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

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Date:   Tue, 5 May 2020 17:40:21 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        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'

Hi,

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

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