 +++++++++++++++++++++++++++++++ scripts/head-object-list.txt | 1 + 9 files changed, 180 insertions(+), 15 deletions(-) create mode 100644 scripts/Makefile.vmlinux_thinlink
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 implementation changes the top-level Makefile with a
macro for generating `vmlinux.o` for distributed ThinLTO
builds. It uses the existing Kbuild infrastructure to
perform two recursive passes through the subdirectories.
The first pass generates LLVM IR object files, similar to
in-process ThinLTO. Following the thin-link stage, a second
pass compiles these IR files into the final native object
files. The build rules and actions for this two-pass process
are primarily implemented in `scripts/Makefile.build`.
Currently, this patch focuses on building the main kernel
image (`vmlinux`) only. Support for building kernel modules
using this method is planned for a subsequent patch.
Tested on the following arch: x86, arm64, loongarch, and
riscv.
Some 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>
---
Changelog since v1:
- Updated the description in arch/Kconfig based on feedback
from Nathan Chancellor
- Revised file suffixes: .final_o -> .o.thinlto.native, and
.final_a -> .a.thinlto.native
- Updated list of ignored files in .gitignore
---
.gitignore | 2 ++
MAINTAINERS | 5 +++
Makefile | 40 ++++++++++++++++++++---
arch/Kconfig | 19 +++++++++++
scripts/Makefile.build | 52 +++++++++++++++++++++++++++---
scripts/Makefile.lib | 7 +++-
scripts/Makefile.vmlinux_o | 16 +++++++---
scripts/Makefile.vmlinux_thinlink | 53 +++++++++++++++++++++++++++++++
scripts/head-object-list.txt | 1 +
9 files changed, 180 insertions(+), 15 deletions(-)
create mode 100644 scripts/Makefile.vmlinux_thinlink
diff --git a/.gitignore b/.gitignore
index f2f63e47fb886..4ac41950311c8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,6 +12,7 @@
#
.*
*.a
+*.a.thinlto_native
*.asn1.[ch]
*.bin
*.bz2
@@ -64,6 +65,7 @@ modules.order
/vmlinux
/vmlinux.32
/vmlinux.map
+/vmlinux.thinlink
/vmlinux.symvers
/vmlinux.unstripped
/vmlinux-gdb.py
diff --git a/MAINTAINERS b/MAINTAINERS
index 5f8688630c014..a492f92c4574b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -5790,6 +5790,11 @@ F: scripts/Makefile.clang
F: scripts/clang-tools/
K: \b(?i:clang|llvm)\b
+CLANG/LLVM THINLTO DISTRIBUTED BUILD
+M: Rong Xu <xur@google.com>
+S: Supported
+F: scripts/Makefile.vmlinux_thinlink
+
CLK API
M: Russell King <linux@armlinux.org.uk>
L: linux-clk@vger.kernel.org
diff --git a/Makefile b/Makefile
index b29cc321ffd9c..212e093fc7411 100644
--- a/Makefile
+++ b/Makefile
@@ -298,7 +298,8 @@ no-dot-config-targets := $(clean-targets) \
outputmakefile rustavailable rustfmt rustfmtcheck
no-sync-config-targets := $(no-dot-config-targets) %install modules_sign kernelrelease \
image_name
-single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.rsi %.s %/
+single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.rsi %.s %.o.thinlto.native \
+ %.a.thinlto.native %.o.thinlto.bc %/
config-build :=
mixed-build :=
@@ -991,10 +992,10 @@ 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 # for CONFIG_LTO_CLANG_THIN or CONFIG_LTO_CLANG_THIN_DIST
+CC_FLAGS_LTO := -flto=thin -fsplit-lto-unit
endif
CC_FLAGS_LTO += -fvisibility=hidden
@@ -1214,8 +1215,34 @@ vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
$(call if_changed,ar_vmlinux.a)
PHONY += vmlinux_o
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+vmlinux.thinlink: vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+ $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux_thinlink
+targets += vmlinux.thinlink
+
+vmlinux.a.thinlto.native := $(patsubst %.a,%.a.thinlto.native,$(KBUILD_VMLINUX_OBJS))
