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[PATCH v2 8/8] kbuild: remove head-y syntax

Masahiro Yamada posted 8 patches 3 years, 7 months ago
There is a newer version of this series
[PATCH v2 8/8] kbuild: remove head-y syntax
Posted by Masahiro Yamada 3 years, 7 months ago 
Kbuild puts the objects listed in head-y at the head of vmlinux.
Conventionally, we do this for head*.S, which contains the kernel entry
point.

A counter approach is to control the section order by the linker script.
Actually, the code marked as __HEAD goes into the ".head.text" section,
which is placed before the normal ".text" section.

I do not know if both of them are needed. From the build system
perspective, head-y is not mandatory. If you can achieve the proper code
placement by the linker script only, it would be cleaner.

I collected the current head-y objects into head-object-list.txt. It is
a whitelist. My hope is it will be reduced in the long run.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

(no changes since v1)

 Documentation/kbuild/makefiles.rst |  9 ++---
 Makefile                           |  4 +--
 arch/alpha/Makefile                |  2 --
 arch/arc/Makefile                  |  2 --
 arch/arm/Makefile                  |  3 --
 arch/arm64/Makefile                |  3 --
 arch/csky/Makefile                 |  2 --
 arch/hexagon/Makefile              |  2 --
 arch/ia64/Makefile                 |  1 -
 arch/loongarch/Makefile            |  2 --
 arch/m68k/Makefile                 |  9 -----
 arch/microblaze/Makefile           |  1 -
 arch/mips/Makefile                 |  2 --
 arch/nios2/Makefile                |  1 -
 arch/openrisc/Makefile             |  2 --
 arch/parisc/Makefile               |  2 --
 arch/powerpc/Makefile              | 12 -------
 arch/riscv/Makefile                |  2 --
 arch/s390/Makefile                 |  2 --
 arch/sh/Makefile                   |  2 --
 arch/sparc/Makefile                |  2 --
 arch/x86/Makefile                  |  5 ---
 arch/xtensa/Makefile               |  2 --
 scripts/head-object-list.txt       | 53 ++++++++++++++++++++++++++++++
 24 files changed, 60 insertions(+), 67 deletions(-)
 create mode 100644 scripts/head-object-list.txt

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index 07c7e5a843c1..528c54d56c09 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -1065,8 +1065,7 @@ When kbuild executes, the following steps are followed (roughly):
    - The values of the above variables are expanded in arch/$(SRCARCH)/Makefile.
 5) All object files are then linked and the resulting file vmlinux is
    located at the root of the obj tree.
-   The very first objects linked are listed in head-y, assigned by
-   arch/$(SRCARCH)/Makefile.
+   The very first objects linked are listed in scripts/head-object-list.txt.
 6) Finally, the architecture-specific part does any required post processing
    and builds the final bootimage.
    - This includes building boot records
@@ -1214,6 +1213,9 @@ When kbuild executes, the following steps are followed (roughly):
 	All object files for vmlinux. They are linked to vmlinux in the same
 	order as listed in KBUILD_VMLINUX_OBJS.
 
+	The objects listed in scripts/head-object-list.txt are exceptions;
+	they are placed before the other objects.
+
     KBUILD_VMLINUX_LIBS
 
 	All .a "lib" files for vmlinux. KBUILD_VMLINUX_OBJS and
@@ -1257,8 +1259,7 @@ When kbuild executes, the following steps are followed (roughly):
 	machinery is all architecture-independent.
 
