Message-ID: <4D935774.302@kernel.org>
Date: Wed, 30 Mar 2011 09:16:52 -0700
From: "H. Peter Anvin" <hpa@kernel.org>
Organization: Linux Kernel Organization, Inc.
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To: Andi Kleen <andi@firstfloor.org>
CC: Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org,
        Christoph Lameter <cl@linux.com>, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Pekka Enberg <penberg@kernel.org>
Subject: Re: [PATCH tip:x86/mm 1/3] x86: A fast way to check capabilities
 of the current cpu
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On 03/30/2011 08:58 AM, H. Peter Anvin wrote:
> On 03/14/2011 12:49 PM, Andi Kleen wrote:
>> Tejun Heo <tj@kernel.org> writes:
>>
>>> Add this_cpu_has() which determines if the current cpu has a certain
>>> ability using a segment prefix and a bit test operation.
>>
>>
>> Hmm: if the CPU capability is really tested in a performance critical
>> path, wouldn't it even be better to just use static_branch() now? 
>>
> 
> We have static_cpu_has() for this specific purpose (it actually predates
> static_branch()).
> 

Note that static_cpu_has() has two caveats:

1. Before alternatives patching, it will always be false.
2. It is the equivalent of boot_cpu_has(), i.e. the intersection of all
the CPU feature sets (available at boot time, obviously.)

	-hpa
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