+quiet_cmd_ar_vmlinux.a.thinlto.native = AR $@
+ cmd_ar_vmlinux.a.thinlto.native = \
+ rm -f $@; \
+ $(AR) cDPrST $@ $(vmlinux.a.thinlto.native); \
+ $(AR) mPiT $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
+
+define rule_gen_vmlinux.a.thinlto.native
+ +$(Q)$(MAKE) $(build)=. need-builtin=1 thinlto_final_pass=1 need-modorder=1 built-in.a.thinlto.native
+ $(call cmd_and_savecmd,ar_vmlinux.a.thinlto.native)
+endef
+
+vmlinux.a.thinlto.native: vmlinux.thinlink scripts/head-object-list.txt FORCE
+ $(call if_changed_rule,gen_vmlinux.a.thinlto.native)
+
+targets += vmlinux.a.thinlto.native
+
+vmlinux_o: vmlinux.a.thinlto.native
+ $(Q)$(MAKE) thinlto_final_pass=1 -f $(srctree)/scripts/Makefile.vmlinux_o
+else
vmlinux_o: vmlinux.a $(KBUILD_VMLINUX_LIBS)
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux_o
+endif
vmlinux.o modules.builtin.modinfo modules.builtin: vmlinux_o
@:
@@ -1573,7 +1600,8 @@ CLEAN_FILES += vmlinux.symvers modules-only.symvers \
modules.builtin.ranges vmlinux.o.map vmlinux.unstripped \
compile_commands.json rust/test \
rust-project.json .vmlinux.objs .vmlinux.export.c \
- .builtin-dtbs-list .builtin-dtb.S
+ .builtin-dtbs-list .builtin-dtb.S \
+ .vmlinux_thinlto_bc_files vmlinux.thinlink
# Directories & files removed with 'make mrproper'
MRPROPER_FILES += include/config include/generated \
@@ -2024,6 +2052,8 @@ clean: $(clean-dirs)
-o -name '*.symtypes' -o -name 'modules.order' \
-o -name '*.c.[012]*.*' \
-o -name '*.ll' \
+ -o -name '*.a.thinlto.native' -o -name '*.o.thinlto.native' \
+ -o -name '*.o.thinlto.bc' \
-o -name '*.gcno' \
\) -type f -print \
-o -name '.tmp_*' -print \
diff --git a/arch/Kconfig b/arch/Kconfig
index b0adb665041f1..5ed243ded6efa 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -810,6 +810,25 @@ 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 .o.thinlto.native 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.build b/scripts/Makefile.build
index 13dcd86e74ca8..49ea3934e80bc 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -50,18 +50,23 @@ endif
# ===========================================================================
+builtin_suffix := $(if $(filter %.a.thinlto.native, $(MAKECMDGOALS)),.a.thinlto.native,.a)
+ifeq ($(thinlto_final_pass),1)
+builtin_suffix :=.a.thinlto.native
+endif
+
# subdir-builtin and subdir-modorder may contain duplications. Use $(sort ...)
-subdir-builtin := $(sort $(filter %/built-in.a, $(real-obj-y)))
+subdir-builtin := $(sort $(filter %/built-in$(builtin_suffix), $(real-obj-y)))
subdir-modorder := $(sort $(filter %/modules.order, $(obj-m)))
targets-for-builtin := $(extra-y)
ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)
-targets-for-builtin += $(obj)/lib.a
+targets-for-builtin += $(obj)/lib$(builtin_suffix)
endif
ifdef need-builtin
-targets-for-builtin += $(obj)/built-in.a
+targets-for-builtin += $(obj)/built-in$(builtin_suffix)
endif
targets-for-modules := $(foreach x, o mod, \
@@ -337,6 +342,10 @@ $(obj)/%.o: $(obj)/%.S FORCE
targets += $(filter-out $(subdir-builtin), $(real-obj-y))
targets += $(filter-out $(subdir-modorder), $(real-obj-m))
targets += $(lib-y) $(always-y)
+ifeq ($(builtin_suffix),.a.thinlto.native)
+native_targets = $(patsubst,%.o,%.o.thinlto.native,$(targets))
+targets += $(native_targets)
+endif
# Linker scripts preprocessor (.lds.S -> .lds)
# ---------------------------------------------------------------------------
@@ -347,6 +356,24 @@ quiet_cmd_cpp_lds_S = LDS $@
$(obj)/%.lds: $(src)/%.lds.S FORCE
$(call if_changed_dep,cpp_lds_S)
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+# Generate .o.thinlto.native (obj) from .o (bitcode) file
+# ---------------------------------------------------------------------------