 
-	head-y, core-y, libs-y, drivers-y
-	    $(head-y) lists objects to be linked first in vmlinux.
+	core-y, libs-y, drivers-y
 
 	    $(libs-y) lists directories where a lib.a archive can be located.
 
diff --git a/Makefile b/Makefile
index f2d06aaaefd3..e96ee00609b7 100644
--- a/Makefile
+++ b/Makefile
@@ -1155,10 +1155,10 @@ quiet_cmd_ar_vmlinux.a = AR      $@
       cmd_ar_vmlinux.a = \
 	rm -f $@; \
 	$(AR) cDPrST $@ $(KBUILD_VMLINUX_OBJS); \
-	$(AR) mPi $$($(AR) t $@ | head -n1) $@ $(head-y)
+	$(AR) mPi $$($(AR) t $@ | head -n1) $@ $$($(AR) t $@ | grep -F --file=$(srctree)/scripts/head-object-list.txt)
 
 targets += vmlinux.a
-vmlinux.a: $(KBUILD_VMLINUX_OBJS) FORCE
+vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
 	$(call if_changed,ar_vmlinux.a)
 
 targets += vmlinux.o
diff --git a/arch/alpha/Makefile b/arch/alpha/Makefile
index 881cb913e23a..45158024085e 100644
--- a/arch/alpha/Makefile
+++ b/arch/alpha/Makefile
@@ -36,8 +36,6 @@ cflags-y				+= $(cpuflags-y)
 # BWX is most important, but we don't really want any emulation ever.
 KBUILD_CFLAGS += $(cflags-y) -Wa,-mev6
 
-head-y := arch/alpha/kernel/head.o
-
 libs-y				+= arch/alpha/lib/
 
 # export what is needed by arch/alpha/boot/Makefile
diff --git a/arch/arc/Makefile b/arch/arc/Makefile
index efc54f3e35e0..329400a1c355 100644
--- a/arch/arc/Makefile
+++ b/arch/arc/Makefile
@@ -82,8 +82,6 @@ KBUILD_CFLAGS	+= $(cflags-y)
 KBUILD_AFLAGS	+= $(KBUILD_CFLAGS)
 KBUILD_LDFLAGS	+= $(ldflags-y)
 
-head-y		:= arch/arc/kernel/head.o
-
 # w/o this dtb won't embed into kernel binary
 core-y		+= arch/arc/boot/dts/
 
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 56f655deebb1..29d15c9a433e 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -134,9 +134,6 @@ KBUILD_AFLAGS	+=$(CFLAGS_ABI) $(AFLAGS_ISA) $(arch-y) $(tune-y) -include asm/uni
 
 CHECKFLAGS	+= -D__arm__
 
-#Default value
-head-y		:= arch/arm/kernel/head$(MMUEXT).o
-
 # Text offset. This list is sorted numerically by address in order to
 # provide a means to avoid/resolve conflicts in multi-arch kernels.
 # Note: the 32kB below this value is reserved for use by the kernel
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 6d9d4a58b898..6e03f15bb041 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -133,9 +133,6 @@ ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_REGS),y)
   CC_FLAGS_FTRACE := -fpatchable-function-entry=2
 endif
 
-# Default value
-head-y		:= arch/arm64/kernel/head.o
-
 ifeq ($(CONFIG_KASAN_SW_TAGS), y)
 KASAN_SHADOW_SCALE_SHIFT := 4
 else ifeq ($(CONFIG_KASAN_GENERIC), y)
diff --git a/arch/csky/Makefile b/arch/csky/Makefile
index 4e1d619fd5c6..0e4237e55758 100644
--- a/arch/csky/Makefile
+++ b/arch/csky/Makefile
@@ -59,8 +59,6 @@ LDFLAGS += -EL
 
 KBUILD_AFLAGS += $(KBUILD_CFLAGS)
 
-head-y := arch/csky/kernel/head.o
-
 core-y += arch/csky/$(CSKYABI)/
 
 libs-y += arch/csky/lib/ \
diff --git a/arch/hexagon/Makefile b/arch/hexagon/Makefile
index 44312bc147d8..92d005958dfb 100644
--- a/arch/hexagon/Makefile
+++ b/arch/hexagon/Makefile
@@ -32,5 +32,3 @@ KBUILD_LDFLAGS += $(ldflags-y)
 TIR_NAME := r19
 KBUILD_CFLAGS += -ffixed-$(TIR_NAME) -DTHREADINFO_REG=$(TIR_NAME) -D__linux__
 KBUILD_AFLAGS += -DTHREADINFO_REG=$(TIR_NAME)
-
-head-y := arch/hexagon/kernel/head.o
diff --git a/arch/ia64/Makefile b/arch/ia64/Makefile
index e55c2f138656..56c4bb276b6e 100644
--- a/arch/ia64/Makefile
+++ b/arch/ia64/Makefile
@@ -44,7 +44,6 @@ quiet_cmd_objcopy = OBJCOPY $@
 cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $(OBJCOPYFLAGS_$(@F)) $< $@
 
