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 36038C04AA6 for ; Mon, 29 Apr 2019 11:06:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0F47620673 for ; Mon, 29 Apr 2019 11:06:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727895AbfD2LG6 (ORCPT ); Mon, 29 Apr 2019 07:06:58 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:53646 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727710AbfD2LG5 (ORCPT ); Mon, 29 Apr 2019 07:06:57 -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 F026280D; Mon, 29 Apr 2019 04:06:56 -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 AE03A3F5AF; Mon, 29 Apr 2019 04:06:53 -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> From: Robin Murphy Message-ID: <90182d27-5764-7676-8ca6-b2773a40cfe1@arm.com> Date: Mon, 29 Apr 2019 12:06:52 +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: <93b3d627-782d-cae0-2175-77a5a8b3fe6e@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 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. Either way I think it would be worth just implementing the straightforward version first, then coming back to consider optimisations later. Robin.