+quiet_cmd_cc_o_bc = CC $(quiet_modtag) $@
+
+cmd_cc_o_bc = $(if $(filter bitcode, $(shell file -b $<)),$(CC) \
+ $(filter-out -Wp% $(LINUXINCLUDE) %.h.gch %.h -D% \
+ -flto=thin, $(c_flags)) \
+ -Wno-unused-command-line-argument \
+ -x ir -fthinlto-index=$<.thinlto.bc -c -o $@ \
+ $(if $(findstring ../,$<), \
+ $$(realpath --relative-to=$(srcroot) $<), $<), \
+ cp $< $@)
+
+$(obj)/%.o.thinlto.native: $(obj)/%.o FORCE
+ $(call if_changed,cc_o_bc)
+endif
+
# ASN.1 grammar
# ---------------------------------------------------------------------------
quiet_cmd_asn1_compiler = ASN.1 $(basename $@).[ch]
@@ -360,7 +387,7 @@ $(obj)/%.asn1.c $(obj)/%.asn1.h: $(src)/%.asn1 $(objtree)/scripts/asn1_compiler
# ---------------------------------------------------------------------------
# To build objects in subdirs, we need to descend into the directories
-$(subdir-builtin): $(obj)/%/built-in.a: $(obj)/% ;
+$(subdir-builtin): $(obj)/%/built-in$(builtin_suffix): $(obj)/% ;
$(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;
#
@@ -377,6 +404,12 @@ quiet_cmd_ar_builtin = AR $@
$(obj)/built-in.a: $(real-obj-y) FORCE
$(call if_changed,ar_builtin)
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+# Rule to compile a set of .o.thinlto.native files into one .a.thinlto.native file.
+$(obj)/built-in.a.thinlto.native: $(patsubst %.o,%.o.thinlto.native,$(real-obj-y)) FORCE
+ $(call if_changed,ar_builtin)
+endif
+
# This is a list of build artifacts from the current Makefile and its
# sub-directories. The timestamp should be updated when any of the member files.
@@ -394,6 +427,14 @@ $(obj)/modules.order: $(obj-m) FORCE
$(obj)/lib.a: $(lib-y) FORCE
$(call if_changed,ar)
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+quiet_cmd_ar_native = AR $@
+ cmd_ar_native = rm -f $@; $(AR) cDPrsT $@ $(patsubst %.o,%.o.thinlto.native,$(real-prereqs))
+
+$(obj)/lib.a.thinlto.native: $(patsubst %.o,%.o.thinlto.native,$(lib-y)) FORCE
+ $(call if_changed,ar_native)
+endif
+
quiet_cmd_ld_multi_m = LD [M] $@
cmd_ld_multi_m = $(LD) $(ld_flags) -r -o $@ @$< $(cmd_objtool)
@@ -459,7 +500,8 @@ $(single-subdir-goals): $(single-subdirs)
PHONY += $(subdir-ym)
$(subdir-ym):
$(Q)$(MAKE) $(build)=$@ \
- need-builtin=$(if $(filter $@/built-in.a, $(subdir-builtin)),1) \
+ need-builtin=$(if $(filter $@/built-in$(builtin_suffix), $(subdir-builtin)),1) \
+ thinlto_final_pass=$(if $(filter .a.thinlto.native, $(builtin_suffix)),1) \
need-modorder=$(if $(filter $@/modules.order, $(subdir-modorder)),1) \
$(filter $@/%, $(single-subdir-goals))
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 2fe73cda0bddb..8e146367f7d16 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -34,8 +34,13 @@ else
obj-m := $(filter-out %/, $(obj-m))
endif
+builtin_suffix := $(if $(filter %.a.thinlto.native, $(MAKECMDGOALS)),.a.thinlto.native,.a)
+ifeq ($(thinlto_final_pass),1)
+ builtin_suffix :=.a.thinlto.native
+endif
+
ifdef need-builtin
-obj-y := $(patsubst %/, %/built-in.a, $(obj-y))
+obj-y := $(patsubst %/, %/built-in$(builtin_suffix), $(obj-y))
else
obj-y := $(filter-out %/, $(obj-y))
endif
diff --git a/scripts/Makefile.vmlinux_o b/scripts/Makefile.vmlinux_o
index 938c7457717ea..0330cf8e0a8c5 100644
--- a/scripts/Makefile.vmlinux_o
+++ b/scripts/Makefile.vmlinux_o
@@ -9,6 +9,14 @@ include $(srctree)/scripts/Kbuild.include
# for objtool
include $(srctree)/scripts/Makefile.lib
+ifeq ($(thinlto_final_pass),1)
+vmlinux_a := vmlinux.a.thinlto.native
+vmlinux_libs := $(patsubst %.a,%.a.thinlto.native,$(KBUILD_VMLINUX_LIBS))
+else
+vmlinux_a := vmlinux.a
+vmlinux_libs := $(KBUILD_VMLINUX_LIBS)
+endif
+