 KBUILD_CFLAGS += $(cflags-y)
-head-y := arch/ia64/kernel/head.o
 
 libs-y				+= arch/ia64/lib/
 
diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index ec3de6191276..131fc210c2bf 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -72,8 +72,6 @@ CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
 	sed -e "s/^\#define /-D'/" -e "s/ /'='/" -e "s/$$/'/" -e 's/\$$/&&/g')
 endif
 
-head-y := arch/loongarch/kernel/head.o
-
 libs-y += arch/loongarch/lib/
 
 ifeq ($(KBUILD_EXTMOD),)
diff --git a/arch/m68k/Makefile b/arch/m68k/Makefile
index e358605b70ba..43e39040d3ac 100644
--- a/arch/m68k/Makefile
+++ b/arch/m68k/Makefile
@@ -86,15 +86,6 @@ ifdef CONFIG_KGDB
 KBUILD_CFLAGS := $(subst -fomit-frame-pointer,,$(KBUILD_CFLAGS)) -g
 endif
 
-#
-# Select the assembler head startup code. Order is important. The default
-# head code is first, processor specific selections can override it after.
-#
-head-y				:= arch/m68k/kernel/head.o
-head-$(CONFIG_SUN3)		:= arch/m68k/kernel/sun3-head.o
-head-$(CONFIG_M68000)		:= arch/m68k/68000/head.o
-head-$(CONFIG_COLDFIRE)		:= arch/m68k/coldfire/head.o
-
 libs-y				+= arch/m68k/lib/
 
 
diff --git a/arch/microblaze/Makefile b/arch/microblaze/Makefile
index 1826d9ce4459..3f8a86c4336a 100644
--- a/arch/microblaze/Makefile
+++ b/arch/microblaze/Makefile
@@ -48,7 +48,6 @@ CPUFLAGS-1 += $(call cc-option,-mcpu=v$(CPU_VER))
 # r31 holds current when in kernel mode
 KBUILD_CFLAGS += -ffixed-r31 $(CPUFLAGS-y) $(CPUFLAGS-1) $(CPUFLAGS-2)
 
-head-y := arch/microblaze/kernel/head.o
 libs-y += arch/microblaze/lib/
 
 boot := arch/microblaze/boot
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index 4d2a3e73fc45..b296e33f8e33 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -324,8 +324,6 @@ endif
 
 OBJCOPYFLAGS		+= --remove-section=.reginfo
 
-head-y := arch/mips/kernel/head.o
-
 libs-y			+= arch/mips/lib/
 libs-$(CONFIG_MIPS_FP_SUPPORT) += arch/mips/math-emu/
 
diff --git a/arch/nios2/Makefile b/arch/nios2/Makefile
index 3f34e6831863..f1ff4ce0f1a2 100644
--- a/arch/nios2/Makefile
+++ b/arch/nios2/Makefile
@@ -37,7 +37,6 @@ KBUILD_CFLAGS += -DUTS_SYSNAME=\"$(UTS_SYSNAME)\"
 KBUILD_CFLAGS += -fno-builtin
 KBUILD_CFLAGS += -G 0
 
-head-y		:= arch/nios2/kernel/head.o
 libs-y		+= arch/nios2/lib/ $(LIBGCC)
 
 INSTALL_PATH ?= /tftpboot
diff --git a/arch/openrisc/Makefile b/arch/openrisc/Makefile
index b446510173cd..68249521db5a 100644
--- a/arch/openrisc/Makefile
+++ b/arch/openrisc/Makefile
@@ -55,8 +55,6 @@ ifeq ($(CONFIG_OPENRISC_HAVE_INST_SEXT),y)
 	KBUILD_CFLAGS += $(call cc-option,-msext)
 endif
 
