linux-kernel - Re: [RFC][PATCH 11/22] x86,xen: Remove .fixup usage

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Message-ID: <20211104223151.k553ng4cx6nbke4z@treble>
Date:   Thu, 4 Nov 2021 15:31:51 -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 11/22] x86,xen: Remove .fixup usage

On Thu, Nov 04, 2021 at 05:47:40PM +0100, Peter Zijlstra wrote:
> Employ the fancy new EX_TYPE_IMM_REG to create EX_TYPE_NEG_REG to
> store '-1' into the designated register and use this to remove some
> Xen .fixup usage.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/include/asm/extable_fixup_types.h |    1 +
>  arch/x86/include/asm/xen/page.h            |   12 ++----------
>  2 files changed, 3 insertions(+), 10 deletions(-)
> 
> --- a/arch/x86/include/asm/extable_fixup_types.h
> +++ b/arch/x86/include/asm/extable_fixup_types.h
> @@ -35,5 +35,6 @@
>  
>  #define	EX_TYPE_IMM_REG			17 /* reg := (long)imm */
>  #define	EX_TYPE_EFAULT_REG		(EX_TYPE_IMM_REG | EX_TYPE_IMM(-EFAULT))
> +#define	EX_TYPE_NEG_REG			(EX_TYPE_IMM_REG | EX_TYPE_IMM(-1))
>  
>  #endif
> --- a/arch/x86/include/asm/xen/page.h
> +++ b/arch/x86/include/asm/xen/page.h
> @@ -96,11 +96,7 @@ static inline int xen_safe_write_ulong(u
>  
>  	asm volatile("1: mov %[val], %[ptr]\n"
>  		     "2:\n"
> -		     ".section .fixup, \"ax\"\n"
> -		     "3: sub $1, %[ret]\n"
> -		     "   jmp 2b\n"
> -		     ".previous\n"
> -		     _ASM_EXTABLE(1b, 3b)
> +		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_NEG_REG, %[ret])
>  		     : [ret] "+r" (ret), [ptr] "=m" (*addr)
>  		     : [val] "r" (val));
>  
> @@ -115,11 +111,7 @@ static inline int xen_safe_read_ulong(co
>  
>  	asm volatile("1: mov %[ptr], %[rval]\n"
>  		     "2:\n"
> -		     ".section .fixup, \"ax\"\n"
> -		     "3: sub $1, %[ret]\n"
> -		     "   jmp 2b\n"
> -		     ".previous\n"
> -		     _ASM_EXTABLE(1b, 3b)
> +		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_NEG_REG, %[ret])
>  		     : [ret] "+r" (ret), [rval] "+r" (rval)
>  		     : [ptr] "m" (*addr));
>  	*val = rval;

-EFAULT is also negative, and presumably more appropriate than -1.

Could we just use EX_TYPE_EFAULT_REG?

-- 
Josh