Re: [PATCH v2 0/4] Introduce %rip-relative addressing to PER_CPU_VAR macro

Uros Bizjak
2023-10-12T20:59:54Z

On Thu, Oct 12, 2023 at 10:53 PM 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@antgroup.com/

Uros.