linux-kernel - Re: [PATCH] crypto: arm64/sha: use %c constraint code in ASM

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Date:   Wed, 5 Apr 2017 18:08:37 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Greg Hackmann <ghackmann@...gle.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Grant Grundler <grundler@...omium.org>,
        Michael Davidson <md@...gle.com>
Subject: Re: [PATCH] crypto: arm64/sha: use %c constraint code in ASM_EXPORT

Hoi Matthias!

On 5 April 2017 at 17:56, Matthias Kaehlcke <mka@...omium.org> wrote:
> From: Greg Hackmann <ghackmann@...gle.com>
>
> The current definition of ASM_EXPORT doesn't work properly with clang,
> according to https://bugs.llvm.org//show_bug.cgi?id=27250#c3 it relies on
> gcc specific behavior. Change the constraint from an intermediate to an
> output expression which works with both gcc and clang.
>
> From: Greg Hackmann <ghackmann@...gle.com>
> Commit-message-by: Matthias Kaehlcke <mka@...omium.org>
> Signed-off-by: Greg Hackmann <ghackmann@...gle.com>
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
>  arch/arm64/crypto/sha1-ce-glue.c | 2 +-
>  arch/arm64/crypto/sha2-ce-glue.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/crypto/sha1-ce-glue.c b/arch/arm64/crypto/sha1-ce-glue.c
> index aefda9868627..c71e94ba0e43 100644
> --- a/arch/arm64/crypto/sha1-ce-glue.c
> +++ b/arch/arm64/crypto/sha1-ce-glue.c
> @@ -18,7 +18,7 @@
>  #include <linux/module.h>
>
>  #define ASM_EXPORT(sym, val) \
> -       asm(".globl " #sym "; .set " #sym ", %0" :: "I"(val));
> +       asm(".globl " #sym "; .set " #sym ", %c0" :: "I"(val));
>
>  MODULE_DESCRIPTION("SHA1 secure hash using ARMv8 Crypto Extensions");
>  MODULE_AUTHOR("Ard Biesheuvel <ard.biesheuvel@...aro.org>");
> diff --git a/arch/arm64/crypto/sha2-ce-glue.c b/arch/arm64/crypto/sha2-ce-glue.c
> index 7cd587564a41..381b5fb2dcb2 100644
> --- a/arch/arm64/crypto/sha2-ce-glue.c
> +++ b/arch/arm64/crypto/sha2-ce-glue.c
> @@ -18,7 +18,7 @@
>  #include <linux/module.h>
>
>  #define ASM_EXPORT(sym, val) \
> -       asm(".globl " #sym "; .set " #sym ", %0" :: "I"(val));
> +       asm(".globl " #sym "; .set " #sym ", %c0" :: "I"(val));
>
>  MODULE_DESCRIPTION("SHA-224/SHA-256 secure hash using ARMv8 Crypto Extensions");
>  MODULE_AUTHOR("Ard Biesheuvel <ard.biesheuvel@...aro.org>");

I am fine with this change, although I would really like to add a
better reference to the commit log. It is *very* difficult to find any
documentation regarding non-trivial uses of inline asm constraints,
and if %c0 is the correct syntax, surely we can quote something better
than a LLVM bugzilla entry? Also, where does the distinction between
'intermediate' vs 'output' expression come from?