linux-kernel - Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree -v1: Eliminate the Linux kernel's "Dependency Hell"

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Date:   Sat, 8 Jan 2022 12:49:04 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Ard Biesheuvel <ardb@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@...iv.linux.org.uk>, llvm@...ts.linux.dev
Subject: Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree
 -v1: Eliminate the Linux kernel's "Dependency Hell"


* Nathan Chancellor <nathan@...nel.org> wrote:

> 5. Build error in arch/arm64/kvm/hyp/nvhe with LTO
> 
> With arm64 + CONFIG_LTO_CLANG_THIN=y, I see:
> 
> $ make -skj"$(nproc)" ARCH=arm64 LLVM=1 defconfig
> 
> $ scripts/config -e LTO_CLANG_THIN
> 
> $ make -skj"$(nproc)" ARCH=arm64 LLVM=1 olddefconfig arch/arm64/kvm/hyp/nvhe/
> ld.lld: error: arch/arm64/kvm/hyp/nvhe/hyp.lds:2: unknown directive: .macro
> >>> .macro __put, val, name
> >>> ^
> make[5]: *** [arch/arm64/kvm/hyp/nvhe/Makefile:51: arch/arm64/kvm/hyp/nvhe/kvm_nvhe.tmp.o] Error 1
> 
> I was not able to figure out the exact include chain but CONFIG_LTO
> causes asm/alternative-macros.h to be included in asm/rwonce.h, which
> eventually gets included in either asm/cache.h or asm/memory.h.
> 
> I managed to solve this with the following diff but I am not sure if
> there is a better or cleaner way to do that.
> 
> diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> index 1bce62fa908a..e19572a205d0 100644
> --- a/arch/arm64/include/asm/rwonce.h
> +++ b/arch/arm64/include/asm/rwonce.h
> @@ -5,7 +5,7 @@
>  #ifndef __ASM_RWONCE_H
>  #define __ASM_RWONCE_H
>  
> -#ifdef CONFIG_LTO
> +#if defined(CONFIG_LTO) && !defined(LINKER_SCRIPT)
>  
>  #include <linux/compiler_types.h>
>  #include <asm/alternative-macros.h>
> @@ -66,7 +66,7 @@
>  })
>  
>  #endif	/* !BUILD_VDSO */
> -#endif	/* CONFIG_LTO */
> +#endif	/* CONFIG_LTO && !LINKER_SCRIPT */

So the error message suggests that the linker script somehow ends up 
including asm-generic/export.h:

  kepler:~/mingo.tip.git> git grep 'macro __put'
  include/asm-generic/export.h:.macro __put, val, name

?

But I'd guess that similar to the __ASSEMBLY__ patterns we have in headers, 
not including the rwonce.h bits if LINKER_SCRIPT is defined is probably 
close to the right solution - but it would also know how such a low level 
header ended up in a linker script. Might have been to pick up some offset 
or size definition somewhere?

I.e. how did the build end up including asm/rwonce.h?

You can generally debug such weird dependency chains by putting a
debug #warning into the affected header - such as the patch below.

This prints a stack of the header dependencies:

    CC      kernel/sched/core.o
  In file included from ./include/linux/compiler.h:263,
                 from ./include/linux/static_call_types.h:7,
                 from ./include/linux/kernel.h:6,
                 from ./include/linux/highmem.h:5,
                 from kernel/sched/core.c:9:
  ./arch/arm64/include/asm/rwonce.h:8:2: warning: #warning debug [-Wcpp]
      8 | #warning debug

... and should in principle also work in the linker script context.

Thanks,

	Ingo

===============>
 arch/arm64/include/asm/rwonce.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
index 1bce62fa908a..5b3305381481 100644
--- a/arch/arm64/include/asm/rwonce.h
+++ b/arch/arm64/include/asm/rwonce.h
@@ -5,6 +5,8 @@
 #ifndef __ASM_RWONCE_H
 #define __ASM_RWONCE_H
 
+#warning debug
+
 #ifdef CONFIG_LTO
 
 #include <linux/compiler_types.h>