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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 B6B7FC433E1 for ; Wed, 3 Jun 2020 11:37:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8D20D206C3 for ; Wed, 3 Jun 2020 11:37:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726167AbgFCLhS (ORCPT ); Wed, 3 Jun 2020 07:37:18 -0400 Received: from foss.arm.com ([217.140.110.172]:60264 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725855AbgFCLhR (ORCPT ); Wed, 3 Jun 2020 07:37:17 -0400 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 1859631B; Wed, 3 Jun 2020 04:37:17 -0700 (PDT) Received: from [10.57.0.45] (unknown [10.57.0.45]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5E3693F305; Wed, 3 Jun 2020 04:37:15 -0700 (PDT) Subject: Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF To: Mathieu Poirier , Sai Prakash Ranjan Cc: Suzuki K Poulose , Mike Leach , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, coresight@lists.linaro.org, Stephen Boyd References: <28123d1e19f235f97555ee36a5ed8b52d20cbdea.1590947174.git.saiprakash.ranjan@codeaurora.org> <20200601212858.GB24287@xps15> From: Robin Murphy Message-ID: Date: Wed, 3 Jun 2020 12:37:14 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200601212858.GB24287@xps15> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-06-01 22:28, Mathieu Poirier wrote: > That being said I'm sure that dependencies on an IOMMU isn't a problem confined > to coresight. I am adding Robin Murphy, who added this commit [1], to the thread > in the hope that he can provide guidance on the right way to do this. Right, it's not specific to CoreSight, and it's not even specific to IOMMUs really. In short, blame kexec ;) The fundamental thing is that devices should stop any DMA activity at shutdown. For a normal poweroff you can typically get away without doing so, but over kexec, ongoing DMA traffic may corrupt memory in the new kernel (at worst, I think even DMA reads could potentially cause unexpected cache behaviour that might lead to mishaps, given the right combination of memory attributes). IOMMUs merely help to make the situation more serious. For similar kexec reasons, they need to disable any existing translations at shutdown (imagine if the second kernel didn't have an IOMMU driver). And at that point, even the normal poweroff case becomes problematic, because any device DMA that hasn't been shut down beforehand is now not necessarily going benignly to memory as it would in the no-IOMMU case above, but potentially to random physical addresses, with all the hilarity ensuing that you would expect from that. Robin.