# Generate a linker script to ensure correct ordering of initcalls for Clang LTO
# ---------------------------------------------------------------------------
@@ -18,7 +26,7 @@ quiet_cmd_gen_initcalls_lds = GEN $@
$(PERL) $(real-prereqs) > $@
.tmp_initcalls.lds: $(srctree)/scripts/generate_initcall_order.pl \
- vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+ $(vmlinux_a) $(vmlinux_libs) FORCE
$(call if_changed,gen_initcalls_lds)
targets := .tmp_initcalls.lds
@@ -59,8 +67,8 @@ quiet_cmd_ld_vmlinux.o = LD $@
$(LD) ${KBUILD_LDFLAGS} -r -o $@ \
$(vmlinux-o-ld-args-y) \
$(addprefix -T , $(initcalls-lds)) \
- --whole-archive vmlinux.a --no-whole-archive \
- --start-group $(KBUILD_VMLINUX_LIBS) --end-group \
+ --whole-archive $(vmlinux_a) --no-whole-archive \
+ --start-group $(vmlinux_libs) --end-group \
$(cmd_objtool)
define rule_ld_vmlinux.o
@@ -68,7 +76,7 @@ define rule_ld_vmlinux.o
$(call cmd,gen_objtooldep)
endef
-vmlinux.o: $(initcalls-lds) vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+vmlinux.o: $(initcalls-lds) $(vmlinux_a) $(vmlinux_libs) FORCE
$(call if_changed_rule,ld_vmlinux.o)
targets += vmlinux.o
diff --git a/scripts/Makefile.vmlinux_thinlink b/scripts/Makefile.vmlinux_thinlink
new file mode 100644
index 0000000000000..13e4026c7d45b
--- /dev/null
+++ b/scripts/Makefile.vmlinux_thinlink
@@ -0,0 +1,53 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+PHONY := __default
+__default: vmlinux.thinlink
+
+include include/config/auto.conf
+include $(srctree)/scripts/Kbuild.include
+
+
+# Generate a linker script to ensure correct ordering of initcalls for Clang LTO
+# ---------------------------------------------------------------------------
+
+quiet_cmd_gen_initcalls_lds = GEN $@
+ cmd_gen_initcalls_lds = \
+ $(PYTHON3) $(srctree)/scripts/jobserver-exec \
+ $(PERL) $(real-prereqs) > $@
+
+.tmp_initcalls_thinlink.lds: $(srctree)/scripts/generate_initcall_order.pl \
+ vmlinux.a FORCE
+ $(call if_changed,gen_initcalls_lds)
+
+targets := .tmp_initcalls_thinlink.lds
+
+initcalls-lds := .tmp_initcalls_thinlink.lds
+
+quiet_cmd_ld_vmlinux.thinlink = LD $@
+ cmd_ld_vmlinux.thinlink = \
+ $(AR) t vmlinux.a > .vmlinux_thinlto_bc_files; \
+ $(LD) ${KBUILD_LDFLAGS} -r $(addprefix -T , $(initcalls-lds)) \
+ --thinlto-index-only @.vmlinux_thinlto_bc_files; \
+ touch vmlinux.thinlink
+
+vmlinux.thinlink: vmlinux.a $(initcalls-lds) FORCE
+ $(call if_changed,ld_vmlinux.thinlink)
+
+targets += vmlinux.thinlink
+
+# 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/head-object-list.txt b/scripts/head-object-list.txt
index 7274dfc65af60..094a4e7c07559 100644
--- a/scripts/head-object-list.txt
+++ b/scripts/head-object-list.txt
@@ -18,6 +18,7 @@ arch/arm/kernel/head.o
arch/csky/kernel/head.o
arch/hexagon/kernel/head.o
arch/loongarch/kernel/head.o
+arch/loongarch/kernel/head.o.thinlto.native
arch/m68k/68000/head.o
arch/m68k/coldfire/head.o
arch/m68k/kernel/head.o
--
2.49.0.987.g0cc8ee98dc-googHi Rong Xu, On 5/8/25 04:55, xur@google.com wrote: > 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 implementation changes the top-level Makefile with a > macro for generating `vmlinux.o` for distributed ThinLTO > builds. It uses the existing Kbuild infrastructure to > perform two recursive passes through the subdirectories. > The first pass generates LLVM IR object files, similar to > in-process ThinLTO. Following the thin-link stage, a second > pass compiles these IR files into the final native object > files. The build rules and actions for this two-pass process > are primarily implemented in `scripts/Makefile.build`. > > Currently, this patch focuses on building the main kernel > image (`vmlinux`) only. Support for building kernel modules > using this method is planned