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

Christophe Leroy
2020-05-05T15:40:27Z

Hi,

Le 05/05/2020 à 16:27, Michael Ellerman a écrit :
> 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 "<>".

It was recommended by Segher, there has been some discussion about it on
v1 of this patch, see
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/4fdc2aba6f5e51887d1cd0fee94be0989eada2cd.1586942312.git.christophe.leroy@c-s.fr/

As far as I understood that's mandatory on recent gcc to get the
pre-update form of the instruction. With older versions "m" was doing
the same, but not anymore. Should we ifdef the "m<>" or "m" based on GCC
version ?

Christophe