-head-y 		:= arch/openrisc/kernel/head.o
-
 libs-y		+= $(LIBGCC)
 
 PHONY += vmlinux.bin
diff --git a/arch/parisc/Makefile b/arch/parisc/Makefile
index e38d993d87f2..a2d8600521f9 100644
--- a/arch/parisc/Makefile
+++ b/arch/parisc/Makefile
@@ -113,8 +113,6 @@ cflags-$(CONFIG_PA7100LC)	+= -march=1.1 -mschedule=7100LC
 cflags-$(CONFIG_PA7300LC)	+= -march=1.1 -mschedule=7300
 cflags-$(CONFIG_PA8X00)		+= -march=2.0 -mschedule=8000
 
-head-y			:= arch/parisc/kernel/head.o 
-
 KBUILD_CFLAGS	+= $(cflags-y)
 LIBGCC		:= $(shell $(CC) -print-libgcc-file-name)
 export LIBGCC
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 02742facf895..89c27827a11f 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -226,18 +226,6 @@ KBUILD_CFLAGS += $(cpu-as-y)
 KBUILD_AFLAGS += $(aflags-y)
 KBUILD_CFLAGS += $(cflags-y)
 
-head-$(CONFIG_PPC64)		:= arch/powerpc/kernel/head_64.o
-head-$(CONFIG_PPC_BOOK3S_32)	:= arch/powerpc/kernel/head_book3s_32.o
-head-$(CONFIG_PPC_8xx)		:= arch/powerpc/kernel/head_8xx.o
-head-$(CONFIG_40x)		:= arch/powerpc/kernel/head_40x.o
-head-$(CONFIG_44x)		:= arch/powerpc/kernel/head_44x.o
-head-$(CONFIG_FSL_BOOKE)	:= arch/powerpc/kernel/head_fsl_booke.o
-
-head-$(CONFIG_PPC64)		+= arch/powerpc/kernel/entry_64.o
-head-$(CONFIG_PPC_FPU)		+= arch/powerpc/kernel/fpu.o
-head-$(CONFIG_ALTIVEC)		+= arch/powerpc/kernel/vector.o
-head-$(CONFIG_PPC_OF_BOOT_TRAMPOLINE)  += arch/powerpc/kernel/prom_init.o
-
 # Default to zImage, override when needed
 all: zImage
 
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 3fa8ef336822..e013df8e7b8b 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -110,8 +110,6 @@ else
 KBUILD_IMAGE	:= $(boot)/Image.gz
 endif
 
-head-y := arch/riscv/kernel/head.o
-
 libs-y += arch/riscv/lib/
 libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
 
diff --git a/arch/s390/Makefile b/arch/s390/Makefile
index 4cb5d17e7ead..de6d8b2ea4d8 100644
--- a/arch/s390/Makefile
+++ b/arch/s390/Makefile
@@ -119,8 +119,6 @@ export KBUILD_CFLAGS_DECOMPRESSOR
 
 OBJCOPYFLAGS	:= -O binary
 
-head-y		:= arch/s390/kernel/head64.o
-
 libs-y		+= arch/s390/lib/
 drivers-y	+= drivers/s390/
 
diff --git a/arch/sh/Makefile b/arch/sh/Makefile
index b39412bf91fb..5c8776482530 100644
--- a/arch/sh/Makefile
+++ b/arch/sh/Makefile
@@ -114,8 +114,6 @@ endif
 
 export ld-bfd
 
-head-y	:= arch/sh/kernel/head_32.o
-
 # Mach groups
 machdir-$(CONFIG_SOLUTION_ENGINE)		+= mach-se
 machdir-$(CONFIG_SH_HP6XX)			+= mach-hp6xx
diff --git a/arch/sparc/Makefile b/arch/sparc/Makefile
index fe58a410b4ce..a4ea5b05f288 100644
--- a/arch/sparc/Makefile
+++ b/arch/sparc/Makefile
@@ -56,8 +56,6 @@ endif
 
 endif
 
-head-y                 := arch/sparc/kernel/head_$(BITS).o
-
 libs-y                 += arch/sparc/prom/
 libs-y                 += arch/sparc/lib/
 
