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 11:15:38 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Tejun Heo <tj@...nel.org>
cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	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 Wed, 30 Mar 2011, Tejun Heo wrote:

> Hello,
>
> On Wed, Mar 30, 2011 at 08:58:23AM -0700, H. Peter Anvin wrote:
> > >> 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()).
>
> These patches have performance benefits but I don't think it would be
> noticeable.  I think it's more of code cleanup.

The main simplification is that the pointer to the cpu_info can be
omitted. Its passed around quite a bit right now in x86 arch code.
Removal of the cpu_info pointer could reduce code size and allow faster
access.


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