linux-kernel - Re: [PATCH] powerpc/uaccess: Use flexible addressing with __put_user()/__get

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Message-ID: <1f5a7975-3b32-3a14-e03e-7c875df57aa3@c-s.fr>
Date:   Thu, 16 Apr 2020 07:50:00 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/uaccess: Use flexible addressing with
 __put_user()/__get_user()

Hi,

Le 16/04/2020 à 00:06, Segher Boessenkool a écrit :
> Hi!
> 
> On Wed, Apr 15, 2020 at 09:20:26AM +0000, Christophe Leroy wrote:
>> At the time being, __put_user()/__get_user() and friends only use
>> register indirect with immediate index addressing, with the index
>> set to 0. Ex:
>>
>> 	lwz	reg1, 0(reg2)
> 
> This is called a "D-form" instruction, or sometimes "offset addressing".
> Don't talk about an "index", it confuses things, because the *other*
> kind is called "indexed" already, also in the ISA docs!  (X-form, aka
> indexed addressing, [reg+reg], where D-form does [reg+imm], and both
> forms can do [reg]).

In the "Programming Environments Manual for 32-Bit Implementations of 
the PowerPC™ Architecture", they list the following addressing modes:

Load and store operations have three categories of effective address 
generation that depend on the
operands specified:
• Register indirect with immediate index mode
• Register indirect with index mode
• Register indirect mode


> 
>> Give the compiler the opportunity to use other adressing modes
>> whenever possible, to get more optimised code.
> 
> Great :-)
> 
>> --- a/arch/powerpc/include/asm/uaccess.h
>> +++ b/arch/powerpc/include/asm/uaccess.h
>> @@ -114,7 +114,7 @@ extern long __put_user_bad(void);
>>    */
>>   #define __put_user_asm(x, addr, err, op)			\
>>   	__asm__ __volatile__(					\
>> -		"1:	" op " %1,0(%2)	# put_user\n"		\
>> +		"1:	" op "%U2%X2 %1,%2	# put_user\n"	\
>>   		"2:\n"						\
>>   		".section .fixup,\"ax\"\n"			\
>>   		"3:	li %0,%3\n"				\
>> @@ -122,7 +122,7 @@ extern long __put_user_bad(void);
>>   		".previous\n"					\
>>   		EX_TABLE(1b, 3b)				\
>>   		: "=r" (err)					\
>> -		: "r" (x), "b" (addr), "i" (-EFAULT), "0" (err))
>> +		: "r" (x), "m" (*addr), "i" (-EFAULT), "0" (err))
> 
> %Un on an "m" operand doesn't do much: you need to make it "m<>" if you
> want pre-modify ("update") insns to be generated.  (You then will want
> to make sure that operand is used in a way GCC can understand; since it
> is used only once here, that works fine).

Ah ? Indeed I got the idea from include/asm/io.h where there is:

#define DEF_MMIO_IN_D(name, size, insn)				\
static inline u##size name(const volatile u##size __iomem *addr)	\
{									\
	u##size ret;							\
	__asm__ __volatile__("sync;"#insn"%U1%X1 %0,%1;twi 0,%0,0;isync"\
		: "=r" (ret) : "m" (*addr) : "memory");			\
	return ret;							\
}

It should be "m<>" there as well ?

> 
>> @@ -130,8 +130,8 @@ extern long __put_user_bad(void);
>>   #else /* __powerpc64__ */
>>   #define __put_user_asm2(x, addr, err)				\
>>   	__asm__ __volatile__(					\
>> -		"1:	stw %1,0(%2)\n"				\
>> -		"2:	stw %1+1,4(%2)\n"			\
>> +		"1:	stw%U2%X2 %1,%2\n"			\
>> +		"2:	stw%U2%X2 %L1,%L2\n"			\
>>   		"3:\n"						\
>>   		".section .fixup,\"ax\"\n"			\
>>   		"4:	li %0,%3\n"				\
>> @@ -140,7 +140,7 @@ extern long __put_user_bad(void);
>>   		EX_TABLE(1b, 4b)				\
>>   		EX_TABLE(2b, 4b)				\
>>   		: "=r" (err)					\
>> -		: "r" (x), "b" (addr), "i" (-EFAULT), "0" (err))
>> +		: "r" (x), "m" (*addr), "i" (-EFAULT), "0" (err))
> 
> Here, it doesn't work.  You don't want two consecutive update insns in
> any case.  Easiest is to just not use "m<>", and then, don't use %Un
> (which won't do anything, but it is confusing).

Can't we leave the Un on the second stw ?

> 
> Same for the reads.
> 
> Rest looks fine, and update should be good with that fixed as said.
> 
> Reviewed-by: Segher Boessenkool <segher@...nel.crashing.org>
> 
> 
> Segher
> 

Thanks for the review
Christophe