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 9E71AC32771 for ; Tue, 21 Jan 2020 00:45:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 781B724125 for ; Tue, 21 Jan 2020 00:45:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728827AbgAUApX (ORCPT ); Mon, 20 Jan 2020 19:45:23 -0500 Received: from mga03.intel.com ([134.134.136.65]:34875 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727009AbgAUApX (ORCPT ); Mon, 20 Jan 2020 19:45:23 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga103.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 20 Jan 2020 16:45:22 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,343,1574150400"; d="scan'208";a="215371656" Received: from allen-box.sh.intel.com (HELO [10.239.159.138]) ([10.239.159.138]) by orsmga007.jf.intel.com with ESMTP; 20 Jan 2020 16:45:20 -0800 Cc: baolu.lu@linux.intel.com, kevin.tian@intel.com, ashok.raj@intel.com, linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, jacob.jun.pan@intel.com, Robin Murphy , Christoph Hellwig Subject: Re: [RFC PATCH 0/4] iommu: Per-group default domain type To: John Garry , Joerg Roedel , Greg Kroah-Hartman , Bjorn Helgaas References: <20200101052648.14295-1-baolu.lu@linux.intel.com> From: Lu Baolu Message-ID: Date: Tue, 21 Jan 2020 08:43:52 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi John, On 1/20/20 5:44 PM, John Garry wrote: > On 01/01/2020 05:26, Lu Baolu wrote: >> An IOMMU group represents the smallest set of devices that are considered >> to be isolated. All devices belonging to an IOMMU group share a default >> domain for DMA APIs. There are two types of default domain: >> IOMMU_DOMAIN_DMA >> and IOMMU_DOMAIN_IDENTITY. The former means IOMMU translation, while the >> latter means IOMMU by-pass. >> >> Currently, the default domain type for the IOMMU groups is determined >> globally. All IOMMU groups use a single default domain type. The global >> default domain type can be adjusted by kernel build configuration or >> kernel parameters. >> >> More and more users are looking forward to a fine grained default domain >> type. For example, with the global default domain type set to >> translation, >> the OEM verndors or end users might want some trusted and fast-speed >> devices >> to bypass IOMMU for performance gains. On the other hand, with global >> default domain type set to by-pass, some devices with limited system >> memory addressing capability might want IOMMU translation to remove the >> bounce buffer overhead. > > Hi Lu Baolu, > > Do you think that it would be a more common usecase to want > kernel-managed devices to be passthrough for performance reasons and > some select devices to be in DMA domain, like those with limited address > cap or whose drivers request huge amounts of memory? > > I just think it would be more manageable to set kernel commandline > parameters for this, i.e. those select few which want DMA domain. > It's just two sides of a coin. Currently, iommu subsystem make DMA domain by default, that's the reason why I selected to let user set which devices are willing to use identity domains. > Thanks, > John Best regards, baolu