Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752829AbbKJMnS (ORCPT ); Tue, 10 Nov 2015 07:43:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54335 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752654AbbKJMnP (ORCPT ); Tue, 10 Nov 2015 07:43:15 -0500 Date: Tue, 10 Nov 2015 14:43:09 +0200 From: "Michael S. Tsirkin" To: Benjamin Herrenschmidt Cc: Andy Lutomirski , Andy Lutomirski , David Woodhouse , "linux-kernel@vger.kernel.org" , "David S. Miller" , sparclinux@vger.kernel.org, Joerg Roedel , Christian Borntraeger , Cornelia Huck , Sebastian Ott , Paolo Bonzini , Christoph Hellwig , KVM , Martin Schwidefsky , linux-s390 , Linux Virtualization Subject: Re: [PATCH v4 0/6] virtio core DMA API conversion Message-ID: <20151110142633-mutt-send-email-mst@redhat.com> References: <20151109133624-mutt-send-email-mst@redhat.com> <1447109937.31884.42.camel@kernel.crashing.org> <1447121076.31884.61.camel@kernel.crashing.org> <1447133316.31884.67.camel@kernel.crashing.org> <1447151874.31884.82.camel@kernel.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1447151874.31884.82.camel@kernel.crashing.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2163 Lines: 51 On Tue, Nov 10, 2015 at 09:37:54PM +1100, Benjamin Herrenschmidt wrote: > On Mon, 2015-11-09 at 21:35 -0800, Andy Lutomirski wrote: > > > > We could do it the other way around: on powerpc, if a PCI device is in > > that range and doesn't have the "bypass" property at all, then it's > > assumed to bypass the IOMMU.??This means that everything that > > currently works continues working.??If someone builds a physical > > virtio device or uses another system in PCIe target mode speaking > > virtio, then it won't work until they upgrade their firmware to set > > bypass=0.??Meanwhile everyone using hypothetical new QEMU also gets > > bypass=0 and no ambiguity. > > > > vfio will presumably notice the bypass and correctly refuse to map any > > current virtio devices. > > > > Would that work? > > That would be extremely strange from a platform perspective. Any device > in that vendor/device range would bypass the iommu unless some new > property "actually-works-like-a-real-pci-device" happens to exist in > the device-tree, which we would then need to define somewhere and > handle accross at least 3 different platforms who get their device-tree > from widly different places. Then we are back to virtio driver telling DMA core whether it wants a 1:1 mapping in the iommu? If that's acceptable to others, I don't think that's too bad. > Also if tomorrow I create a PCI device that implements virtio-net and > put it in a machine running IBM proprietary firmware (or Apple's or > Sun's), it won't have that property... > > This is not hypothetical. People are using virtio to do point-to-point > communication between machines via PCIe today. > > Cheers, > Ben. But not virtio-pci I think - that's broken for that usecase since we use weaker barriers than required for real IO, as these have measureable overhead. We could have a feature "is a real PCI device", that's completely reasonable. -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/