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

[Uros Bizjak]

On Thu, Oct 12, 2023 at 11:08 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> On 10/12/23 13:59, Uros Bizjak wrote:
> > On Thu, Oct 12, 2023 at 10:53 PM Dave Hansen <dave.hansen@xxxxxxxxx> 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@xxxxxxxxxxxx/
> >
>
> Are you PIC-adjusting the percpu variables as well?

After this patch (and after fixing percpu_stable_op to use "a" operand
modifier on GCC), the only *one* remaining absolute reference to
percpu variable remain in xen-head.S, where:

    movq    $INIT_PER_CPU_VAR(fixed_percpu_data),%rax

should be changed to use leaq.

All others should then be (%rip)-relative.

Uros.