Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932817AbdGXRYJ (ORCPT ); Mon, 24 Jul 2017 13:24:09 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:35640 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753400AbdGXRXY (ORCPT ); Mon, 24 Jul 2017 13:23:24 -0400 Subject: Re: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM SMMUv3 driver To: Alex Williamson Cc: Will Deacon , Anup Patel , Joerg Roedel , Baptiste Reynal , Scott Branden , Linux Kernel , Linux ARM Kernel , Linux IOMMU , kvm@vger.kernel.org, BCM Kernel Feedback References: <1500456838-18405-1-git-send-email-anup.patel@broadcom.com> <1500456838-18405-4-git-send-email-anup.patel@broadcom.com> <20170719112524.GF13642@arm.com> <20170719113325.GI13642@arm.com> <20170719115333.GJ13642@arm.com> <20170720091003.GA17837@arm.com> <8e82d8f5-e5e2-dd09-c774-29f9eda2ecdd@arm.com> <20170724111621.7f1c3a85@w520.home> From: Robin Murphy Message-ID: <6468f359-1682-b9b0-5a4d-72738939cb84@arm.com> Date: Mon, 24 Jul 2017 18:23:20 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170724111621.7f1c3a85@w520.home> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2688 Lines: 54 On 24/07/17 18:16, Alex Williamson wrote: > On Thu, 20 Jul 2017 12:17:12 +0100 > Robin Murphy wrote: > >> On 20/07/17 10:10, Will Deacon wrote: >>> On Thu, Jul 20, 2017 at 09:32:00AM +0530, Anup Patel wrote: >>>> On Wed, Jul 19, 2017 at 5:23 PM, Will Deacon wrote: >>>>> There are two things here: >>>>> >>>>> 1. iommu_present() is pretty useless, because it applies to a "bus" which >>>>> doesn't actually tell you what you need to know for things like the >>>>> platform_bus, where some masters might be upstream of an SMMU and >>>>> others might not be. >>>> >>>> I agree with you. The iommu_present() check in vfio_iommu_group_get() >>>> is not much useful. We only reach line which checks iommu_present() >>>> when iommu_group_get() returns NULL for given "struct device *". If there >>>> is no IOMMU group for a "struct device *" then it means there is no IOMMU >>>> HW doing translations for such device. >>>> >>>> If we drop the iommu_present() check (due to above reasons) in >>>> vfio_iommu_group_get() then we don't require the IOMMU_CAP_BYPASS >>>> and we can happily drop PATCH1, PATCH2, and PATCH3. >>>> >>>> I will remove the iommu_present() check in vfio_iommu_group_get() >>>> because it is only comes into actions when VFIO_NOIOMMU is >>>> enabled. This will also help us drop PATCH1-to-PATCH3. >>> >>> I don't think that's the right answer. Whilst iommu_present has obvious >>> shortcomings, its intention is clear: it should tell you whether a given >>> *device* is upstream of an IOMMU. So the right fix is to make this >>> per-device, instead of per-bus. Removing it altogether is worse than leaving >>> it like it is. >> >> Not really - if there is an IOMMU up and running to the point of setting >> bus ops, every device it cares about can be expected to have a group >> already (there are only a couple of drivers left that don't use groups, >> and they're hardly relevant to VFIO). Thus iommu_group_get() already is >> the de-facto per-device IOMMU check. >> >> And having looked into it, I'm now spinning a couple of patches to >> finish off making groups truly mandatory so that that can be less >> de-facto ;) > > No, look at vfio-noiommu and even vfio-mdev devices for devices which > have an iommu group but there is no physical iommu supporting them. > iommu_present() is how we can distinguish these groups and therefore > not generate a segfault in trying to use the full IOMMU API on them. OK, so that means that the combination of vfio-noiommu and vfio-platform is simply unusable, because iommu_present(&platform_bus_type) can give such dangerous false positives too. Robin.