linux-kernel - Re: [PATCH] arm64: remove special treatment for the link order of head.o

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Message-ID: <ZBzAp457rrO52FPy@aurel32.net>
Date:   Thu, 23 Mar 2023 22:12:07 +0100
From:   Aurelien Jarno <aurelien@...el32.net>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        Nicolas Schier <nicolas@...sle.eu>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: remove special treatment for the link order of
 head.o

Hi,

On 2023-03-22 15:51, Ard Biesheuvel wrote:
> On Tue, 21 Mar 2023 at 23:26, Aurelien Jarno <aurelien@...el32.net> wrote:
> >
> > Hi,
> >
> > On 2022-10-13 08:35, Masahiro Yamada wrote:
> > > In the previous discussion (see the Link tag), Ard pointed out that
> > > arm/arm64/kernel/head.o does not need any special treatment - the only
> > > piece that must appear right at the start of the binary image is the
> > > image header which is emitted into .head.text.
> > >
> > > The linker script does the right thing to do. The build system does
> > > not need to manipulate the link order of head.o.
> > >
> > > Link: https://lore.kernel.org/lkml/CAMj1kXH77Ja8bSsq2Qj8Ck9iSZKw=1F8Uy-uAWGVDm4-CG=EuA@mail.gmail.com/
> > > Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> > > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > > ---
> > >
> > >  scripts/head-object-list.txt | 1 -
> > >  1 file changed, 1 deletion(-)
> > >
> > > diff --git a/scripts/head-object-list.txt b/scripts/head-object-list.txt
> > > index b16326a92c45..f226e45e3b7b 100644
> > > --- a/scripts/head-object-list.txt
> > > +++ b/scripts/head-object-list.txt
> > > @@ -15,7 +15,6 @@ arch/alpha/kernel/head.o
> > >  arch/arc/kernel/head.o
> > >  arch/arm/kernel/head-nommu.o
> > >  arch/arm/kernel/head.o
> > > -arch/arm64/kernel/head.o
> > >  arch/csky/kernel/head.o
> > >  arch/hexagon/kernel/head.o
> > >  arch/ia64/kernel/head.o
> >
> > This patch causes a significant increase of the arch/arm64/boot/Image
> > size. For instance the generic arm64 Debian kernel went from 31 to 39 MB
> > after this patch has been applied to the 6.1 stable tree.
> >
> > In turn this causes issues with some bootloaders, for instance U-Boot on
> > a Raspberry Pi limits the kernel size to 36 MB.
> >
> 
> I cannot reproduce this with mainline
> 
> With the patch
> 
> $ size vmlinux
>    text    data     bss     dec     hex filename
> 24567309 14752630 621680 39941619 26175f3 vmlinux
> 
> With the patch reverted
> 
> $ size vmlinux
>    text    data     bss     dec     hex filename
> 24567309 14752694 621680 39941683 2617633 vmlinux

I have tried with the current mainline, this is what I get, using GCC 12.2.0
and binutils 2.40:

   text    data     bss     dec     hex filename
32531655        8192996  621968 41346619        276e63b vmlinux.orig
25170610        8192996  621968 33985574        2069426 vmlinux.revert

> It would help to compare the resulting vmlinux ELF images from both
> builds to see where the extra space is being allocated

At a first glance, it seems the extra space is allocated in the BTF
section. I have uploaded the resulting files as well as the config file
I used there:
https://temp.aurel32.net/linux-arm64-size-head.o.tar.gz

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@...el32.net                 http://www.aurel32.net