linux-kernel - Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

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Message-ID: <87ikuj30g8.fsf@linux.intel.com>
Date: Wed, 25 Sep 2024 14:09:11 -0700
From: Andi Kleen <ak@...ux.intel.com>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org,  Ard Biesheuvel <ardb@...nel.org>,
  x86@...nel.org,  "H. Peter Anvin" <hpa@...or.com>,  Andy Lutomirski
 <luto@...nel.org>,  Peter Zijlstra <peterz@...radead.org>,  Uros Bizjak
 <ubizjak@...il.com>,  Dennis Zhou <dennis@...nel.org>,  Tejun Heo
 <tj@...nel.org>,  Christoph Lameter <cl@...ux.com>,  Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>,  Paolo Bonzini <pbonzini@...hat.com>,
  Vitaly Kuznetsov <vkuznets@...hat.com>,  Juergen Gross <jgross@...e.com>,
  Boris Ostrovsky <boris.ostrovsky@...cle.com>,  Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>,  Arnd Bergmann <arnd@...db.de>,  Masahiro
 Yamada <masahiroy@...nel.org>,  Kees Cook <kees@...nel.org>,  Nathan
 Chancellor <nathan@...nel.org>,  Keith Packard <keithp@...thp.com>,
  Justin Stitt <justinstitt@...gle.com>,  Josh Poimboeuf
 <jpoimboe@...nel.org>,  Arnaldo Carvalho de Melo <acme@...nel.org>,
  Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

Ard Biesheuvel <ardb+git@...gle.com> writes:
> This substantially reduces the number of relocations that need to be
> processed when booting a relocatable KASLR kernel.
>
> Before (size in bytes of the reloc table):
>
>   797372 arch/x86/boot/compressed/vmlinux.relocs
>
> After:
>
>   400252 arch/x86/boot/compressed/vmlinux.relocs

I don't know why anybody would care about the size of the relocation table?

What matters is what it does to general performance.

Traditionally even on x86-64 PIC/E has a cost and the kernel model
was intended to avoid that.

>From my perspective this patch kit doesn't fix a real problem,
it's all risk of performance regression with no gain.

-Andi