From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EFB19C2BB55 for ; Fri, 17 Apr 2020 06:50:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DAA9821D82 for ; Fri, 17 Apr 2020 06:50:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728181AbgDQGu7 (ORCPT ); Fri, 17 Apr 2020 02:50:59 -0400 Received: from verein.lst.de ([213.95.11.211]:55946 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726050AbgDQGu6 (ORCPT ); Fri, 17 Apr 2020 02:50:58 -0400 Received: by verein.lst.de (Postfix, from userid 2407) id 22EAA68BEB; Fri, 17 Apr 2020 08:50:55 +0200 (CEST) Date: Fri, 17 Apr 2020 08:50:54 +0200 From: Christoph Hellwig To: Lu Baolu Cc: Christoph Hellwig , Joerg Roedel , ashok.raj@intel.com, jacob.jun.pan@linux.intel.com, kevin.tian@intel.com, Sai Praneeth Prakhya , iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Daniel Drake , Derrick Jonathan , Jerry Snitselaar , Robin Murphy Subject: Re: [PATCH v3 1/3] iommu/vt-d: Allow 32bit devices to uses DMA domain Message-ID: <20200417065054.GA18880@lst.de> References: <20200416062354.10307-1-baolu.lu@linux.intel.com> <20200416062354.10307-2-baolu.lu@linux.intel.com> <20200416070102.GA12588@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 16, 2020 at 03:40:38PM +0800, Lu Baolu wrote: >> description. I'd need to look at the final code, but it seems like >> this will still cause bounce buffering instead of using dynamic >> mapping, which still seems like an awful idea. > > Yes. If the user chooses to use identity domain by default through > kernel command, identity domain will be applied for all devices. For > those devices with limited addressing capability, bounce buffering will > be used when they try to access the memory beyond their address > capability. This won't cause any kernel regression as far as I can see. > > Switching domain during runtime with drivers loaded will cause real > problems as I said in the commit message. That's the reason why I am > proposing to remove it. If we want to keep it, we have to make sure that > switching domain for one device should not impact other devices which > share the same domain with it. Furthermore, it's better to implement it > in the generic layer to keep device driver behavior consistent on all > architectures. I don't disagree with the technical points. What I pointed out is that a) the actual technical change is not in the commit log, which it should be b) that I still think taking away the ability to dynamically map devices in the identify domain after all the time we allowed for that is going to cause nasty regressions.