Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Sat, 5 Jan 2002 23:08:53 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Sat, 5 Jan 2002 23:08:34 -0500 Received: from ip216-26-43-158.dsl.du.teleport.com ([216.26.43.158]:21749 "EHLO w-gerrit2") by vger.kernel.org with ESMTP id ; Sat, 5 Jan 2002 23:08:28 -0500 To: Alan Cox cc: znmeb@aracnet.com (M. Edward Borasky), harald.holzer@eunet.at (Harald Holzer), linux-kernel@vger.kernel.org Reply-To: Gerrit Huizenga From: Gerrit Huizenga Subject: Re: i686 SMP systems with more then 12 GB ram with 2.4.x kernel ? In-Reply-To: Your message of Tue, 01 Jan 2002 18:46:40 GMT. Date: Wed, 02 Jan 2002 13:17:59 -0800 Message-Id: Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org In message , > : Alan Cox writes: > > 2. Isn't the boundary at 2^30 really irrelevant and the three "correct" > > zones are (0 - 2^24-1), (2^24 - 2^32-1) and (2^32 - 2^36-1)? > > Nope. The limit for directly mapped memory is 2^30. The limit *per L1 Page Table Base Pointer*, that is. You could in theory have a different L1 Page Table base pointer for each task (including each proc 0 in linux). You can also pull a few tricks such as instantiating a 4 GB kernel virtual address space while in kernel mode (using a virtual windowing mechanism as is used for high mem today to map in user space for copying in data from user space if/when needed). The latter takes some tricky code to get mapping correct but it wasn't a lot of code in PTX. Just needed a lot of careful thought, review, testing, etc. I don't know if there are real examples of large memory systems exhausting the ~1 GB of kernel virtual address space on machines with > 12-32 GB of physical memory (we had this problem in PTX which created the need for a larger kernel virtual address space in some contexts). > > 3. On a system without ISA DMA devices, can DMA and low be merged into a > > single zone? > > Rarely. PCI vendors are not exactly angels when it comes to implementing > all 32bits of a DMA transfer Would be nice to have a config option like "CONFIG_PCI_36" to imply that all devices on a PAE system were able to access all of memory, globally removing the need for bounce buffering and allowing a native PCI setup for mapping memory addresses... gerrit - 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/