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author | Masahiro Yamada <masahiroy@kernel.org> | 2022-05-27 19:01:49 +0900 |
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committer | Masahiro Yamada <masahiroy@kernel.org> | 2022-05-29 18:39:35 +0900 |
commit | c25e1c55822f9b3b53ccbf88b85644317a525752 (patch) | |
tree | 0a71732cc734866099013c08aafef8f7d4437619 /scripts/Makefile.build | |
parent | 0cfd90060d714e3d1e0d1b30cfd1c46b71f4e721 (diff) | |
download | lwn-c25e1c55822f9b3b53ccbf88b85644317a525752.tar.gz lwn-c25e1c55822f9b3b53ccbf88b85644317a525752.zip |
kbuild: do not create *.prelink.o for Clang LTO or IBT
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)
|
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>
Diffstat (limited to 'scripts/Makefile.build')
-rw-r--r-- | scripts/Makefile.build | 63 |
1 files changed, 20 insertions, 43 deletions
diff --git a/scripts/Makefile.build b/scripts/Makefile.build index f80196eef03a..cc7061258e42 100644 --- a/scripts/Makefile.build +++ b/scripts/Makefile.build @@ -88,10 +88,6 @@ endif targets-for-modules := $(foreach x, o mod $(if $(CONFIG_TRIM_UNUSED_KSYMS), usyms), \ $(patsubst %.o, %.$x, $(filter %.o, $(obj-m)))) -ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),) -targets-for-modules += $(patsubst %.o, %.prelink.o, $(filter %.o, $(obj-m))) -endif - ifdef need-modorder targets-for-modules += $(obj)/modules.order endif @@ -152,8 +148,18 @@ $(obj)/%.ll: $(src)/%.c FORCE # The C file is compiled and updated dependency information is generated. # (See cmd_cc_o_c + relevant part of rule_cc_o_c) +is-single-obj-m = $(and $(part-of-module),$(filter $@, $(obj-m)),y) + +# When a module consists of a single object, there is no reason to keep LLVM IR. +# Make $(LD) covert LLVM IR to ELF here. +ifdef CONFIG_LTO_CLANG +cmd_ld_single_m = $(if $(is-single-obj-m), ; $(LD) $(ld_flags) -r -o $(tmp-target) $@; mv $(tmp-target) $@) +endif + quiet_cmd_cc_o_c = CC $(quiet_modtag) $@ - cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $< $(cmd_objtool) + cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $< \ + $(cmd_ld_single_m) \ + $(cmd_objtool) ifdef CONFIG_MODVERSIONS # When module versioning is enabled the following steps are executed: @@ -219,7 +225,7 @@ objtool_args = \ $(if $(CONFIG_STACK_VALIDATION), --stackval) \ $(if $(CONFIG_HAVE_STATIC_CALL_INLINE), --static-call) \ --uaccess \ - $(if $($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT)), --link) \ + $(if $(delay-objtool), --link) \ $(if $(part-of-module), --module) \ $(if $(CONFIG_GCOV_KERNEL), --no-unreachable) @@ -228,21 +234,15 @@ cmd_gen_objtooldep = $(if $(objtool-enabled), { echo ; echo '$@: $$(wildcard $(o endif # CONFIG_OBJTOOL -ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),) - -# Skip objtool for LLVM bitcode -$(obj)/%.o: objtool-enabled := - -else - # 'OBJECT_FILES_NON_STANDARD := y': skip objtool checking for a directory # 'OBJECT_FILES_NON_STANDARD_foo.o := 'y': skip objtool checking for a file # 'OBJECT_FILES_NON_STANDARD_foo.o := 'n': override directory skip for a file -$(obj)/%.o: objtool-enabled = $(if $(filter-out y%, \ - $(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y) +is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y) -endif +delay-objtool := $(or $(CONFIG_LTO_CLANG),$(CONFIG_X86_KERNEL_IBT)) + +$(obj)/%.o: objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y)) ifdef CONFIG_TRIM_UNUSED_KSYMS cmd_gen_ksymdeps = \ @@ -271,24 +271,6 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE $(call if_changed_rule,cc_o_c) $(call cmd,force_checksrc) -ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),) -# Module .o files may contain LLVM bitcode, compile them into native code -# before ELF processing -quiet_cmd_cc_prelink_modules = LD [M] $@ - cmd_cc_prelink_modules = \ - $(LD) $(ld_flags) -r -o $@ \ - --whole-archive $(filter-out FORCE,$^) \ - $(cmd_objtool) - -# objtool was skipped for LLVM bitcode, run it now that we have compiled -# modules into native code -$(obj)/%.prelink.o: objtool-enabled = y -$(obj)/%.prelink.o: part-of-module := y - -$(obj)/%.prelink.o: $(obj)/%.o FORCE - $(call if_changed,cc_prelink_modules) -endif - cmd_mod = echo $(addprefix $(obj)/, $(call real-search, $*.o, .o, -objs -y -m)) | \ $(AWK) -v RS='( |\n)' '!x[$$0]++' > $@ @@ -298,7 +280,7 @@ $(obj)/%.mod: FORCE # List module undefined symbols cmd_undefined_syms = $(NM) $< | sed -n 's/^ *U //p' > $@ -$(obj)/%.usyms: $(obj)/%$(mod-prelink-ext).o FORCE +$(obj)/%.usyms: $(obj)/%.o FORCE $(call if_changed,undefined_syms) quiet_cmd_cc_lst_c = MKLST $@ @@ -420,16 +402,11 @@ $(obj)/modules.order: $(obj-m) FORCE $(obj)/lib.a: $(lib-y) FORCE $(call if_changed,ar) -ifneq ($(CONFIG_LTO_CLANG)$(CONFIG_X86_KERNEL_IBT),) -quiet_cmd_link_multi-m = AR [M] $@ -cmd_link_multi-m = \ - rm -f $@; \ - $(AR) cDPrsT $@ @$(patsubst %.o,%.mod,$@) -else quiet_cmd_link_multi-m = LD [M] $@ - cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ @$(patsubst %.o,%.mod,$@) -endif + cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ @$(patsubst %.o,%.mod,$@) $(cmd_objtool) +$(multi-obj-m): objtool-enabled := $(delay-objtool) +$(multi-obj-m): part-of-module := y $(multi-obj-m): %.o: %.mod FORCE $(call if_changed,link_multi-m) $(call multi_depend, $(multi-obj-m), .o, -objs -y -m) |