linux-kernel - Re: [PATCH 2/3] MIPS: boot/compressed: Build without LTO

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Date:   Tue, 14 Dec 2021 11:43:28 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nathan Chancellor <nathan@...nel.org>, list@...ndingux.net,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 2/3] MIPS: boot/compressed: Build without LTO

On Mon, Dec 13, 2021 at 2:49 PM Paul Cercueil <paul@...pouillou.net> wrote:
>
> We need a valid ELF object for dummy.o, so that objcopy can insert the
> vmlinuz payload.
>
> Therefore, we must build the decompresser program without LTO, otherwise
> dummy.o will be LLVM bytecode instead of a ELF object file.

While the below may work and follows the pattern observed in
drivers/firmware/efi/libstub/Makefile
43:KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO), $(KBUILD_CFLAGS))

I can't help but wonder whether we should instead be doing something like:
arch/x86/power/Makefile
9:CFLAGS_REMOVE_cpu.o := $(CC_FLAGS_LTO)
scripts/mod/Makefile
3:CFLAGS_REMOVE_empty.o += $(CC_FLAGS_LTO)
drivers/misc/lkdtm/Makefile
19:CFLAGS_REMOVE_rodata.o               += $(CC_FLAGS_LTO)

but for dummy.o?  ie.
CFLAGS_REMOVE_dummy.o += $(CC_FLAGS_LTO)

Can you please try that rather than disabling LTO for a large subset of files?

>
> Building the decompresser with LTO wouldn't make much sense anyway,
> unlike building the vmlinuz itself with LTO.

Why not? We do it for x86 and arm64 AFAICT.

>
> Signed-off-by: Paul Cercueil <paul@...pouillou.net>
> ---
>  arch/mips/boot/compressed/Makefile | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/mips/boot/compressed/Makefile b/arch/mips/boot/compressed/Makefile
> index 4c9ecfbb0ef4..2d01c50fb0b1 100644
> --- a/arch/mips/boot/compressed/Makefile
> +++ b/arch/mips/boot/compressed/Makefile
> @@ -27,6 +27,9 @@ ifdef CONFIG_CPU_LOONGSON64
>  KBUILD_CFLAGS := $(filter-out -march=loongson3a, $(KBUILD_CFLAGS)) -march=mips64r2
>  endif
>
> +# Disable LTO
> +KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO), $(KBUILD_CFLAGS))
> +
>  KBUILD_CFLAGS := $(KBUILD_CFLAGS) -mno-abicalls -D__KERNEL__ -D__DISABLE_EXPORTS \
>         -DBOOT_HEAP_SIZE=$(BOOT_HEAP_SIZE) -D"VMLINUX_LOAD_ADDRESS_ULL=$(VMLINUX_LOAD_ADDRESS)ull"
>
> --
> 2.33.0
>


-- 
Thanks,
~Nick Desaulniers