linux-kernel - Re: [PATCH 2/5] arm: LLVMLinux: use current_stack

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Message-ID: <yw1xioyd1t1d.fsf@unicorn.mansr.com>
Date:	Fri, 06 Sep 2013 23:31:26 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	behanw@...verseincode.com
Cc:	linux@....linux.org.uk, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mark Charlebois <charlebm@...il.com>,
	linux-arm-kernel@...ts.infradead.org (moderated list:ARM PORT),
	linux-kernel@...r.kernel.org (open list)
Subject: Re: [PATCH 2/5] arm: LLVMLinux: use current_stack_pointer for percpu

behanw@...verseincode.com writes:

> From: Behan Webster <behanw@...verseincode.com>
>
> The existing code uses named registers to get the value of the stack pointer.
> The new current_stack_pointer macro is more readable and allows for a central
> portable implementation of how to get the stack pointer with ASM.  This change
> supports being able to compile the kernel with both gcc and Clang.
>
> Signed-off-by: Mark Charlebois <charlebm@...il.com>
> Signed-off-by: Behan Webster <behanw@...verseincode.com>
> Reviewed-by: Jan-Simon Möller <dl9pf@....de>
> ---
>  arch/arm/include/asm/percpu.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/percpu.h b/arch/arm/include/asm/percpu.h
> index 209e650..629a975 100644
> --- a/arch/arm/include/asm/percpu.h
> +++ b/arch/arm/include/asm/percpu.h
> @@ -30,14 +30,14 @@ static inline void set_my_cpu_offset(unsigned long off)
>  static inline unsigned long __my_cpu_offset(void)
>  {
>  	unsigned long off;
> -	register unsigned long *sp asm ("sp");
> +	unsigned long sp = current_stack_pointer;
>
>  	/*
>  	 * Read TPIDRPRW.
>  	 * We want to allow caching the value, so avoid using volatile and
>  	 * instead use a fake stack read to hazard against barrier().
>  	 */
> -	asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (off) : "Q" (*sp));
> +	asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (off) : "Q" (sp));

This doesn't do quite the same thing.  The existing code pretends to
read something from the stack in order to create a barrier of some
sort.  Your new code stores the value of the stack pointer to a location
on the stack for consumption by the "Q" memory constraint.  This store
is not necessary and should preferably be avoided.

-- 
Måns Rullgård
mans@...sr.com
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