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

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Message-ID: <20180405192109.GA130399@google.com>
Date:   Thu, 5 Apr 2018 12:21:09 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Greg Hackmann <ghackmann@...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>,
        Manoj Gupta <manojgupta@...gle.com>
Subject: Re: [PATCH] arm/arm64: smccc: Use xN for arm64 register constraints
 with clang

On Thu, Apr 05, 2018 at 06:43:05PM +0000, Nick Desaulniers wrote:
> On Thu, Mar 22, 2018 at 4:58 PM Matthias Kaehlcke <mka@...omium.org> 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 (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.
> 
> 
> Manoj fixed this in:
> https://reviews.llvm.org/rL328829
> https://bugs.llvm.org/show_bug.cgi?id=36862
> 
> 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?

We should be good, unless somebody wants to look into a patch that
fixes clang pre-6.0.1 builds and doesn't look too ugly.