Re: [RFC][PATCH 0/5] kbuild changes, thin archives, --gc-sections

On Sun, 07 Aug 2016 22:23:02 +0200
Arnd Bergmann [off-list ref] wrote:

On Friday, August 5, 2016 10:11:58 PM CEST Nicholas Piggin wrote:

quoted

Hello,

I have 3 different things in this patchset. All arch specific, but all
involve kbuild changes, so I'd like to discuss them with kbuild
maintainers. The goal has been to improve long standing linking
difficulties with the powerpc kernel.

* First, building kernel using thin archives rather than incremental
  linking. This seems quite clean and is per-arch, so I hope it should
  not be too controversial.

* Second, building kernel using -ffunction-sections -fdata-sections,
  --gc-sections. Yes, I'm spinning the wheel again. It was motivated
  by tiny codesize regression in the first patch, but the results seem
  too good to ignore.

* Third, allowing architecture to run a tool over module after it has
  been linked. Powerpc wants to use it in order to relocate "alternate
  code" instructions that get don't get linked at their runtime
  address. No idea if this is the right approach wrt kbuild, but it
  seems to work.

I have included the powerpc code for the first two as a reference. The
third is much bigger and mostly uninteresting for this cc list, but it
can be found here:

 https://patchwork.ozlabs.org/patch/651006/

Comments appreciated.

  

I've started tested this a bit on ARM now. The first things I noticed
are:

1. /home/arnd/cross-gcc/bin/arm-linux-gnueabi-ld: warning: drivers/xen/efi.o uses 2-byte wchar_t yet the output is to use 4-byte wchar_t; use of wchar_t values across objects may fail

(actually this one has been present since the first version that
stopped using recursive linking, I did 971a69db7dc0 ("Xen: don't
warn about 2-byte wchar_t in efi") when I first saw the bug, but
that fix no longer works and we have to do this differently

2. big-endian builds on ARM stopped working, I now get

22:53:02   CC      init/do_mounts_md.o
22:53:02   LD      init/mounts.o
/home/arnd/cross-gcc/bin/arm-linux-gnueabi-ld: init/do_mounts.o: compiled for a big endian system and target is little endian
/home/arnd/cross-gcc/bin/arm-linux-gnueabi-ld: failed to merge target specific data of file init/do_mounts.o

The problem seems to be that we don't pass the correct linker
flags any more, it should be using --be8 from

arch/arm/Makefile:LDFLAGS_vmlinux       += --be8
arch/arm/Makefile:LDFLAGS_MODULE        += --be8

but that somehow is lost.

These both seem like linker flags are being lost for some reason, but I
don't really know why. Maybe ARM's Makefile. I don't have an ARM build setup
but I'll try to help track it down when I get some time.

3. drivers/firmware/efi/libstub/lib.a: error adding symbols: Archive has no index; run ranlib to add one

haven't investigated at all, turned off EFI for now.

Okay this looks like a bug in patch 2 (caused by me, not Stephen!)

For thin archives, cmd_link_l_target should be:

cmd_link_l_target = rm -f $@; $(AR) rcT$(KBUILD_ARFLAGS) $@ $(lib-y)

(note no "S" option).

Thanks,
Nick