Subject: Re: [RFC PATCH v4 11/17] kbuild: Add generic hook for architectures to use before the final vmlinux link

From: "Nicholas Piggin" <[email-masked]>
Date: Mon, 15 Jul 2024 17:29:53 +1000

On Sun Jul 14, 2024 at 6:27 PM AEST, Naveen N Rao wrote:
> On powerpc, we would like to be able to make a pass on vmlinux.o and
> generate a new object file to be linked into vmlinux. Add a generic pass
> in Makefile.vmlinux that architectures can use for this purpose.
>
> Architectures need to select CONFIG_ARCH_WANTS_PRE_LINK_VMLINUX and must
> provide arch/<arch>/tools/Makefile with .arch.vmlinux.o target, which
> will be invoked prior to the final vmlinux link step.

Maybe POSTLINK should move to more like this with explicit config
option too rather than just picking up Makefile.postlink...

>
> Signed-off-by: Naveen N Rao <[email-masked]>
> ---
>  arch/Kconfig             |  6 ++++++
>  scripts/Makefile.vmlinux |  8 ++++++++
>  scripts/link-vmlinux.sh  | 11 ++++++++---
>  3 files changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 975dd22a2dbd..ef868ff8156a 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1643,4 +1643,10 @@ config CC_HAS_SANE_FUNCTION_ALIGNMENT
>  config ARCH_NEED_CMPXCHG_1_EMU
>  	bool
>  
> +config ARCH_WANTS_PRE_LINK_VMLINUX
> +	def_bool n
> +	help
> +	  An architecture can select this if it provides arch/<arch>/tools/Makefile
> +	  with .arch.vmlinux.o target to be linked into vmlinux.

Someone bikeshedded me before for putting comments for putting comment
for non-user-selectable option in 'help'. Even though heaps of options
are like that here, apparently they preferred # comment above the option
for developer comments. I personally thought this looks nicer but do
Kconfig maintainers prefer #?

> +
>  endmenu
> diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
> index 49946cb96844..6410e0be7f52 100644
> --- a/scripts/Makefile.vmlinux
> +++ b/scripts/Makefile.vmlinux
> @@ -22,6 +22,14 @@ targets += .vmlinux.export.o
>  vmlinux: .vmlinux.export.o
>  endif
>  
> +ifdef CONFIG_ARCH_WANTS_PRE_LINK_VMLINUX
> +targets += .arch.vmlinux.o
> +.arch.vmlinux.o: vmlinux.o FORCE
> +	$(Q)$(MAKE) $(build)=arch/$(SRCARCH)/tools .arch.vmlinux.o
> +
> +vmlinux: .arch.vmlinux.o
> +endif

Does .vmlinux.arch.o follow convention better? I guess the btf does
not. So, nevermind.

Could this just be done entirely in link-vmlinux.sh like kallsyms and
btf?

> +
>  ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
>  
>  # Final link of vmlinux with optional arch pass after final link
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 518c70b8db50..aafaed1412ea 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -122,7 +122,7 @@ gen_btf()
>  		return 1
>  	fi
>  
> -	vmlinux_link ${1}
> +	vmlinux_link ${1} ${arch_vmlinux_o}
>  
>  	info "BTF" ${2}
>  	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}

BTF generation needs  the prelink .o?

> @@ -178,7 +178,7 @@ kallsyms_step()
>  	kallsymso=${kallsyms_vmlinux}.o
>  	kallsyms_S=${kallsyms_vmlinux}.S
>  
> -	vmlinux_link ${kallsyms_vmlinux} "${kallsymso_prev}" ${btf_vmlinux_bin_o}
> +	vmlinux_link ${kallsyms_vmlinux} "${kallsymso_prev}" ${btf_vmlinux_bin_o} ${arch_vmlinux_o}
>  	mksysmap ${kallsyms_vmlinux} ${kallsyms_vmlinux}.syms
>  	kallsyms ${kallsyms_vmlinux}.syms ${kallsyms_S}
>  

> @@ -223,6 +223,11 @@ fi
>  
>  ${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init init/version-timestamp.o
>  
> +arch_vmlinux_o=""
> +if is_enabled CONFIG_ARCH_WANTS_PRE_LINK_VMLINUX; then
> +	arch_vmlinux_o=.arch.vmlinux.o
> +fi
> +
>  btf_vmlinux_bin_o=""
>  if is_enabled CONFIG_DEBUG_INFO_BTF; then
>  	btf_vmlinux_bin_o=.btf.vmlinux.bin.o
> @@ -273,7 +278,7 @@ if is_enabled CONFIG_KALLSYMS; then
>  	fi
>  fi
>  
> -vmlinux_link vmlinux "${kallsymso}" ${btf_vmlinux_bin_o}
> +vmlinux_link vmlinux "${kallsymso}" ${btf_vmlinux_bin_o} ${arch_vmlinux_o}
>  
>  # fill in BTF IDs
>  if is_enabled CONFIG_DEBUG_INFO_BTF && is_enabled CONFIG_BPF; then

I guess it looks okay, similar to btf although I'm not a kbuild expert.

Thanks,
Nick