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

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Message-ID: <bd336b0f-9cf7-e2ce-e0a8-1891599638d1@csgroup.eu>
Date:   Wed, 6 May 2020 20:10:57 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, npiggin@...il.com,
        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'



Le 06/05/2020 à 19:58, Segher Boessenkool a écrit :
> On Wed, May 06, 2020 at 10:58:55AM +1000, Michael Ellerman wrote:
>>>> The "m<>" here is breaking GCC 4.6.3, which we allegedly still support.
>>>
>>> [ You shouldn't use 4.6.3, there has been 4.6.4 since a while.  And 4.6
>>>    is nine years old now.  Most projects do not support < 4.8 anymore, on
>>>    any architecture.  ]
>>
>> Moving up to 4.6.4 wouldn't actually help with this though would it?
> 
> Nope.  But 4.6.4 is a bug-fix release, 91 bugs fixed since 4.6.3, so you
> should switch to it if you can :-)
> 
>> Also I have 4.6.3 compilers already built, I don't really have time to
>> rebuild them for 4.6.4.
>>
>> The kernel has a top-level minimum version, which I'm not in charge of, see:
>>
>> https://www.kernel.org/doc/html/latest/process/changes.html?highlight=gcc
> 
> Yes, I know.  And it is much preferred not to have stricter requirements
> for Power, I know that too.  Something has to give though :-/
> 
>> There were discussions about making 4.8 the minimum, but I'm not sure
>> where they got to.
> 
> Yeah, just petered out I think?
> 
> All significant distros come with a 4.8 as system compiler.
> 
>>>> Plain "m" works, how much does the "<>" affect code gen in practice?
>>>>
>>>> A quick diff here shows no difference from removing "<>".
>>>
>>> It will make it impossible to use update-form instructions here.  That
>>> probably does not matter much at all, in this case.
>>>
>>> If you remove the "<>" constraints, also remove the "%Un" output modifier?
>>
>> So like this?
>>
>> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
>> index 62cc8d7640ec..ca847aed8e45 100644
>> --- a/arch/powerpc/include/asm/uaccess.h
>> +++ b/arch/powerpc/include/asm/uaccess.h
>> @@ -207,10 +207,10 @@ do {								\
>>   
>>   #define __put_user_asm_goto(x, addr, label, op)			\
>>   	asm volatile goto(					\
>> -		"1:	" op "%U1%X1 %0,%1	# put_user\n"	\
>> +		"1:	" op "%X1 %0,%1	# put_user\n"		\
>>   		EX_TABLE(1b, %l2)				\
>>   		:						\
>> -		: "r" (x), "m<>" (*addr)				\
>> +		: "r" (x), "m" (*addr)				\
>>   		:						\
>>   		: label)
> 
> Like that.  But you will have to do that to *all* places we use the "<>"
> constraints, or wait for more stuff to fail?  And, there probably are
> places we *do* want update form insns used (they do help in some loops,
> for example)?
> 

AFAICT, git grep "m<>" provides no result.

However many places have %Ux:

arch/powerpc/boot/io.h:	__asm__ __volatile__("lbz%U1%X1 %0,%1; twi 
0,%0,0; isync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("stb%U0%X0 %1,%0; sync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("lhz%U1%X1 %0,%1; twi 
0,%0,0; isync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("sth%U0%X0 %1,%0; sync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("lwz%U1%X1 %0,%1; twi 
0,%0,0; isync"
arch/powerpc/boot/io.h:	__asm__ __volatile__("stw%U0%X0 %1,%0; sync"
arch/powerpc/include/asm/atomic.h:	__asm__ __volatile__("lwz%U1%X1 
%0,%1" : "=r"(t) : "m"(v->counter));
arch/powerpc/include/asm/atomic.h:	__asm__ __volatile__("stw%U0%X0 
%1,%0" : "=m"(v->counter) : "r"(i));
arch/powerpc/include/asm/atomic.h:	__asm__ __volatile__("ld%U1%X1 %0,%1" 
: "=r"(t) : "m"(v->counter));
arch/powerpc/include/asm/atomic.h:	__asm__ __volatile__("std%U0%X0 
%1,%0" : "=m"(v->counter) : "r"(i));
arch/powerpc/include/asm/book3s/32/pgtable.h:		stw%U0%X0 %2,%0\n\
arch/powerpc/include/asm/book3s/32/pgtable.h:		stw%U0%X0 %L2,%1"
arch/powerpc/include/asm/io.h:	__asm__ __volatile__("sync;"#insn"%U1%X1 
%0,%1;twi 0,%0,0;isync"\
arch/powerpc/include/asm/io.h:	__asm__ __volatile__("sync;"#insn"%U0%X0 
%1,%0"			\
arch/powerpc/include/asm/nohash/pgtable.h:			stw%U0%X0 %2,%0\n\
arch/powerpc/include/asm/nohash/pgtable.h:			stw%U0%X0 %L2,%1"
arch/powerpc/kvm/powerpc.c:	asm ("lfs%U1%X1 0,%1; stfd%U0%X0 0,%0" : 
"=m" (fprd) : "m" (fprs)
arch/powerpc/kvm/powerpc.c:	asm ("lfd%U1%X1 0,%1; stfs%U0%X0 0,%0" : 
"=m" (fprs) : "m" (fprd)


Christophe