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Date:   Thu, 05 Apr 2018 18:43:05 +0000
From:   Nick Desaulniers <>
To:     Matthias Kaehlcke <>
Cc:     Greg Hackmann <>,,
        Catalin Marinas <>,
        Robin Murphy <>,,
        LKML <>,
        Christoffer Dall <>,, Andrey Konovalov <>,
        Dmitry Vyukov <>,
        Kostya Serebryany <>,
        Ard Biesheuvel <>,
        Manoj Gupta <>
Subject: Re: [PATCH] arm/arm64: smccc: Use xN for arm64 register constraints
 with clang

On Thu, Mar 22, 2018 at 4:58 PM Matthias Kaehlcke <> wrote:

> El Thu, Mar 22, 2018 at 04:19:42PM -0700 Greg Hackmann ha dit:
> > NAK.  There's a reason I didn't send my change upstream.
> >
> > As Marc pointed out (, the "r"
> > prefix tells gcc to pick the appropriate register width.  "x" makes it
> > unconditionally use the entire 64-bit register width.  Just swapping out
> > one for the other changes the macro's semantics.
> >
> > Unfortunately since this was breaking builds in android-4.14 and we
> > didn't have an immediate-term fix, I bit the bullet and added the above
> > commit -- but *only* as a short-term workaround.  For the one caller we
> > currently have in 4.14.y, gcc was using the entire 64-bit width for all
> > its inputs anyway, so "r" vs. "x" didn't make a difference.  But that
> > might not be true if/when someone introduces other SMCCC 1.1 callers.
> >
> > Unfortunately I don't see a better way to deal with this than waiting
> > for clang to support "r"-style constraints on ARM64.

> Thanks for the clarification! From the other thread
> ( I had the impression that ARM
> folks saw the option of a mergeable fix.

> Given the fact that clang support for kernel builds is still
> recent/WIP I guess it's not the end of the world if we have to raise
> the minimum clang version to 7.x for newer kernels.

Manoj fixed this in:

Looks set to ride the Clang 6.0 train.  mka@ if you're planning another
state of the union email, it would be good to note the clang 6.0
requirement for arm64.

Is there anything left to do here?
~Nick Desaulniers

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