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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 5554AC04AA8 for ; Thu, 2 May 2019 01:54:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 28196208C3 for ; Thu, 2 May 2019 01:54:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726338AbfEBByQ (ORCPT ); Wed, 1 May 2019 21:54:16 -0400 Received: from mga18.intel.com ([134.134.136.126]:61668 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726152AbfEBByQ (ORCPT ); Wed, 1 May 2019 21:54:16 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 01 May 2019 18:54:15 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.60,419,1549958400"; d="scan'208";a="145299326" Received: from allen-box.sh.intel.com (HELO [10.239.159.136]) ([10.239.159.136]) by fmsmga008.fm.intel.com with ESMTP; 01 May 2019 18:54:12 -0700 Cc: baolu.lu@linux.intel.com, David Woodhouse , Joerg Roedel , ashok.raj@intel.com, jacob.jun.pan@intel.com, alan.cox@intel.com, kevin.tian@intel.com, mika.westerberg@linux.intel.com, pengfei.xu@intel.com, Konrad Rzeszutek Wilk , Marek Szyprowski , iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free To: Robin Murphy , Christoph Hellwig References: <20190421011719.14909-1-baolu.lu@linux.intel.com> <20190421011719.14909-3-baolu.lu@linux.intel.com> <20190422164555.GA31181@lst.de> <0c6e5983-312b-0d6b-92f5-64861cd6804d@linux.intel.com> <20190423061232.GB12762@lst.de> <20190424144532.GA21480@lst.de> <20190426150433.GA19930@lst.de> <93b3d627-782d-cae0-2175-77a5a8b3fe6e@linux.intel.com> <90182d27-5764-7676-8ca6-b2773a40cfe1@arm.com> <1361b6ab-c3cf-d8ab-5f6b-9d9b7797bf02@linux.intel.com> From: Lu Baolu Message-ID: <998eadf0-0435-1a6b-7234-71554d95bb70@linux.intel.com> Date: Thu, 2 May 2019 09:47:53 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: 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 Robin, On 4/30/19 5:53 PM, Robin Murphy wrote: > On 30/04/2019 03:02, Lu Baolu wrote: >> Hi Robin, >> >> On 4/29/19 7:06 PM, Robin Murphy wrote: >>> On 29/04/2019 06:10, Lu Baolu wrote: >>>> Hi Christoph, >>>> >>>> On 4/26/19 11:04 PM, Christoph Hellwig wrote: >>>>> On Thu, Apr 25, 2019 at 10:07:19AM +0800, Lu Baolu wrote: >>>>>> This is not VT-d specific. It's just how generic IOMMU works. >>>>>> >>>>>> Normally, IOMMU works in paging mode. So if a driver issues DMA with >>>>>> IOVA  0xAAAA0123, IOMMU can remap it with a physical address >>>>>> 0xBBBB0123. >>>>>> But we should never expect IOMMU to remap 0xAAAA0123 with physical >>>>>> address of 0xBBBB0000. That's the reason why I said that IOMMU >>>>>> will not >>>>>> work there. >>>>> >>>>> Well, with the iommu it doesn't happen.  With swiotlb it obviosuly >>>>> can happen, so drivers are fine with it.  Why would that suddenly >>>>> become an issue when swiotlb is called from the iommu code? >>>>> >>>> >>>> I would say IOMMU is DMA remapping, not DMA engine. :-) >>> >>> I'm not sure I really follow the issue here - if we're copying the >>> buffer to the bounce page(s) there's no conceptual difference from >>> copying it to SWIOTLB slot(s), so there should be no need to worry >>> about the original in-page offset. >>> >>>  From the reply up-thread I guess you're trying to include an >>> optimisation to only copy the head and tail of the buffer if it spans >>> multiple pages, and directly map the ones in the middle, but AFAICS >>> that's going to tie you to also using strict mode for TLB >>> maintenance, which may not be a win overall depending on the balance >>> between invalidation bandwidth vs. memcpy bandwidth. At least if we >>> use standard SWIOTLB logic to always copy the whole thing, we should >>> be able to release the bounce pages via the flush queue to allow >>> 'safe' lazy unmaps. >>> >> >> With respect, even we use the standard SWIOTLB logic, we need to use >> the strict mode for TLB maintenance. >> >> Say, some swiotbl slots are used by untrusted device for bounce page >> purpose. When the device driver unmaps the IOVA, the slots are freed but >> the mapping is still cached in IOTLB, hence the untrusted device is >> still able to access the slots. Then the slots are allocated to other >> devices. This makes it possible for the untrusted device to access >> the data buffer of other devices. > > Sure, that's indeed how it would work right now - however since the > bounce pages will be freed and reused by the DMA API layer itself (at > the same level as the IOVAs) I see no technical reason why we couldn't > investigate deferred freeing as a future optimisation. Yes, agreed. Best regards, Lu Baolu