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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 23 Dec 2019 18:27:40 -0800
Message-ID: <CALCETrW9hsrVVzudvRY22AqakcsrVzqp=SdwOTwW2zRBK+kEaA@mail.gmail.com>
Subject: Re: [RFC PATCH v2 04/10] lib: vdso: get pointer to vdso data from the arch
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Arnd Bergmann <arnd@arndb.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Andrew Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        X86 ML <x86@kernel.org>
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On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy
<christophe.leroy@c-s.fr> wrote:
>
> On powerpc, __arch_get_vdso_data() clobbers the link register,
> requiring the caller to set a stack frame in order to save it.
>
> As the parent function already has to set a stack frame and save
> the link register to call the C vdso function, retriving the
> vdso data pointer there is lighter.

I'm confused.  Can't you inline __arch_get_vdso_data()?  Or is the
issue that you can't retrieve the program counter on power without
clobbering the link register?

I would imagine that this patch generates worse code on any
architecture with PC-relative addressing modes (which includes at
least x86_64, and I would guess includes most modern architectures).

--Andy