From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752410AbeDOMME (ORCPT ); Sun, 15 Apr 2018 08:12:04 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:36282 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751190AbeDOMMC (ORCPT ); Sun, 15 Apr 2018 08:12:02 -0400 Date: Sun, 15 Apr 2018 05:11:57 -0700 From: Christoph Hellwig To: Benjamin Herrenschmidt Cc: Christoph Hellwig , Anshuman Khandual , "Michael S. Tsirkin" , robh@kernel.org, aik@ozlabs.ru, jasowang@redhat.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, joe@perches.com, david@gibson.dropbear.id.au, linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net Subject: Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU Message-ID: <20180415121157.GA17726@infradead.org> References: <20180405105631.9514-1-khandual@linux.vnet.ibm.com> <3e1b113b-79ca-b700-5be9-10c66d74aabe@linux.vnet.ibm.com> <20180405175326-mutt-send-email-mst@kernel.org> <1522940983.21446.205.camel@kernel.crashing.org> <20180405213042-mutt-send-email-mst@kernel.org> <1522963113.21446.211.camel@kernel.crashing.org> <70cb433f-a8f7-5199-3c96-a760c7624804@linux.vnet.ibm.com> <20180406071634.GA31108@infradead.org> <1523003838.21446.238.camel@kernel.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1523003838.21446.238.camel@kernel.crashing.org> User-Agent: Mutt/1.9.2 (2017-12-15) 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 On Fri, Apr 06, 2018 at 06:37:18PM +1000, Benjamin Herrenschmidt wrote: > > > implemented as DMA API which the virtio core understands. There is no > > > need for an IOMMU to be involved for the device representation in this > > > case IMHO. > > > > This whole virtio translation issue is a mess. I think we need to > > switch it to the dma API, and then quirk the legacy case to always > > use the direct mapping inside the dma API. > > Fine with using a dma API always on the Linux side, but we do want to > special case virtio still at the arch and qemu side to have a "direct > mapping" mode. Not sure how (special flags on PCI devices) to avoid > actually going through an emulated IOMMU on the qemu side, because that > slows things down, esp. with vhost. > > IE, we can't I think just treat it the same as a physical device. We should have treated it like a physical device from the start, but that device has unfortunately sailed. But yes, we'll need a per-device quirk that says 'don't attach an iommu'.