for a subsequent patch. > > Tested on the following arch: x86, arm64, loongarch, and > riscv. > > Some 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> > --- > Changelog since v1: > - Updated the description in arch/Kconfig based on feedback > from Nathan Chancellor > - Revised file suffixes: .final_o -> .o.thinlto.native, and > .final_a -> .a.thinlto.native > - Updated list of ignored files in .gitignore > --- > .gitignore | 2 ++ > MAINTAINERS | 5 +++ > Makefile | 40 ++++++++++++++++++++--- > arch/Kconfig | 19 +++++++++++ > scripts/Makefile.build | 52 +++++++++++++++++++++++++++--- > scripts/Makefile.lib | 7 +++- > scripts/Makefile.vmlinux_o | 16 +++++++--- > scripts/Makefile.vmlinux_thinlink | 53 +++++++++++++++++++++++++++++++ > scripts/head-object-list.txt | 1 + > 9 files changed, 180 insertions(+), 15 deletions(-) > create mode 100644 scripts/Makefile.vmlinux_thinlink > I noticed that both Makefile.autofdo and Makefile.propeller add extra linker flags when building with ThinLTO. Did you miss updating that or is the omission there intentional? -- Regards, Eric
On Mon, May 19, 2025 at 10:22 PM Eric Naim <dnaim@cachyos.org> wrote: > > Hi Rong Xu, > > On 5/8/25 04:55, xur@google.com wrote: > > 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 implementation changes the top-level Makefile with a > > macro for generating `vmlinux.o` for distributed ThinLTO > > builds. It uses the existing Kbuild infrastructure to > > perform two recursive passes through the subdirectories. > > The first pass generates LLVM IR object files, similar to > > in-process ThinLTO. Following the thin-link stage, a second > > pass compiles these IR files into the final native object > > files. The build rules and actions for this two-pass process > > are primarily implemented in `scripts/Makefile.build`. > > > > Currently, this patch focuses on building the main kernel > > image (`vmlinux`) only. Support for building kernel modules > > using this method is planned for a subsequent patch. > > > > Tested on the following arch: x86, arm64, loongarch, and > > riscv. > > > > Some 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> > > --- > > Changelog since v1: > > - Updated the description in arch/Kconfig based on feedback > > from Nathan Chancellor > > - Revised file suffixes: .final_o -> .o.thinlto.native, and > > .final_a -> .a.thinlto.native > > - Updated list of ignored files in .gitignore > > --- > > .gitignore | 2 ++ > > MAINTAINERS | 5 +++ > > Makefile | 40 ++++++++++++++++++++--- > > arch/Kconfig | 19 +++++++++++ > > scripts/Makefile.build | 52 +++++++++++++++++++++++++++--- > > scripts/Makefile.lib | 7 +++- > > scripts/Makefile.vmlinux_o | 16 +++++++--- > > scripts/Makefile.vmlinux_thinlink | 53 +++++++++++++++++++++++++++++++ > > scripts/head-object-list.txt | 1 + > > 9 files changed, 180 insertions(+), 15 deletions(-) > > create mode 100644 scripts/Makefile.vmlinux_thinlink > > > > I noticed that both Makefile.autofdo and Makefile.propeller add extra linker flags when building with ThinLTO. Did you miss updating that or is the omission there intentional? Thanks for catching this! One good aspect of distributed build mode is that we no longer need the extra linker flags -- most of them are just to pass the options to the BE compilation. So this patch does not need these linker options. But for the Propeller build, we still need to pass one of the two profiles to the final link, and I'll be sure to incorporate that into the patch. However, I do need to make a change regarding file suffixes. The is_kernel_obj macro in the Makefile.build uses the basename command. The issue is that basename extracts everything before the last period in a filename. So, for a file named "foo.o.thinlto.native", basename returns "foo.o.thinlto", but kbuild expects it to return "foo". To fix this, I'll adjust the suffixes to ".a_thinlto_native" and ".o_thinlto_native". I'll send the patch v3 shortly. Thanks, -Rong > > -- > Regards, > Eric
