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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
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c25e1c5582
When CONFIG_LTO_CLANG=y, additional intermediate *.prelink.o is created
for each module. Also, objtool is postponed until LLVM IR is converted
to ELF.
CONFIG_X86_KERNEL_IBT works in a similar way to postpone objtool until
objects are merged together.
This commit stops generating *.prelink.o, so the build flow will look
similar with/without LTO.
The following figures show how the LTO build currently works, and
how this commit is changing it.
Current build flow
==================
[1] single-object module
$(LD)
$(CC) +objtool $(LD)
foo.c --------------------> foo.o -----> foo.prelink.o -----> foo.ko
(LLVM IR) (ELF) | (ELF)
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foo.mod.o --/
(LLVM IR)
[2] multi-object module
$(LD)
$(CC) $(AR) +objtool $(LD)
foo1.c -----> foo1.o -----> foo.o -----> foo.prelink.o -----> foo.ko
| (archive) (ELF) | (ELF)
foo2.c -----> foo2.o --/ |
(LLVM IR) foo.mod.o --/
(LLVM IR)
One confusion is that foo.o in multi-object module is an archive
despite of its suffix.
New build flow
==============
[1] single-object module
Since there is only one object, there is no need to keep the LLVM IR.
Use $(CC)+$(LD) to generate an ELF object in one build rule. When LTO
is disabled, $(LD) is unneeded because $(CC) produces an ELF object.
$(CC)+$(LD)+objtool $(LD)
foo.c ----------------------------> foo.o ---------> foo.ko
(ELF) | (ELF)
|
foo.mod.o --/
(LLVM IR)
[2] multi-object module
Previously, $(AR) was used to combine LLVM IR files into an archive,
but there was no technical reason to do so. Use $(LD) to merge them
into a single ELF object.
$(LD)
$(CC) +objtool $(LD)
foo1.c ---------> foo1.o ---------> foo.o ---------> foo.ko
| (ELF) | (ELF)
foo2.c ---------> foo2.o ----/ |
(LLVM IR) foo.mod.o --/
(LLVM IR)
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
81 lines
2.7 KiB
Makefile
81 lines
2.7 KiB
Makefile
# SPDX-License-Identifier: GPL-2.0-only
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# ===========================================================================
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# Module final link
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# ===========================================================================
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PHONY := __modfinal
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__modfinal:
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include include/config/auto.conf
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include $(srctree)/scripts/Kbuild.include
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# for c_flags
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include $(srctree)/scripts/Makefile.lib
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# find all modules listed in modules.order
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modules := $(sort $(shell cat $(MODORDER)))
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__modfinal: $(modules)
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@:
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# modname and part-of-module are set to make c_flags define proper module flags
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modname = $(notdir $(@:.mod.o=))
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part-of-module = y
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quiet_cmd_cc_o_c = CC [M] $@
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cmd_cc_o_c = $(CC) $(filter-out $(CC_FLAGS_CFI), $(c_flags)) -c -o $@ $<
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%.mod.o: %.mod.c FORCE
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$(call if_changed_dep,cc_o_c)
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ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
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quiet_cmd_ld_ko_o = LD [M] $@
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cmd_ld_ko_o += \
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$(LD) -r $(KBUILD_LDFLAGS) \
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$(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) \
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-T scripts/module.lds -o $@ $(filter %.o, $^); \
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$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
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quiet_cmd_btf_ko = BTF [M] $@
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cmd_btf_ko = \
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if [ -f vmlinux ]; then \
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LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) --btf_base vmlinux $@; \
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$(RESOLVE_BTFIDS) -b vmlinux $@; \
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else \
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printf "Skipping BTF generation for %s due to unavailability of vmlinux\n" $@ 1>&2; \
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fi;
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# Same as newer-prereqs, but allows to exclude specified extra dependencies
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newer_prereqs_except = $(filter-out $(PHONY) $(1),$?)
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# Same as if_changed, but allows to exclude specified extra dependencies
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if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check), \
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$(cmd); \
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printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)
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# Re-generate module BTFs if either module's .ko or vmlinux changed
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$(modules): %.ko: %.o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
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+$(call if_changed_except,ld_ko_o,vmlinux)
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ifdef CONFIG_DEBUG_INFO_BTF_MODULES
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+$(if $(newer-prereqs),$(call cmd,btf_ko))
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endif
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targets += $(modules) $(modules:.ko=.mod.o)
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# Add FORCE to the prequisites of a target to force it to be always rebuilt.
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# ---------------------------------------------------------------------------
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PHONY += FORCE
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FORCE:
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# Read all saved command lines and dependencies for the $(targets) we
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# may be building above, using $(if_changed{,_dep}). As an
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# optimization, we don't need to read them if the target does not
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# exist, we will rebuild anyway in that case.
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existing-targets := $(wildcard $(sort $(targets)))
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-include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
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.PHONY: $(PHONY)
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