Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754378AbYGIWai (ORCPT ); Wed, 9 Jul 2008 18:30:38 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751522AbYGIWaa (ORCPT ); Wed, 9 Jul 2008 18:30:30 -0400 Received: from terminus.zytor.com ([198.137.202.10]:52506 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751227AbYGIWaa (ORCPT ); Wed, 9 Jul 2008 18:30:30 -0400 Message-ID: <48753A71.2030006@zytor.com> Date: Wed, 09 Jul 2008 18:23:45 -0400 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.14 (X11/20080501) MIME-Version: 1.0 To: "Eric W. Biederman" CC: Jeremy Fitzhardinge , Arjan van de Ven , Ingo Molnar , Mike Travis , Andrew Morton , Christoph Lameter , Jack Steiner , linux-kernel@vger.kernel.org Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses References: <20080709165129.292635000@polaris-admin.engr.sgi.com> <20080709200757.GD14009@elte.hu> <48751B57.8030605@goop.org> <20080709133958.612635f0@infradead.org> <4875231F.1020506@zytor.com> <487524A0.6020304@goop.org> <487529AE.3060505@zytor.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1415 Lines: 35 Eric W. Biederman wrote: >>> >> 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. > > Also on x86_64 CONFIG_PHYSICAL_START is irrelevant as the kernel text segment > is liked at a fixed address -2G and the option only determines the virtual > to physical address mapping. > No, it's not irrelevant; we currently base the kernel at virtual address -2 GB (KERNEL_IMAGE_START) + CONFIG_PHYSICAL_START, in order to have the proper alignment for large pages. Now, it probably wouldn't hurt moving KERNEL_IMAGE_START up a bit to have low positive values safer to use. > That said the idea may not be too far off. > > Potentially we could put the percpu area at our fixed -2G address and then > we have a constant (instead of an address) we could subtract from this address. We can't put it at -2 GB since the offset +40 for the stack sentinel is hard-coded into gcc. This leaves growing upward from +48 (or another small positive number), or growing down from zero (or +40) as realistic options. Unfortunately, GNU ld handles grow-down not at all. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/