On Wed, May 21, 2025 at 9:40 AM Rong Xu <xur@google.com> wrote: > > On Mon, May 19, 2025 at 10:22 PM Eric Naim <dnaim@cachyos.org> wrote: > > > > Hi Rong Xu, > > > > On 5/8/25 04:55, xur@google.com wrote: > > > 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 implementation changes the top-level Makefile with a > > > macro for generating `vmlinux.o` for distributed ThinLTO > > > builds. It uses the existing Kbuild infrastructure to > > > perform two recursive passes through the subdirectories. > > > The first pass generates LLVM IR object files, similar to > > > in-process ThinLTO. Following the thin-link stage, a second > > > pass compiles these IR files into the final native object > > > files. The build rules and actions for this two-pass process > > > are primarily implemented in `scripts/Makefile.build`. > > > > > > Currently, this patch focuses on building the main kernel > > > image (`vmlinux`) only. Support for building kernel modules > > > using this method is planned for a subsequent patch. > > > > > > Tested on the following arch: x86, arm64, loongarch, and > > > riscv. > > > > > > Some 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> > > > --- > > > Changelog since v1: > > > - Updated the description in arch/Kconfig based on feedback > > > from Nathan Chancellor > > > - Revised file suffixes: .final_o -> .o.thinlto.native, and > > > .final_a -> .a.thinlto.native > > > - Updated list of ignored files in .gitignore > > > --- > > > .gitignore | 2 ++ > > > MAINTAINERS | 5 +++ > > > Makefile | 40 ++++++++++++++++++++--- > > > arch/Kconfig | 19 +++++++++++ > > > scripts/Makefile.build | 52 +++++++++++++++++++++++++++--- > > > scripts/Makefile.lib | 7 +++- > > > scripts/Makefile.vmlinux_o | 16 +++++++--- > > > scripts/Makefile.vmlinux_thinlink | 53 +++++++++++++++++++++++++++++++ > > > scripts/head-object-list.txt | 1 + > > > 9 files changed, 180 insertions(+), 15 deletions(-) > > > create mode 100644 scripts/Makefile.vmlinux_thinlink > > > > > > > I noticed that both Makefile.autofdo and Makefile.propeller add extra linker flags when building with ThinLTO. Did you miss updating that or is the omission there intentional? > > Thanks for catching this! One good aspect of distributed build mode is > that we no longer need the extra linker flags -- most of them are just > to pass the options to the BE compilation. > So this patch does not need these linker options. But for the > Propeller build, we still need to pass one of the two profiles to the > final link, and I'll be sure to incorporate that into the patch. One clarification: We don't need to change scripts/Makefile.propeller as the link ordering profile is already passed to the final link under CLANG_PROPELLER_PROFILE_PREFIX. All the --lto-* options are only for in-process thinlto builds. > > However, I do need to make a change regarding file suffixes. The > is_kernel_obj macro in the Makefile.build uses the basename command. > The issue is that basename extracts everything before the last period > in a filename. So, for a file named "foo.o.thinlto.native", basename > returns "foo.o.thinlto", but kbuild expects it to return "foo". > > To fix this, I'll adjust the suffixes to ".a_thinlto_native" and > ".o_thinlto_native". I'll send the patch v3 shortly. > > Thanks, > > -Rong > > > > -- > > Regards, > > Eric
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