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=-10.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 E7514C07E96 for ; Thu, 8 Jul 2021 14:43:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C23356162D for ; Thu, 8 Jul 2021 14:43:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231933AbhGHOqe (ORCPT ); Thu, 8 Jul 2021 10:46:34 -0400 Received: from foss.arm.com ([217.140.110.172]:60604 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229738AbhGHOqd (ORCPT ); Thu, 8 Jul 2021 10:46:33 -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 A4B321424; Thu, 8 Jul 2021 07:43:51 -0700 (PDT) Received: from [10.57.35.192] (unknown [10.57.35.192]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C709D3F66F; Thu, 8 Jul 2021 07:43:50 -0700 (PDT) Subject: Re: [PATCH] iommu/amd: Enable swiotlb if any device supports iommu v2 and uses identity mapping To: Kai-Heng Feng Cc: will@kernel.org, "open list:AMD IOMMU (AMD-VI)" , open list , Joerg Roedel References: <20210708074232.924844-1-kai.heng.feng@canonical.com> From: Robin Murphy Message-ID: Date: Thu, 8 Jul 2021 15:43:42 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-07-08 14:57, Kai-Heng Feng wrote: > On Thu, Jul 8, 2021 at 6:18 PM Robin Murphy wrote: >> >> On 2021-07-08 10:28, Joerg Roedel wrote: >>> On Thu, Jul 08, 2021 at 03:42:32PM +0800, Kai-Heng Feng wrote: >>>> @@ -344,6 +344,9 @@ static int iommu_init_device(struct device *dev) >>>> >>>> iommu = amd_iommu_rlookup_table[dev_data->devid]; >>>> dev_data->iommu_v2 = iommu->is_iommu_v2; >>>> + >>>> + if (dev_data->iommu_v2) >>>> + swiotlb = 1; >>> >>> This looks like the big hammer, as it will affect all other systems >>> where the AMD GPUs are in their own group. >>> >>> What is needed here is an explicit check whether a non-iommu-v2 device >>> is direct-mapped because it shares a group with the GPU, and only enable >>> swiotlb in this case. >> >> Right, it's basically about whether any DMA-limited device might at any >> time end up in an IOMMU_DOMAIN_IDENTITY domain. And given the >> possibility of device hotplug and the user being silly with the sysfs >> interface, I don't think we can categorically determine that at boot time. >> >> Also note that Intel systems are likely to be similarly affected (in >> fact intel-iommu doesn't even have the iommu_default_passthough() check >> so it's probably even easier to blow up). > > swiotlb is enabled by pci_swiotlb_detect_4gb() and intel-iommu doesn't > disable it. Oh, right... I did say I found this dance hard to follow. Clearly I shouldn't have trusted what I thought I remembered from looking at it yesterday :) Also not helped by the fact that it sets iommu_detected which *does* disable SWIOTLB, but only on IA-64. > I wonder if we can take the same approach in amd-iommu? Certainly if there's a precedent for leaving SWIOTLB enabled even if it *might* be redundant, that seems like the easiest option (it's what we do on arm64 too, but then we have system topologies where some devices may not be behind IOMMUs even when others are). More fun would be to try to bring it up at the first sign of IOMMU_DOMAIN_IDENTITY if it was disabled previously, but I don't have the highest hope of that being practical. Robin.