linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Date:	Thu, 10 Jul 2008 12:42:25 -0500
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Jeremy Fitzhardinge wrote:

> If %gs:0 points to start of your percpu area, then all the offsets off
> %gs are going to be no larger than the amount of percpu memory you
> have.  The gs base itself can be any 64-bit address, so it doesn't
> matter where it is within overall kernel memory.  Using zero-based
> percpu area means that you must set a non-zero %gs base before you can
> access the percpu area.

Correct.

> If the layout of the percpu area is done by the linker by packing all
> the percpu variables into one section, then any address computation
> using a percpu variable symbol will generate an offset which is
> appropriate to apply to a %gs: addressing mode.

Of course.

> The nice thing about the non-zero-based scheme i386 uses is that setting
> gs-base to zero means that percpu variables accesses get directly to the
> prototype percpu data area, which simplifies boot time setup (which is
> doubly awkward on 32-bit because you need to generate a GDT entry rather
> than just load an MSR as you do in 64-bit).

Great but it causes trouble in other ways as discussed. Its best to consistently access
per cpu variables using the segment registers.
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