linux-kernel - Re: [PATCH] arm64: support _

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Date:   Tue, 31 Oct 2017 12:17:02 +0000
From:   Will Deacon <will.deacon@....com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] arm64: support __int128 on gcc 5+

Hi Jason,

On Tue, Oct 31, 2017 at 12:57:29PM +0100, Jason A. Donenfeld wrote:
> On Tue, Oct 31, 2017 at 12:51 PM, Will Deacon <will.deacon@....com> wrote:
> > Which code in the kernel actually uses 128-bit types directly? I know we
> > have some unfortunate occurences in our headers (including uapi) for the
> > vector registers, but I thought we generally used asm or copy routines to
> > access those.
> 
> math64.h provides it, and various things throughout use those
> functions. Notably, the scheduler and kvm use those the __int128
> functions. There's also an elliptic curve implementation that uses it,
> which makes a big difference. And soon I'll be adding an
> implementation of curve25519 that will make heavy use of these
> instructions for significant speedups as well.

Thanks for the info. It does indeed look like mul_u64_u64_shr improves
pretty drastically with this patch, so that sounds good to me. We don't
ever call that function on arm64, but hey.

> Generally, adding this CONFIG_ARCH key is a hint for current and
> future bits of code, so that they can use the faster 128-bit
> implementations when available. Not providing it when it's there and
> available would be silly. This wasn't originally added to the
> architecture, because when the Kconfig symbol was added, gcc didn't
> have sane support for it on aarch64. But now it does, so let's support
> it.

Sure, I'll wait for your patch that matches the relevant compiler versions.

Will