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 A9920C76190 for ; Fri, 26 Jul 2019 01:57:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 88A4E22C7C for ; Fri, 26 Jul 2019 01:57:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726109AbfGZB5c (ORCPT ); Thu, 25 Jul 2019 21:57:32 -0400 Received: from mga12.intel.com ([192.55.52.136]:24504 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725851AbfGZB5c (ORCPT ); Thu, 25 Jul 2019 21:57:32 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga106.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 25 Jul 2019 18:57:31 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,308,1559545200"; d="scan'208";a="181722703" Received: from allen-box.sh.intel.com (HELO [10.239.159.136]) ([10.239.159.136]) by orsmga002.jf.intel.com with ESMTP; 25 Jul 2019 18:57:26 -0700 Cc: baolu.lu@linux.intel.com, David Woodhouse , Joerg Roedel , Bjorn Helgaas , ashok.raj@intel.com, jacob.jun.pan@intel.com, alan.cox@intel.com, kevin.tian@intel.com, mika.westerberg@linux.intel.com, Ingo Molnar , Greg Kroah-Hartman , pengfei.xu@intel.com, Konrad Rzeszutek Wilk , Marek Szyprowski , Robin Murphy , Jonathan Corbet , Boris Ostrovsky , Juergen Gross , Stefano Stabellini , Steven Rostedt , iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Jacob Pan Subject: Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops To: Christoph Hellwig References: <20190725031717.32317-1-baolu.lu@linux.intel.com> <20190725031717.32317-3-baolu.lu@linux.intel.com> <20190725054413.GC24527@lst.de> <20190725114348.GA30957@lst.de> From: Lu Baolu Message-ID: Date: Fri, 26 Jul 2019 09:56:51 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20190725114348.GA30957@lst.de> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 7/25/19 7:43 PM, Christoph Hellwig wrote: > On Thu, Jul 25, 2019 at 03:18:03PM +0800, Lu Baolu wrote: >>> Don't we need to keep this bit so that we still allow the IOMMU >>> to act if the device has a too small DMA mask to address all memory in >>> the system, even if if it should otherwise be identity mapped? >>> >> >> This checking happens only when device is using an identity mapped >> domain. If the device has a small DMA mask, swiotlb will be used for >> high memory access. >> >> This is supposed to be handled in dma_direct_map_page(): >> >> if (unlikely(!dma_direct_possible(dev, dma_addr, size)) && >> !swiotlb_map(dev, &phys, &dma_addr, size, dir, attrs)) { >> report_addr(dev, dma_addr, size); >> return DMA_MAPPING_ERROR; >> } > > Well, yes. But the point is that the current code uses dynamic iommu > mappings even if the devices is in the identity mapped domain when the > dma mask іs too small to map all memory directly. Your change means it > will now use swiotlb which is most likely going to be a lot more By default, we use DMA domain. The privileged users are able to change this with global kernel parameter or per-group default domain type under discussion. In another word, use of identity domain is a choice of the privileged user who should consider the possible bounce buffer overhead. I think current code doesn't do the right thing. The user asks the iommu driver to use identity domain for a device, but the driver force it back to DMA domain because of the device address capability. > expensive. I don't think that this change is a good idea, and even if > we decide that this is a good idea after all that should be done in a > separate prep patch that explains the rationale. Yes. Make sense. Best regards, Baolu