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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 7938AC433E6 for ; Mon, 15 Mar 2021 01:15:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5266564E7A for ; Mon, 15 Mar 2021 01:15:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229578AbhCOBOs (ORCPT ); Sun, 14 Mar 2021 21:14:48 -0400 Received: from mga07.intel.com ([134.134.136.100]:23170 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229476AbhCOBOn (ORCPT ); Sun, 14 Mar 2021 21:14:43 -0400 IronPort-SDR: 45mmlxA4VBKJkn3x3gMLVvquQJQwS7XitgCE/93r+fvhR7YZcDeCneHhNNu+D0hS6Pdt+UY1ug tJcfw4twMgGQ== X-IronPort-AV: E=McAfee;i="6000,8403,9923"; a="253038255" X-IronPort-AV: E=Sophos;i="5.81,249,1610438400"; d="scan'208";a="253038255" Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Mar 2021 18:14:42 -0700 IronPort-SDR: 6WehyGfaZkgN3csKE1b1b22pkbFj42af9jfpdGakLe0uNVVQ5rKAYCG99RlHI6oErO2bfhEPnz aGsgOE7aPyyQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.81,249,1610438400"; d="scan'208";a="601256882" Received: from unknown (HELO [10.239.154.55]) ([10.239.154.55]) by fmsmga006.fm.intel.com with ESMTP; 14 Mar 2021 18:14:38 -0700 Subject: Re: [PATCH v7] i2c: virtio: add a virtio i2c frontend driver To: Arnd Bergmann Cc: Linux I2C , virtualization@lists.linux-foundation.org, "linux-kernel@vger.kernel.org" , "Michael S. Tsirkin" , Wolfram Sang , Jason Wang , Wolfram Sang , Andy Shevchenko , conghui.chen@intel.com, kblaiech@mellanox.com, jarkko.nikula@linux.intel.com, Sergey Semin , Mike Rapoport , loic.poulain@linaro.org, Tali Perry , =?UTF-8?Q?Uwe_Kleine-K=c3=b6nig?= , Bjorn Andersson , yu1.wang@intel.com, shuo.a.liu@intel.com, Viresh Kumar , Stefan Hajnoczi , Paolo Bonzini References: From: Jie Deng Message-ID: <8070f03d-8233-636b-5ea9-395e723f7a2c@intel.com> Date: Mon, 15 Mar 2021 09:14:37 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-i2c@vger.kernel.org On 2021/3/12 16:58, Arnd Bergmann wrote: > On Fri, Mar 12, 2021 at 2:33 PM Jie Deng wrote: > >> + >> +/** >> + * struct virtio_i2c_req - the virtio I2C request structure >> + * @out_hdr: the OUT header of the virtio I2C message >> + * @buf: the buffer into which data is read, or from which it's written >> + * @in_hdr: the IN header of the virtio I2C message >> + */ >> +struct virtio_i2c_req { >> + struct virtio_i2c_out_hdr out_hdr; >> + uint8_t *buf; >> + struct virtio_i2c_in_hdr in_hdr; >> +}; > The simpler request structure clearly looks better than the previous version, > but I think I found another problem here, at least a theoretical one: > > When you map the headers into the DMA address space, they should > be in separate cache lines, to allow the DMA mapping interfaces to > perform cache management on each one without accidentally clobbering > another member. > > So far I think there is an assumption that virtio buffers are always > on cache-coherent devices, but if you ever have a virtio-i2c device > backend on a physical interconnect that is not cache coherent (e.g. a > microcontroller that shares the memory bus), this breaks down. > > You could avoid this by either allocating arrays of each type separately, > or by marking each member that you pass to the device as > ____cacheline_aligned. > > Arnd The virtio devices are software emulated. The backend software may need to consider this since it may exchange data with physical devices. But I don't think we need it for this interface, because no DMA operation is involved between the frontend driver and backend driver. Regards, Jie