linux-kernel - Re: [PATCH] x86: Replace uses of current_cpu_data with this

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Date:	Wed, 08 Dec 2010 18:57:06 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>, akpm@...ux-foundation.org,
	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] x86: Replace uses of current_cpu_data with this_cpu ops

Hello, Christoph.

On 12/08/2010 06:33 PM, Christoph Lameter wrote:
> Beginnings of a patch to avoid bitops. If you think this is the right way
> then I will complete it for all uses of cpu_has.
> 
> ---
>  arch/x86/include/asm/cpufeature.h |   15 +++++++++++++++
>  arch/x86/kernel/apic/apic.c       |    2 +-
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/arch/x86/include/asm/cpufeature.h
> ===================================================================
> --- linux-2.6.orig/arch/x86/include/asm/cpufeature.h	2010-12-08 09:39:31.000000000 -0600
> +++ linux-2.6/arch/x86/include/asm/cpufeature.h	2010-12-08 09:40:02.000000000 -0600
> @@ -221,6 +221,21 @@ extern const char * const x86_power_flag
>  	  ? 1 :								\
>  	 test_cpu_cap(c, bit))
> 
> +#define this_cpu_has(bit)							\
> +	(__builtin_constant_p(bit) &&					\
> +	 ( (((bit)>>5)==0 && (1UL<<((bit)&31) & REQUIRED_MASK0)) ||	\
> +	   (((bit)>>5)==1 && (1UL<<((bit)&31) & REQUIRED_MASK1)) ||	\
> +	   (((bit)>>5)==2 && (1UL<<((bit)&31) & REQUIRED_MASK2)) ||	\
> +	   (((bit)>>5)==3 && (1UL<<((bit)&31) & REQUIRED_MASK3)) ||	\
> +	   (((bit)>>5)==4 && (1UL<<((bit)&31) & REQUIRED_MASK4)) ||	\
> +	   (((bit)>>5)==5 && (1UL<<((bit)&31) & REQUIRED_MASK5)) ||	\
> +	   (((bit)>>5)==6 && (1UL<<((bit)&31) & REQUIRED_MASK6)) ||	\
> +	   (((bit)>>5)==7 && (1UL<<((bit)&31) & REQUIRED_MASK7)) ||	\
> +	   (((bit)>>5)==8 && (1UL<<((bit)&31) & REQUIRED_MASK8)) ||	\
> +	   (((bit)>>5)==9 && (1UL<<((bit)&31) & REQUIRED_MASK9)) )	\
> +	  ? 1 :								\

It would be nice to factor out the above so that it's not repated for
cpu_has() and this_cpu_has().

> +	 (this_cpu_read(cpu_info.x86_capabilty) & (1 << bit)))

bit can go beyond unsigned, so 1 << bit doesn't work.  Why not just
start with a simple wrapper?  ie. sth like

#define this_cpu_has(bit)	cpu_has(__this_cpu_ptr(&cpu_info), bit)

We can later optimize in a separate patch along the line of...

#define this_cpu_has(bit) ({						\
	__builtin_constant_p(bit) ?					\
		(cpu_required_bit_test(bit) ||				\
		 this_cpu_read(cpu_info.x86_capability + ((bit) >> 5)) & \
		 (1 << ((bit) & 31))) :					\
		cpu_has(__this_cpu_ptr(&cpu_info), bit);		\
})

Thanks.

-- 
tejun
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