linux-kernel - Re: [PATCH v2 11/23] x86,xen: Remove .fixup usage

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Message-ID: <42933d6b-c6f4-7420-1d0f-7f5d6ec17d8e@suse.com>
Date:   Wed, 10 Nov 2021 11:35:37 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        mark.rutland@....com, dvyukov@...gle.com, seanjc@...gle.com,
        pbonzini@...hat.com, mbenes@...e.cz
Subject: Re: [PATCH v2 11/23] x86,xen: Remove .fixup usage

On 10.11.21 11:01, Peter Zijlstra wrote:
> Employ the fancy new EX_TYPE_IMM_REG to store -EFAULT in the return
> register and use this to remove some Xen .fixup usage.
> 
> All callers of these functions only test for 0 return, so the actual
> return value change from -1 to -EFAULT is immaterial.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>   arch/x86/include/asm/xen/page.h |   14 +++-----------
>   1 file changed, 3 insertions(+), 11 deletions(-)
> 
> --- 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_EFAULT_REG, %[ret])
>   		     : [ret] "+r" (ret), [ptr] "=m" (*addr)
>   		     : [val] "r" (val));
>   
> @@ -110,16 +106,12 @@ static inline int xen_safe_write_ulong(u
>   static inline int xen_safe_read_ulong(const unsigned long *addr,
>   				      unsigned long *val)
>   {
> -	int ret = 0;
>   	unsigned long rval = ~0ul;
> +	int ret = 0;

Spurious change.

>   
>   	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_EFAUL_REG, %[ret])

I guess you didn't compile that? There is a typo: s/EFAUL/EFAULT/

>   		     : [ret] "+r" (ret), [rval] "+r" (rval)
>   		     : [ptr] "m" (*addr));
>   	*val = rval;
> 
> 


Juergen

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