From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 40R6lz0TV8zF1yH for ; Thu, 19 Apr 2018 02:20:14 +1000 (AEST) Date: Wed, 18 Apr 2018 19:20:10 +0300 From: "Michael S. Tsirkin" To: Anshuman Khandual Cc: Christoph Hellwig , Benjamin Herrenschmidt , 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: <20180418191722-mutt-send-email-mst@kernel.org> References: <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> <20180415121157.GA17726@infradead.org> <002f153f-452d-f64b-4fc7-7f3383b39162@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <002f153f-452d-f64b-4fc7-7f3383b39162@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, Apr 18, 2018 at 08:47:10AM +0530, Anshuman Khandual wrote: > On 04/15/2018 05:41 PM, Christoph Hellwig wrote: > > 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'. > > How about doing it per platform basis as suggested in this RFC through > an arch specific callback. Because all the virtio devices in the given > platform would require and exercise this option (to avail bounce buffer > mechanism for secure guests as an example). So the flag basically is a > platform specific one not a device specific one. That's not the case. A single platform can have a mix of virtio and non-virtio devices. Same applies even within virtio, e.g. the balloon device always bypasses an iommu. Further, QEMU supports out of process devices some of which might bypass the IOMMU. -- MST