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index bafbd905e6e7..9afd323c6916 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -234,11 +234,6 @@ archheaders:
 ###
 # Kernel objects
 
-head-y := arch/x86/kernel/head_$(BITS).o
-head-y += arch/x86/kernel/head$(BITS).o
-head-y += arch/x86/kernel/ebda.o
-head-y += arch/x86/kernel/platform-quirks.o
-
 libs-y  += arch/x86/lib/
 
 # drivers-y are linked after core-y
diff --git a/arch/xtensa/Makefile b/arch/xtensa/Makefile
index 5097caa7bf0c..bfd8e433ed62 100644
--- a/arch/xtensa/Makefile
+++ b/arch/xtensa/Makefile
@@ -55,8 +55,6 @@ KBUILD_CPPFLAGS += $(patsubst %,-I$(srctree)/%include,$(vardirs) $(plfdirs))
 
 KBUILD_DEFCONFIG := iss_defconfig
 
-head-y		:= arch/xtensa/kernel/head.o
-
 libs-y		+= arch/xtensa/lib/
 
 boot		:= arch/xtensa/boot
diff --git a/scripts/head-object-list.txt b/scripts/head-object-list.txt
new file mode 100644
index 000000000000..dd2ba2eda636
--- /dev/null
+++ b/scripts/head-object-list.txt
@@ -0,0 +1,53 @@
+# Head objects
+#
+# The objects listed here are placed at the head of vmlinux. A typical use-case
+# is an object that contains the entry point. This is kept for compatibility
+# with head-y, which Kbuild used to support.
+#
+# A counter approach is to control the section placement by the linker script.
+# The code marked as __HEAD goes into the ".head.text" section, which is placed
+# before the normal ".text" section.
+#
+# If you can achieve the correct code ordering by linker script, please delete
+# the entry from this file.
+#
+arch/alpha/kernel/head.o
+arch/arc/kernel/head.o
+arch/arm/kernel/head-nommu.o
+arch/arm/kernel/head.o
+arch/arm64/kernel/head.o
+arch/csky/kernel/head.o
+arch/hexagon/kernel/head.o
+arch/ia64/kernel/head.o
+arch/loongarch/kernel/head.o
+arch/m68k/68000/head.o
+arch/m68k/coldfire/head.o
+arch/m68k/kernel/head.o
+arch/m68k/kernel/sun3-head.o
+arch/microblaze/kernel/head.o
+arch/mips/kernel/head.o
+arch/nios2/kernel/head.o
+arch/openrisc/kernel/head.o
+arch/parisc/kernel/head.o
+arch/powerpc/kernel/head_40x.o
+arch/powerpc/kernel/head_44x.o
+arch/powerpc/kernel/head_64.o
+arch/powerpc/kernel/head_8xx.o
+arch/powerpc/kernel/head_book3s_32.o
+arch/powerpc/kernel/head_fsl_booke.o
+arch/powerpc/kernel/entry_64.o
+arch/powerpc/kernel/fpu.o
+arch/powerpc/kernel/vector.o
+arch/powerpc/kernel/prom_init.o
+arch/riscv/kernel/head.o
+arch/s390/kernel/head64.o
+arch/sh/kernel/head_32.o
+arch/sparc/kernel/head_32.o
+arch/sparc/kernel/head_64.o
+arch/x86/kernel/head_32.o
+arch/x86/kernel/head_64.o
+arch/x86/kernel/head32.o
+arch/x86/kernel/head64.o
+arch/x86/kernel/ebda.o
+arch/x86/kernel/platform-quirks.o
+arch/xtensa/kernel/head.o
-- 
2.34.1
Re: [PATCH v2 8/8] kbuild: remove head-y syntax
Posted by Arnd Bergmann 3 years, 7 months ago 
On Tue, Sep 6, 2022, at 8:13 AM, Masahiro Yamada wrote:
> Kbuild puts the objects listed in head-y at the head of vmlinux.
> Conventionally, we do this for head*.S, which contains the kernel entry
> point.
>
> A counter approach is to control the section order by the linker script.
> Actually, the code marked as __HEAD goes into the ".head.text" section,
> which is placed before the normal ".text" section.
>
> I do not know if both of them are needed. From the build system
> perspective, head-y is not mandatory. If you can achieve the proper code
> placement by the linker script only, it would be cleaner.
>
> I collected the current head-y objects into head-object-list.txt. It is
> a whitelist. My hope is it will be reduced in the long run.
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---

