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

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Message-ID: <CAFULd4Yj9LfTnWFu=c1M7Eh44+XFk0ibwL57r5H7wZjvKZ8yaA@mail.gmail.com>
Date: Sat, 5 Oct 2024 10:31:37 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Ard Biesheuvel <ardb@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Dennis Zhou <dennis@...nel.org>, 
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	llvm@...ts.linux.dev
Subject: Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

On Fri, Oct 4, 2024 at 11:06 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> On 10/3/24 04:13, Ard Biesheuvel wrote:
> >
> >> That said, doing changes like changing "mov $sym" to "lea sym(%rip)" I
> >> feel are a complete no-brainer and should be done regardless of any
> >> other code generation issues.
> >
> > Yes, this is the primary reason I ended up looking into this in the
> > first place. Earlier this year, we ended up having to introduce
> > RIP_REL_REF() to emit those RIP-relative references explicitly, in
> > order to prevent the C code that is called via the early 1:1 mapping
> > from exploding. The amount of C code called in that manner has been
> > growing steadily over time with the introduction of 5-level paging and
> > SEV-SNP and TDX support, which need to play all kinds of tricks before
> > the normal kernel mappings are created.
> >
>
> movq $sym to leaq sym(%rip) which you said ought to be smaller (and in
> reality appears to be the same size, 7 bytes) seems like a no-brainer
> and can be treated as a code quality issue -- in other words, file bug
> reports against gcc and clang.

It is the kernel assembly source that should be converted to
rip-relative form, gcc (and probably clang) have nothing with it.

Uros.