Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'

Michael Ellerman
2020-05-05T14:27:51Z

Christophe Leroy <christophe.leroy@c-s.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@c-s.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