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

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Message-ID: <CALCETrWOENu2k3aGNO-oiY1Sj8=cG9mMZ8eOepHOzdu25vFVVA@mail.gmail.com>
Date:   Thu, 16 Jan 2020 12:22:41 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        Andrew Lutomirski <luto@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        X86 ML <x86@...nel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>
Subject: Re: [RFC PATCH v3 08/12] lib: vdso: allow arches to provide vdso data pointer

On Thu, Jan 16, 2020 at 2:35 AM Thomas Gleixner <tglx@...utronix.de> 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.