On Thu, Jan 16, 2020 at 2:35 AM Thomas Gleixner [off-list ref] 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.