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From:   Uros Bizjak <ubizjak@gmail.com>
Date:   Thu, 12 Oct 2023 22:59:37 +0200
Message-ID: <CAFULd4byzHyoz4xM1gL_T1wFkNE-ab8K4upXfPirxM9PdX5JWg@mail.gmail.com>
Subject: Re: [PATCH v2 0/4] Introduce %rip-relative addressing to PER_CPU_VAR macro
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     x86@kernel.org, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, Juergen Gross <jgross@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
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On Thu, Oct 12, 2023 at 10:53=E2=80=AFPM Dave Hansen <dave.hansen@intel.com=
> wrote:
>
> On 10/12/23 13:12, Uros Bizjak wrote:
> > The last patch introduces (%rip) suffix and uses it for x86_64 target,
> > resulting in a small code size decrease: text data bss dec hex filename
> > 25510677 4386685 808388 30705750 1d48856 vmlinux-new.o 25510629 4386685
> > 808388 30705702 1d48826 vmlinux-old.o
>
> I feel like I'm missing some of the motivation here.
>
> 50 bytes is great and all, but it isn't without the cost of changing
> some rules and introducing potential PER_CPU_ARG() vs. PER_CPU_VAR()
> confusion.
>
> Are there some other side benefits?  What else does this enable?

These changes are necessary to build the kernel as Position
Independent Executable (PIE) on x86_64 [1]. And since I was working in
percpu area I thought that it was worth implementing them.

[1] https://lore.kernel.org/lkml/cover.1682673542.git.houwenlong.hwl@antgro=
up.com/

Uros.