linux-kernel - Re: [PATCH v2] arm64: Add ASM modifier for xN register operands

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Date:   Fri, 28 Apr 2017 15:47:45 +0100
From:   Will Deacon <will.deacon@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        KVM devel mailing list <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Grant Grundler <grundler@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Michael Davidson <md@...gle.com>
Subject: Re: [PATCH v2] arm64: Add ASM modifier for xN register operands

On Fri, Apr 28, 2017 at 03:43:56PM +0100, Ard Biesheuvel wrote:
> On 28 April 2017 at 15:33, Mark Rutland <mark.rutland@....com> wrote:
> > On Fri, Apr 28, 2017 at 08:18:52AM +0100, Ard Biesheuvel wrote:
> OK, good. That is a departure from previous behavior of Clang, which
> was causing build errors before due to the fact that msr/mrs
> instructions involving 32-bit values must still use x registers.
> 
> > * If an operand type is 64 bits in size, clang will not produce a warning
> >   regarding the operand size.
> >
> > * If an x or w modifier is used, clang will not produce a warning
> >   regarding the operand size, regardless of whether it matches the
> >   register size. Clang is happy for %wN to be used on a pointer type.
> >
> > * If an operand type is less than 64 bits in size, and neither an x or
> >   w modifier is used, clang will produce a warning as above.
> >
> > * If an operand type is greater than 64 bits in size, clang encounters
> >   an internal error.
> >
> > Given that, I think we *should not* use the x modifier to suppress this
> > warning, as I think for those cases we have a potential bug as outlined
> > in my prior reply.
> >
> > Instead, we should use a temporary 64-bit variable (or cast input
> > operands to 64-bit), which avoids that and makes clang happy.
> >
> 
> Yes, I think that makes sense.

Likewise, we could even raise a feature request against GCC because these
warnings actually sound useful. Thanks for getting to the bottom of this.

Will