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

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Date:   Mon, 24 Apr 2017 18:00:09 +0100
From:   Will Deacon <will.deacon@....com>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     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>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...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] arm64: Add ASM modifier for xN register operands

Hi Matthias,

On Thu, Apr 20, 2017 at 11:30:53AM -0700, Matthias Kaehlcke wrote:
> Many inline assembly statements don't include the 'x' modifier when
> using xN registers as operands. This is perfectly valid, however it
> causes clang to raise warnings like this:
> 
> warning: value size does not match register size specified by the
>   constraint and modifier [-Wasm-operand-widths]
> ...
> arch/arm64/include/asm/barrier.h:62:23: note: expanded from macro
>   '__smp_store_release'
>     asm volatile ("stlr %1, %0"

If I understand this correctly, then the warning is emitted when we pass
in a value smaller than 64-bit, but refer to %<n> without a modifier
in the inline asm.

However, if that's the case then I don't understand why:

> diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
> index 0c00c87bb9dd..021e1733da0c 100644
> --- a/arch/arm64/include/asm/io.h
> +++ b/arch/arm64/include/asm/io.h
> @@ -39,33 +39,33 @@
>  #define __raw_writeb __raw_writeb
>  static inline void __raw_writeb(u8 val, volatile void __iomem *addr)
>  {
> -	asm volatile("strb %w0, [%1]" : : "rZ" (val), "r" (addr));
> +	asm volatile("strb %w0, [%x1]" : : "rZ" (val), "r" (addr));

is necessary. addr is a pointer type, so is 64-bit.

Given that the scattergun nature of this patch implies that you've been
fixing the places where warnings are reported, then I'm confused as to
why a warning is generated for the case above.

What am I missing?

Will