The scripts/head-object-list.txt approach feels a little awkward,
so overall I'm not convinced that this is an improvement as long
as there is no final decision for what should be done instead.

If the .head.text section approach works, maybe convert at
a minimum the x86 and arm64 architectures to provide an example
of what it should look like in the end, otherwise I doubt that
any architecture maintainers are going to work on removing their
architectures from the head-object-list.txt file.

> +arch/alpha/kernel/head.o
> +arch/arc/kernel/head.o
> +arch/arm/kernel/head-nommu.o
> +arch/arm/kernel/head.o
> +arch/arm64/kernel/head.o
> +arch/csky/kernel/head.o
> +arch/hexagon/kernel/head.o
> +arch/ia64/kernel/head.o
> +arch/loongarch/kernel/head.o
> +arch/m68k/68000/head.o
> +arch/m68k/coldfire/head.o
> +arch/m68k/kernel/head.o
> +arch/m68k/kernel/sun3-head.o
> +arch/microblaze/kernel/head.o
> +arch/mips/kernel/head.o
> +arch/nios2/kernel/head.o
> +arch/openrisc/kernel/head.o
> +arch/parisc/kernel/head.o
> +arch/powerpc/kernel/head_40x.o
> +arch/powerpc/kernel/head_44x.o
> +arch/powerpc/kernel/head_64.o
> +arch/powerpc/kernel/head_8xx.o
> +arch/powerpc/kernel/head_book3s_32.o
> +arch/powerpc/kernel/head_fsl_booke.o
> +arch/powerpc/kernel/entry_64.o
> +arch/powerpc/kernel/fpu.o
> +arch/powerpc/kernel/vector.o
> +arch/powerpc/kernel/prom_init.o
> +arch/riscv/kernel/head.o
> +arch/s390/kernel/head64.o
> +arch/sh/kernel/head_32.o
> +arch/sparc/kernel/head_32.o
> +arch/sparc/kernel/head_64.o
> +arch/x86/kernel/head_32.o
> +arch/x86/kernel/head_64.o
> +arch/x86/kernel/head32.o
> +arch/x86/kernel/head64.o
> +arch/x86/kernel/ebda.o
> +arch/x86/kernel/platform-quirks.o
> +arch/xtensa/kernel/head.o

Seeing that almost all of these have the same naming
convention, another alternative would be to have a
special case exclusively for arch/*/kernel/head.S and
make that either an assembly file that includes all
the other files from your current list, or use
an intermediate object to link head-*.o into head.o
before putting that first.

     Arnd
Re: [PATCH v2 8/8] kbuild: remove head-y syntax
Posted by Masahiro Yamada 3 years, 7 months ago 
On Tue, Sep 6, 2022 at 5:00 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Tue, Sep 6, 2022, at 8:13 AM, Masahiro Yamada wrote:
> > Kbuild puts the objects listed in head-y at the head of vmlinux.
> > Conventionally, we do this for head*.S, which contains the kernel entry
> > point.
> >
> > A counter approach is to control the section order by the linker script.
> > Actually, the code marked as __HEAD goes into the ".head.text" section,
> > which is placed before the normal ".text" section.
> >
> > I do not know if both of them are needed. From the build system
> > perspective, head-y is not mandatory. If you can achieve the proper code
> > placement by the linker script only, it would be cleaner.
> >
> > I collected the current head-y objects into head-object-list.txt. It is
> > a whitelist. My hope is it will be reduced in the long run.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> > ---
>
> The scripts/head-object-list.txt approach feels a little awkward,
> so overall I'm not convinced that this is an improvement as long
> as there is no final decision for what should be done instead.
>
> If the .head.text section approach works, maybe convert at
> a minimum the x86 and arm64 architectures to provide an example
> of what it should look like in the end, otherwise I doubt that
> any architecture maintainers are going to work on removing their
> architectures from the head-object-list.txt file.



