linux-kernel - Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put

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Date:   Wed, 06 May 2020 00:27:58 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, npiggin@...il.com,
        segher@...nel.crashing.org
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'

Christophe Leroy <christophe.leroy@....fr> writes:
> unsafe_put_user() is designed to take benefit of 'asm goto'.
>
> Instead of using the standard __put_user() approach and branch
> based on the returned error, use 'asm goto' and make the
> exception code branch directly to the error label. There is
> no code anymore in the fixup section.
>
> This change significantly simplifies functions using
> unsafe_put_user()
>
...
>
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  arch/powerpc/include/asm/uaccess.h | 61 +++++++++++++++++++++++++-----
>  1 file changed, 52 insertions(+), 9 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index 9cc9c106ae2a..9365b59495a2 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -196,6 +193,52 @@ do {								\
>  })
>  
>  
> +#define __put_user_asm_goto(x, addr, label, op)			\
> +	asm volatile goto(					\
> +		"1:	" op "%U1%X1 %0,%1	# put_user\n"	\
> +		EX_TABLE(1b, %l2)				\
> +		:						\
> +		: "r" (x), "m<>" (*addr)				\

The "m<>" here is breaking GCC 4.6.3, which we allegedly still support.

Plain "m" works, how much does the "<>" affect code gen in practice?

A quick diff here shows no difference from removing "<>".

cheers