Re: [RFC PATCH v3 08/12] lib: vdso: allow arches to provide vdso data pointer

[Andy Lutomirski]

On Thu, Jan 16, 2020 at 2:35 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> static __maybe_unused int
> __cvdso_data_clock_gettime(clockid_t clock, struct __kernel_timespec *ts,
>                            const struct vdso_data *vd)
> {
>         .....
> }
>
> static __maybe_unused int
> __cvdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts)
> {
>         const struct vdso_data *vd = __arch_get_vdso_data();
>
>         return __cvdso_data_clock_gettime(clock, ts, vd);
> }
>
> and then use __cvdso_data_clock_gettime on PPC and let the other archs
> unmodified.
>
>

FWIW, I did some experiments on x86 with gcc 9.2.  gcc 9.2 uses
rip-relative accesses if I simplify the config enough and otherwise
materializes the pointer.  Presumably it decides that the code size
reduction is worth it if there are a lot of accesses.

I suspect that tglx's suggestion will be fine or at worst will add
negligible overhead on x86_64.