Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S265418AbTGCWGM (ORCPT ); Thu, 3 Jul 2003 18:06:12 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S265429AbTGCWGM (ORCPT ); Thu, 3 Jul 2003 18:06:12 -0400 Received: from dsl2.external.hp.com ([192.25.206.7]:24591 "EHLO dsl2.external.hp.com") by vger.kernel.org with ESMTP id S265418AbTGCWFA (ORCPT ); Thu, 3 Jul 2003 18:05:00 -0400 Date: Thu, 3 Jul 2003 16:19:27 -0600 From: Grant Grundler To: Andi Kleen Cc: Alan Cox , Grant Grundler , James Bottomley , axboe@suse.de, davem@redhat.com, suparna@in.ibm.com, Linux Kernel Mailing List , alex_williamson@hp.com, bjorn_helgaas@hp.com Subject: Re: [RFC] block layer support for DMA IOMMU bypass mode II Message-ID: <20030703221927.GC12433@dsl2.external.hp.com> References: <1057077975.2135.54.camel@mulgrave> <20030702015701.6007ac26.ak@suse.de> <20030702165510.GC11739@dsl2.external.hp.com> <1057180598.20318.32.camel@dhcp22.swansea.linux.org.uk> <20030702235619.GA21567@wotan.suse.de> <1057263988.21508.18.camel@dhcp22.swansea.linux.org.uk> <20030703212415.GA30277@wotan.suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20030703212415.GA30277@wotan.suse.de> User-Agent: Mutt/1.3.28i X-Home-Page: http://www.parisc-linux.org/ Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1046 Lines: 25 On Thu, Jul 03, 2003 at 11:24:15PM +0200, Andi Kleen wrote: ... > Also it's likely cheaper just submit more segments than to have the IOMMU > overhead It depends on the device. If using something like 8237A to master DMA cycles, then CPU cost of merging is relatively cheap. If sending the SG list is just a sequence of MMIO space writes, then passing the raw list is cheaper. ZX1 and PARISC IOMMUs clearly add some overhead both in terms of CPU utilization (manage IOMMU) and DMA latency (IOMMU TLB misses sometimes). ... > (at least for sane devices, if not it may be worth to artificially limit the > dma mask of the device to force IOMMU on IA64 and x86-64) Agreed. We are only doing that until BIO code and IOMMU code can agree on how merging works without requiring the IOMMU. thanks, grant - 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/