linux-kernel - Re: [RFC][PATCH 10/22] x86,uaccess: Remove .fixup usage

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Date:   Thu, 4 Nov 2021 15:28:23 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mark.rutland@....com,
        dvyukov@...gle.com, seanjc@...gle.com, pbonzini@...hat.com,
        mbenes@...e.cz
Subject: Re: [RFC][PATCH 10/22] x86,uaccess: Remove .fixup usage

On Thu, Nov 04, 2021 at 05:47:39PM +0100, Peter Zijlstra wrote:
>  #define __get_user_asm_u64(x, ptr, retval) \
> -	 __get_user_asm(x, ptr, retval, "q", "=r")
> +	 __get_user_asm(x, ptr, retval, "q", "=a")
>  #endif
>  
>  #define __get_user_size(x, ptr, size, retval)				\
> @@ -379,14 +377,14 @@ do {									\
>  	__chk_user_ptr(ptr);						\
>  	switch (size) {							\
>  	case 1:								\
> -		__get_user_asm(x_u8__, ptr, retval, "b", "=q");		\
> +		__get_user_asm(x_u8__, ptr, retval, "b", "=a");		\
>  		(x) = x_u8__;						\
>  		break;							\
>  	case 2:								\
> -		__get_user_asm(x, ptr, retval, "w", "=r");		\
> +		__get_user_asm(x, ptr, retval, "w", "=a");		\
>  		break;							\
>  	case 4:								\
> -		__get_user_asm(x, ptr, retval, "l", "=r");		\
> +		__get_user_asm(x, ptr, retval, "l", "=a");		\
>  		break;							\
>  	case 8:								\
>  		__get_user_asm_u64(x, ptr, retval);			\
> @@ -400,16 +398,13 @@ do {									\
>  	asm volatile("\n"						\
>  		     "1:	mov"itype" %[umem],%[output]\n"		\
>  		     "2:\n"						\
> -		     ".section .fixup,\"ax\"\n"				\
> -		     "3:	mov %[efault],%[errout]\n"		\
> -		     "	xorl %k[output],%k[output]\n"			\
> -		     "	jmp 2b\n"					\
> -		     ".previous\n"					\
> -		     _ASM_EXTABLE_UA(1b, 3b)				\
> +		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG | \
> +					   EX_FLAG_CLR_AX,		\
> +					   %[errout])			\
>  		     : [errout] "=r" (err),				\
>  		       [output] ltype(x)				\
>  		     : [umem] "m" (__m(addr)),				\
> -		       [efault] "i" (-EFAULT), "0" (err))
> +		       "0" (err))

Since this legacy __get_user_asm() now has a hard dependency on "=a",
might as well just make that explicit by hard-coding the constraint here
and removing the 'ltype' arg.

-- 
Josh