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 41jh4G1bCXzF3H9 for ; Sun, 5 Aug 2018 10:10:04 +1000 (AEST) Date: Sun, 5 Aug 2018 03:09:55 +0300 From: "Michael S. Tsirkin" To: Christoph Hellwig Cc: Benjamin Herrenschmidt , Will Deacon , Anshuman Khandual , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, aik@ozlabs.ru, robh@kernel.org, joe@perches.com, elfring@users.sourceforge.net, david@gibson.dropbear.id.au, jasowang@redhat.com, mpe@ellerman.id.au, linuxram@us.ibm.com, haren@linux.vnet.ibm.com, paulus@samba.org, srikar@linux.vnet.ibm.com, robin.murphy@arm.com, jean-philippe.brucker@arm.com, marc.zyngier@arm.com Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Message-ID: <20180805030326-mutt-send-email-mst@kernel.org> References: <26c1d3d50d8e081eed44fe9940fbefed34598cbd.camel@kernel.crashing.org> <20180802182959-mutt-send-email-mst@kernel.org> <82ccef6ec3d95ee43f3990a4a2d0aea87eb45e89.camel@kernel.crashing.org> <20180802200646-mutt-send-email-mst@kernel.org> <20180802225738-mutt-send-email-mst@kernel.org> <20180803070507.GA1344@infradead.org> <20180803221634-mutt-send-email-mst@kernel.org> <20180804081500.GA1455@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20180804081500.GA1455@infradead.org> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Sat, Aug 04, 2018 at 01:15:00AM -0700, Christoph Hellwig wrote: > On Fri, Aug 03, 2018 at 10:17:32PM +0300, Michael S. Tsirkin wrote: > > It seems reasonable to teach a platform to override dma-range > > for a specific device e.g. in case it knows about bugs in ACPI. > > A platform will be able override dma-range using the dev->bus_dma_mask > field starting in 4.19. But we'll still need a way how to > > a) document in the virtio spec that all bus dma quirks are to be > applied I agree it's a good idea. In particular I suspect that PLATFORM_IOMMU should be extended to cover that. But see below. > b) a way to document in a virtio-related spec how the bus handles > dma for Ben's totally fucked up hypervisor. Without that there > is not way we'll get interoperable implementations. So in this case however I'm not sure what exactly do we want to add. It seems that from point of view of the device, there is nothing special - it just gets a PA and writes there. It also seems that guest does not need to get any info from the device either. Instead guest itself needs device to DMA into specific addresses, for its own reasons. It seems that the fact that within guest it's implemented using a bounce buffer and that it's easiest to do by switching virtio to use the DMA API isn't something virtio spec concerns itself with. I'm open to suggestions. -- MST