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

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Date:	Wed, 09 Jul 2008 17:12:14 -0400
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Jeremy Fitzhardinge wrote:
> H. Peter Anvin wrote:
>> 1. it means pda references are invalid if their offsets are ever more 
>> than CONFIG_PHYSICAL_BASE (which I do not think is likely, but still...)
> 
> Why?
> 
> As an aside, could we solve the problems by making CONFIG_PHYSICAL_BASE 
> 0 - putting the percpu variables as the first thing in the kernel - and 
> relocating on load?  That would avoid having to make a special PT_LOAD 
> segment at 0.  Hm, would that result in the pda and the boot params 
> getting mushed together?
> 

CONFIG_PHYSICAL_START rather.  And no, it can't be zero!  Realistically 
we should make it 16 MB by default (currently 2 MB), to keep the DMA 
zone clear.

Either way, I really suspect that the right thing to do is to use 
negative offsets, with the possible exception of a handful of things (40 
bytes or less, perhaps like current) which can get small positive 
offsets and end up in the "super hot" cacheline.

The sucky part is that I don't believe GNU ld has native support for a 
"hanging down" section (one which has a fixed endpoint rather than a 
starting point), so it requires extra magic around the link (or finding 
some way to do it with linker script functions.)  Let me see if I can 
cook up something in linker script that would actually work.

	-hpa

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