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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5BFF0C636CC for ; Wed, 15 Feb 2023 11:09:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230212AbjBOLJd (ORCPT ); Wed, 15 Feb 2023 06:09:33 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50878 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229503AbjBOLJb (ORCPT ); Wed, 15 Feb 2023 06:09:31 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id C41A21F5EB for ; Wed, 15 Feb 2023 03:09:30 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2FCBB1042; Wed, 15 Feb 2023 03:10:13 -0800 (PST) Received: from [10.57.90.25] (unknown [10.57.90.25]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id CAC213F703; Wed, 15 Feb 2023 03:09:28 -0800 (PST) Message-ID: Date: Wed, 15 Feb 2023 11:09:22 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:102.0) Gecko/20100101 Thunderbird/102.7.2 Subject: Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment Content-Language: en-GB To: Baolu Lu , "Tian, Kevin" , Jason Gunthorpe Cc: "iommu@lists.linux.dev" , Joerg Roedel , Christoph Hellwig , Will Deacon , "linux-kernel@vger.kernel.org" References: <20230213074941.919324-1-baolu.lu@linux.intel.com> <20230213074941.919324-3-baolu.lu@linux.intel.com> <4ac228af-fb74-2d89-b8d7-462186170e12@linux.intel.com> From: Robin Murphy In-Reply-To: <4ac228af-fb74-2d89-b8d7-462186170e12@linux.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023-02-15 07:28, Baolu Lu wrote: > On 2023/2/15 14:56, Tian, Kevin wrote: >>> From: Baolu Lu >>> Sent: Wednesday, February 15, 2023 1:51 PM >>> >>> On 2/13/23 10:19 PM, Jason Gunthorpe wrote: >>>> On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote: >>>>> @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct >>> iommu_group *group, >>>>>        else >>>>>            return -EINVAL; >>>>> >>>>> +    if (req_type != IOMMU_DOMAIN_DMA_FQ || >>>>> +        group->default_domain->type != IOMMU_DOMAIN_DMA) { >>>>> +        ret = iommu_group_claim_dma_owner(group, (void *)buf); >>>>> +        if (ret) >>>>> +            return ret; >>>>> +        group_owner_claimed = true; >>>>> +    } >>>> I don't get it, this should be done unconditionally. If we couldn't >>>> take ownership then we simply can't progress. >>> The existing code allows the user to switch the default domain from >>> strict to lazy invalidation mode. The default domain is not changed, >>> hence it should be seamless and transparent to the device driver. >> Is there real usage relying on this transition for a bound device? >> >> In concept strict->lazy transition implies relaxed DMA security. It's >> hard >> to think of a motivation of doing so while the device might be doing >> in-fly DMAs. >> >> Presumably such perf/security tradeoff should be planned way before >> binding device/driver together. >> >> btw if strict->lazy is allowed why lazy->strict is prohibited? >> > > We all know, strict vs. lazy is a tradeoff between performance and > security. > > strict -> lazy: driver works in secure mode. This transition trades off > security for better performance. > > lazy->strict: The driver is already working in non-safety mode. This > transition only results in worse performance. It makes no sense. If user > want to put the driver in a secure mode, they need to unbind the driver, > reset the device and do the lazy->strict transition. > > Robin might have better insights. Yes, this was added for a definite use-case in ChromeOS, where strict->lazy needs to support being done "live" since the device in question is the storage controller for the already-mounted root filesystem. Your reasoning seems to match what I summarised in the original commit message :) Thanks, Robin.