linux-kernel - Re: [PATCH v2 0/4] Introduce %rip-relative addressing to PER_CPU

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Date:   Thu, 12 Oct 2023 14:08:00 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Uros Bizjak <ubizjak@...il.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     x86@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2 0/4] Introduce %rip-relative addressing to PER_CPU_VAR
 macro

On 10/12/23 13:59, Uros Bizjak wrote:
> On Thu, Oct 12, 2023 at 10:53 PM Dave Hansen <dave.hansen@...el.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@antgroup.com/
> 

Are you PIC-adjusting the percpu variables as well?

	-hpa