Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753262AbcD0PLF (ORCPT ); Wed, 27 Apr 2016 11:11:05 -0400 Received: from mail-oi0-f45.google.com ([209.85.218.45]:36756 "EHLO mail-oi0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752205AbcD0PLC (ORCPT ); Wed, 27 Apr 2016 11:11:02 -0400 MIME-Version: 1.0 In-Reply-To: <20160427175031-mutt-send-email-mst@redhat.com> References: <1461245745-6710-1-git-send-email-mst@redhat.com> <20160421135416.GE11775@citrix.com> <1461759501.118304.149.camel@infradead.org> <20160427153345-mutt-send-email-mst@redhat.com> <20160427142331.GH17926@8bytes.org> <20160427173603-mutt-send-email-mst@redhat.com> <20160427175031-mutt-send-email-mst@redhat.com> From: Andy Lutomirski Date: Wed, 27 Apr 2016 08:10:42 -0700 Message-ID: Subject: Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api To: "Michael S. Tsirkin" Cc: Joerg Roedel , David Woodhouse , Kevin Wolf , Wei Liu , Andy Lutomirski , qemu-block@nongnu.org, Christian Borntraeger , Jason Wang , Stefano Stabellini , "qemu-devel@nongnu.org Developers" , peterx@redhat.com, "linux-kernel@vger.kernel.org" , Amit Shah , iommu@lists.linux-foundation.org, Stefan Hajnoczi , kvm list , Cornelia Huck , Paolo Bonzini , Linux Virtualization , Anthony PERARD Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3227 Lines: 68 On Wed, Apr 27, 2016 at 7:54 AM, Michael S. Tsirkin wrote: > On Wed, Apr 27, 2016 at 07:43:07AM -0700, Andy Lutomirski wrote: >> On Wed, Apr 27, 2016 at 7:38 AM, Michael S. Tsirkin wrote: >> > On Wed, Apr 27, 2016 at 07:31:43AM -0700, Andy Lutomirski wrote: >> >> On Wed, Apr 27, 2016 at 7:23 AM, Joerg Roedel wrote: >> >> > On Wed, Apr 27, 2016 at 04:37:04PM +0300, Michael S. Tsirkin wrote: >> >> >> One correction: it's a feature of the device in the system. >> >> >> There could be a mix of devices bypassing and not >> >> >> bypassing the IOMMU. >> >> > >> >> > No, it really is not. A device can't chose to bypass the IOMMU. But the >> >> > IOMMU can chose to let the device bypass. So any fix here belongs >> >> > into the platform/iommu code too and not into some driver. >> >> > >> >> >> Sounds good. And a way to detect appropriate devices could >> >> >> be by looking at the feature flag, perhaps? >> >> > >> >> > Again, no! The way to detect that is to look into the iommu description >> >> > structures provided by the firmware. They provide everything necessary >> >> > to tell the iommu code which devices are not translated. >> >> > >> >> >> >> Except on PPC and SPARC. As far as I know, those are the only >> >> problematic platforms. >> >> >> >> Is it too late to *disable* QEMU's q35-iommu thingy until it can be >> >> fixed to report correct data in the DMAR tables? >> >> >> >> --Andy >> > >> > Meaning virtio or assigned devices? >> > For virtio - it's way too late since these are working configurations. >> > For assigned devices - they don't work on x86 so it doesn't have >> > to be disabled, it's safe to ignore. >> >> I mean actually prevent QEMU from running in q35-iommu mode with any >> virtio devices attached or maybe even turn off q35-iommu mode entirely >> [1]. Doesn't it require that the user literally pass the word >> "experimental" into QEMU right now? It did at some point IIRC. >> >> The reason I'm asking is that, other than q35-iommu, QEMU's virtio >> devices *don't* bypass the IOMMU except on PPC and SPARC, simply >> because there is no other configuration AFAICT that has virtio and and >> IOMMU. So maybe the right solution is to fix q35-iommu to use DMAR >> correctly (thus breaking q35-iommu users with older guest kernels, >> which hopefully don't actually exist) and to come up with a PPC- and >> SPARC-specific solution, or maybe OpenFirmware-specific solution, to >> handle PPC and SPARC down the road. >> >> [1] I'm pretty sure I emailed the QEMU list before q35-iommu ever >> showed up in a release asking the QEMU team to please not do that >> until this issue was resolved. Sadly, that email was ignored :( >> >> --Andy > > Sorry, I didn't make myself clear. > Point is, QEMU is not the only virtio implementation out there. > So we can't know no virtio implementations have an IOMMU as long as > linux supports this IOMMU. > virtio always used physical addresses since it was born and if it > changes that it must do this in a way that does not break existing > users. Is there any non-QEMU virtio implementation can provide an IOMMU-bypassing virtio device on a platform that has a nontrivial IOMMU? --Andy