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

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Date:   Wed, 15 Apr 2020 17:06:52 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
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!

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]).

> 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).

> @@ -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).

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