linux-kernel - Re: [PATCH tip:x86/mm 1/3] x86: A fast way to check capabilities of the current cpu

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Date:	Wed, 30 Mar 2011 09:16:52 -0700
From:	"H. Peter Anvin" <hpa@...nel.org>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH tip:x86/mm 1/3] x86: A fast way to check capabilities
 of the current cpu

On 03/30/2011 08:58 AM, H. Peter Anvin wrote:
> On 03/14/2011 12:49 PM, Andi Kleen wrote:
>> Tejun Heo <tj@...nel.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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