I wish I had done this before fa96b57c149061f71a70bd6582d995f6424fbbf4.
Maybe I would have been able to avoid loongarch adding a head-y.

When people port a new arch, they mimic other arch implementations,
and apparently they assume having head-y is mandatory, and the right
thing to do.

So, we need something to make arch maintainers realize
"when you add a new head-y entry, very likely you are doing wrong".

So, collecting the current ones into a whitelist can prevent the list
from growing at least, even if nobody strives to reduce it.


I did a similar approach for

  scripts/headers_install.sh
  usr/include/Makefile

It was successful because we did not get a new breakage.




Do you have a better idea to avoid bad code slipping in?




See the next arch port.


If somebody upsteams arch/kvx/, they will be very likely to add
arch/kvx/kernel/head.o to head-y.

They are already doing it.
https://github.com/kalray/linux_coolidge/blob/coolidge/arch/kvx/Makefile




People never re-think "why do we need head-y in the first place?"







>
> > +arch/alpha/kernel/head.o
> > +arch/arc/kernel/head.o
> > +arch/arm/kernel/head-nommu.o
> > +arch/arm/kernel/head.o
> > +arch/arm64/kernel/head.o
> > +arch/csky/kernel/head.o
> > +arch/hexagon/kernel/head.o
> > +arch/ia64/kernel/head.o
> > +arch/loongarch/kernel/head.o
> > +arch/m68k/68000/head.o
> > +arch/m68k/coldfire/head.o
> > +arch/m68k/kernel/head.o
> > +arch/m68k/kernel/sun3-head.o
> > +arch/microblaze/kernel/head.o
> > +arch/mips/kernel/head.o
> > +arch/nios2/kernel/head.o
> > +arch/openrisc/kernel/head.o
> > +arch/parisc/kernel/head.o
> > +arch/powerpc/kernel/head_40x.o
> > +arch/powerpc/kernel/head_44x.o
> > +arch/powerpc/kernel/head_64.o
> > +arch/powerpc/kernel/head_8xx.o
> > +arch/powerpc/kernel/head_book3s_32.o
> > +arch/powerpc/kernel/head_fsl_booke.o
> > +arch/powerpc/kernel/entry_64.o
> > +arch/powerpc/kernel/fpu.o
> > +arch/powerpc/kernel/vector.o
> > +arch/powerpc/kernel/prom_init.o
> > +arch/riscv/kernel/head.o
> > +arch/s390/kernel/head64.o
> > +arch/sh/kernel/head_32.o
> > +arch/sparc/kernel/head_32.o
> > +arch/sparc/kernel/head_64.o
> > +arch/x86/kernel/head_32.o
> > +arch/x86/kernel/head_64.o
> > +arch/x86/kernel/head32.o
> > +arch/x86/kernel/head64.o
> > +arch/x86/kernel/ebda.o
> > +arch/x86/kernel/platform-quirks.o
> > +arch/xtensa/kernel/head.o
>
> Seeing that almost all of these have the same naming
> convention, another alternative would be to have a
> special case exclusively for arch/*/kernel/head.S and
> make that either an assembly file that includes all
> the other files from your current list, or use
> an intermediate object to link head-*.o into head.o
> before putting that first.
>
>      Arnd


I prefer a simple list of objects, so that people
can work on it one by one.



As Ard pointed out, we can remove
arch/arm64/kernel/head.o
but it requires deep arch-specific knowledge.



