Re: [PATCH] kbuild: Disable extra debugging info in .s output

[Borislav Petkov]

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On Mon, Jan 21, 2019 at 10:53:38AM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@xxxxxxx>
> 
> Modern gcc adds view assignments, reset assertion checking in .loc
> directives and a couple more additional debug markers, which clutters
> the asm output unnecessarily:
> 
> For example:
> 
>   bsp_resume:
>   .LFB3466:
>           .loc 1 1868 1 is_stmt 1 view -0
>           .cfi_startproc
>           .loc 1 1869 2 view .LVU73
>   # arch/x86/kernel/cpu/common.c:1869:    if (this_cpu->c_bsp_resume)
>           .loc 1 1869 14 is_stmt 0 view .LVU74
>           movq    this_cpu(%rip), %rax    # this_cpu, this_cpu
>           movq    64(%rax), %rax  # this_cpu.94_1->c_bsp_resume, _2
>   # arch/x86/kernel/cpu/common.c:1869:    if (this_cpu->c_bsp_resume)
>           .loc 1 1869 5 view .LVU75
>           testq   %rax, %rax      # _2
>           je      .L8     #,
>           .loc 1 1870 3 is_stmt 1 view .LVU76
>           movq    $boot_cpu_data, %rdi    #,
>           jmp     __x86_indirect_thunk_rax
> 
> or
>   	.loc 2 57 9 view .LVU478
>   	.loc 2 57 9 view .LVU479
>   	.loc 2 57 9 view .LVU480
>   	.loc 2 57 9 view .LVU481
>   .LBB1385:
>   .LBB1383:
>   .LBB1379:
>   .LBB1377:
>   .LBB1375:
>   	.loc 2 57 9 view .LVU482
>   	.loc 2 57 9 view .LVU483
>   	movl	%edi, %edx	# cpu, cpu
>   .LVL87:
>   	.loc 2 57 9 is_stmt 0 view .LVU484
> 
> That MOV in there is drowned in debugging information and latter makes
> it hard to follow the asm. And that DWARF info is not really needed for
> asm output staring.
> 
> Disable the debug information generation which clutters the asm output
> unnecessarily:
> 
>   bsp_resume:
>   .LFB3466:
>           .loc 1 1868 1
>   # arch/x86/kernel/cpu/common.c:1869:    if (this_cpu->c_bsp_resume)
>           .loc 1 1869 14
>           movq    this_cpu(%rip), %rax    # this_cpu, this_cpu
>           movq    64(%rax), %rax  # this_cpu.94_1->c_bsp_resume, _2
>   # arch/x86/kernel/cpu/common.c:1869:    if (this_cpu->c_bsp_resume)
>           .loc 1 1869 5
>           testq   %rax, %rax      # _2
>           je      .L8     #,
>   # arch/x86/kernel/cpu/common.c:1870:            this_cpu->c_bsp_resume(&boot_cpu_data);
>           .loc 1 1870 3
>           movq    $boot_cpu_data, %rdi    #,
>           jmp     __x86_indirect_thunk_rax
> 
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> Cc: Michal Marek <michal.lkml@xxxxxxxxxxx>
> Cc: linux-kbuild@xxxxxxxxxxxxxxx
> ---
>  scripts/Makefile.build | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index fd03d60f6c5a..4f33fdab89d2 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -103,8 +103,12 @@ modkern_cflags =                                          \
>  		$(KBUILD_CFLAGS_KERNEL) $(CFLAGS_KERNEL))
>  quiet_modtag = $(if $(part-of-module),[M],   )
>  
> +disable_extra_cc_dbg := $(call cc-option,-gno-as-locview-support)
> +disable_extra_cc_dbg += $(call cc-option,-fno-dwarf2-cfi-asm)
> +disable_extra_cc_dbg += $(call cc-option,-feliminate-unused-debug-symbols)
> +disable_extra_cc_dbg += $(call cc-option,-gno-statement-frontiers)
>  quiet_cmd_cc_s_c = CC $(quiet_modtag)  $@
> -cmd_cc_s_c       = $(CC) $(c_flags) $(DISABLE_LTO) -fverbose-asm -S -o $@ $<
> +cmd_cc_s_c       = $(CC) $(c_flags) $(DISABLE_LTO) $(disable_extra_cc_dbg) -fverbose-asm -S -o $@ $<
>  
>  $(obj)/%.s: $(src)/%.c FORCE
>  	$(call if_changed_dep,cc_s_c)
> -- 
> 2.19.1
> 

-- 
Regards/Gruss,
    Boris.

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