linux-kernel - Re: [PATCH] arm/arm64: smccc: Use xN for arm64 register constraints with clang

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Message-ID: <20180322235811.GC78232@google.com>
Date:   Thu, 22 Mar 2018 16:58:11 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Greg Hackmann <ghackmann@...gle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>, marc.zyngier@....com,
        Catalin Marinas <catalin.marinas@....com>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Dave.Martin@....com, Andrey Konovalov <andreyknvl@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] arm/arm64: smccc: Use xN for arm64 register constraints
 with clang

El Thu, Mar 22, 2018 at 04:19:42PM -0700 Greg Hackmann ha dit:

> On 03/22/2018 03:44 PM, Matthias Kaehlcke wrote:
> > El Thu, Mar 22, 2018 at 10:26:18PM +0000 Nick Desaulniers ha dit:
> > 
> >> Note that a patch in this form has previously been implemented by:
> >>
> >> Andrey Konovalov <andreyknvl@...gle.com>:
> >> https://gist.github.com/xairy/ee11682ea86044a45c0291c528cd936f
> >>
> >> and another by:
> >>
> >> Greg Hackmann <ghackmann@...gle.com>:
> >> https://android-review.googlesource.com/c/kernel/common/+/645181
> >>
> >> If you used either as a reference, you may want to credit them with a
> >> `Suggested-by:` in the commit message.
> > 
> > Not really, but I think I prefer Greg's version over mine and might
> > use it in a respin if nobody raises objections.
> 
> NAK.  There's a reason I didn't send my change upstream.
> 
> As Marc pointed out (https://lkml.org/lkml/2018/3/16/987), 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
(https://lkml.org/lkml/2018/3/1/268) 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.