Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934818AbbKSXiS (ORCPT ); Thu, 19 Nov 2015 18:38:18 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:36627 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934665AbbKSXiQ (ORCPT ); Thu, 19 Nov 2015 18:38:16 -0500 Message-ID: <1447976286.145626.122.camel@infradead.org> Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff From: David Woodhouse To: Andy Lutomirski , "Michael S. Tsirkin" Cc: Benjamin Herrenschmidt , Christian Borntraeger , Paolo Bonzini , "linux-kernel@vger.kernel.org" , Martin Schwidefsky , Sebastian Ott , linux-s390 , Cornelia Huck , Joerg Roedel , Linux Virtualization , Christoph Hellwig , KVM Date: Thu, 19 Nov 2015 23:38:06 +0000 In-Reply-To: References: <20151119153821-mutt-send-email-mst@redhat.com> Content-Type: multipart/signed; micalg="sha-1"; protocol="application/x-pkcs7-signature"; boundary="=-jROUKBSz9BgaeBGw8pIB" X-Mailer: Evolution 3.18.1 (3.18.1-1.fc23) Mime-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9730 Lines: 157 --=-jROUKBSz9BgaeBGw8pIB Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2015-11-19 at 13:59 -0800, Andy Lutomirski wrote: >=20 > > > > So thinking hard about it, I don't see any real drawbacks to making thi= s > > conditional on a new feature bit, that Xen can then set.. >=20 > Can you elaborate?=C2=A0 If I run QEMU, hosting Xen, hosting Linux, and t= he > virtio device is provided by QEMU, then how does Xen set the bit? > Similarly, how would Xen set the bit for a real physical device? Right. This is *not* a fundamental characteristic of the device. This is all about how your *particular* hypervisor (in the set of turtles- all-the-way-down) happened to expose the thing to you. This is why it lives in the DMAR table, in the Intel world, which *tells* you which devices are behind which IOMMU (and which are not). And why I keep repeating myself that it has nothing to do with the actual device or the virtio drivers. I understand that POWER and other platforms don't currently have a clean way to indicate that certain device don't have translation. And I understand that we may end up with a *quirk* which ensures that the DMA API does the right thing (i.e. nothing) in certain cases. But we should *NOT* be involving the virtio device drivers in that quirk, in any way. And putting a feature bit in the virtio device itself doesn't seem at all sane either. Bear in mind that qemu-system-x86_64 currently has the *same* problem with assigned physical devices. 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