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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Christophe Leroy <christophe.leroy@c-s.fr>, luto@kernel.org
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, arnd@arndb.de,
        vincenzo.frascino@arm.com, x86@kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org
Subject: Re: [RFC PATCH v3 08/12] lib: vdso: allow arches to provide vdso data pointer
In-Reply-To: <b5fddcf8-99ff-fc0d-40c0-0eb81ad4e94a@c-s.fr>
References: <cover.1578934751.git.christophe.leroy@c-s.fr> <381e547dbb3c48fd39d6cf208033bba38ad048fb.1578934751.git.christophe.leroy@c-s.fr> <87ftghbpuu.fsf@nanos.tec.linutronix.de> <d2de3211-9d7c-513e-fe0f-8bdce623fb65@c-s.fr> <b5fddcf8-99ff-fc0d-40c0-0eb81ad4e94a@c-s.fr>
Date:   Thu, 16 Jan 2020 11:35:17 +0100
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Christophe Leroy <christophe.leroy@c-s.fr> writes:
> Le 15/01/2020 à 07:15, Christophe Leroy a écrit :
>  From your point of view, what should I do:
> A/ __arch_get_vdso_data() handled entirely at arch level and arches 
> handing over the vdso data pointer to generic C VDSO functions all the 
> time (as in my v2 series) ?

No. That's again moving the same code to all architectures.

> B/ Data pointer being handed over all the way up for arches wanting to 
> do so, no changes at all for others (as in my v3 series) ?

Too much ifdeffery

> C/ __arch_get_vdso_data() being called at the outermost generic level 
> for arches not interested in handling data pointer from the caller (as 
> suggested by Thomas) ?
>
> Andy, with A/ you were concerned about arches being able to do PC 
> related accesses. Would it be an issue for C/ as well ? If not, I guess 
> C/ would be cleaner than B/ allthought not as clean as A which doesn't 
> add any #ifdefery at all.

You can avoid ifdeffery with C if you do:

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.

Thanks,

        tglx