-- 
Best Regards
Masahiro Yamada
Re: [PATCH v2 8/8] kbuild: remove head-y syntax
Posted by Ard Biesheuvel 3 years, 7 months ago 
On Tue, 6 Sept 2022 at 10:06, Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Tue, Sep 6, 2022, at 8:13 AM, Masahiro Yamada wrote:
> > Kbuild puts the objects listed in head-y at the head of vmlinux.
> > Conventionally, we do this for head*.S, which contains the kernel entry
> > point.
> >
> > A counter approach is to control the section order by the linker script.
> > Actually, the code marked as __HEAD goes into the ".head.text" section,
> > which is placed before the normal ".text" section.
> >
> > I do not know if both of them are needed. From the build system
> > perspective, head-y is not mandatory. If you can achieve the proper code
> > placement by the linker script only, it would be cleaner.
> >
> > I collected the current head-y objects into head-object-list.txt. It is
> > a whitelist. My hope is it will be reduced in the long run.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> > ---
>
> The scripts/head-object-list.txt approach feels a little awkward,
> so overall I'm not convinced that this is an improvement as long
> as there is no final decision for what should be done instead.
>

Agree with Arnd here - having an exhaustive list that spans all arch/*
subdirectories goes against the divide-and-conquer nature of arch/,
where each architecture manages such things themselves

> If the .head.text section approach works, maybe convert at
> a minimum the x86 and arm64 architectures to provide an example
> of what it should look like in the end, otherwise I doubt that
> any architecture maintainers are going to work on removing their
> architectures from the head-object-list.txt file.
>

For the arm64 case, I seriously doubt whether head.o needs any special
treatment - the layout is section based, and the only piece that must
appear right at the start of the binary image is the image header
(which is emitted into .head.text IIRC), everything else appearing in
head.S is placed in .text, .idmap.text or .init.text, and does not
require any special treatment.

> > +arch/alpha/kernel/head.o
> > +arch/arc/kernel/head.o
> > +arch/arm/kernel/head-nommu.o
> > +arch/arm/kernel/head.o
> > +arch/arm64/kernel/head.o
> > +arch/csky/kernel/head.o
> > +arch/hexagon/kernel/head.o
> > +arch/ia64/kernel/head.o
> > +arch/loongarch/kernel/head.o
> > +arch/m68k/68000/head.o
> > +arch/m68k/coldfire/head.o
> > +arch/m68k/kernel/head.o
> > +arch/m68k/kernel/sun3-head.o
> > +arch/microblaze/kernel/head.o
> > +arch/mips/kernel/head.o
> > +arch/nios2/kernel/head.o
> > +arch/openrisc/kernel/head.o
> > +arch/parisc/kernel/head.o
> > +arch/powerpc/kernel/head_40x.o
> > +arch/powerpc/kernel/head_44x.o
> > +arch/powerpc/kernel/head_64.o
> > +arch/powerpc/kernel/head_8xx.o
> > +arch/powerpc/kernel/head_book3s_32.o
> > +arch/powerpc/kernel/head_fsl_booke.o
> > +arch/powerpc/kernel/entry_64.o
> > +arch/powerpc/kernel/fpu.o
> > +arch/powerpc/kernel/vector.o
> > +arch/powerpc/kernel/prom_init.o
> > +arch/riscv/kernel/head.o
> > +arch/s390/kernel/head64.o
> > +arch/sh/kernel/head_32.o
> > +arch/sparc/kernel/head_32.o
> > +arch/sparc/kernel/head_64.o
> > +arch/x86/kernel/head_32.o
> > +arch/x86/kernel/head_64.o
> > +arch/x86/kernel/head32.o
> > +arch/x86/kernel/head64.o
> > +arch/x86/kernel/ebda.o
> > +arch/x86/kernel/platform-quirks.o
> > +arch/xtensa/kernel/head.o
>
> Seeing that almost all of these have the same naming
> convention, another alternative would be to have a
> special case exclusively for arch/*/kernel/head.S and
> make that either an assembly file that includes all
> the other files from your current list, or use
> an intermediate object to link head-*.o into head.o
> before putting that first.
>
>      Arnd

Patchew 2.1-devel-ea86937

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