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 883C8C43219 for ; Tue, 30 Apr 2019 09:53:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6128E21707 for ; Tue, 30 Apr 2019 09:53:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726948AbfD3Jxx (ORCPT ); Tue, 30 Apr 2019 05:53:53 -0400 Received: from foss.arm.com ([217.140.101.70]:42942 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726262AbfD3Jxw (ORCPT ); Tue, 30 Apr 2019 05:53:52 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 1579D80D; Tue, 30 Apr 2019 02:53:52 -0700 (PDT) Received: from [10.1.196.75] (e110467-lin.cambridge.arm.com [10.1.196.75]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B97DC3F5C1; Tue, 30 Apr 2019 02:53:49 -0700 (PDT) Subject: Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free To: Lu Baolu , Christoph Hellwig Cc: 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 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: Robin Murphy Message-ID: Date: Tue, 30 Apr 2019 10:53:48 +0100 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: <1361b6ab-c3cf-d8ab-5f6b-9d9b7797bf02@linux.intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Robin.