From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DDC39C28CF6 for ; Wed, 1 Aug 2018 22:35:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8B71320894 for ; Wed, 1 Aug 2018 22:35:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8B71320894 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732151AbeHBAXd (ORCPT ); Wed, 1 Aug 2018 20:23:33 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:50268 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727002AbeHBAXd (ORCPT ); Wed, 1 Aug 2018 20:23:33 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 70DDE7262C; Wed, 1 Aug 2018 22:35:31 +0000 (UTC) Received: from redhat.com (ovpn-121-8.rdu2.redhat.com [10.10.121.8]) by smtp.corp.redhat.com (Postfix) with SMTP id D600A1C679; Wed, 1 Aug 2018 22:35:27 +0000 (UTC) Date: Thu, 2 Aug 2018 01:35:27 +0300 From: "Michael S. Tsirkin" To: Christoph Hellwig Cc: Will Deacon , Benjamin Herrenschmidt , 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: <20180802013230-mutt-send-email-mst@kernel.org> References: <20180720035941.6844-1-khandual@linux.vnet.ibm.com> <20180727095804.GA25592@arm.com> <20180730093414.GD26245@infradead.org> <20180730125100-mutt-send-email-mst@kernel.org> <20180730111802.GA9830@infradead.org> <20180730155633-mutt-send-email-mst@kernel.org> <20180731173052.GA17153@infradead.org> <3d6e81511571260de1c8047aaffa8ac4df093d2e.camel@kernel.crashing.org> <20180801081637.GA14438@arm.com> <20180801083639.GF26378@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180801083639.GF26378@infradead.org> X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Wed, 01 Aug 2018 22:35:31 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Wed, 01 Aug 2018 22:35:31 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'mst@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 01, 2018 at 01:36:39AM -0700, Christoph Hellwig wrote: > On Wed, Aug 01, 2018 at 09:16:38AM +0100, Will Deacon wrote: > > On arm/arm64, the problem we have is that legacy virtio devices on the MMIO > > transport (so definitely not PCI) have historically been advertised by qemu > > as not being cache coherent, but because the virtio core has bypassed DMA > > ops then everything has happened to work. If we blindly enable the arch DMA > > ops, > > No one is suggesting that as far as I can tell. > > > we'll plumb in the non-coherent ops and start getting data corruption, > > so we do need a way to quirk virtio as being "always coherent" if we want to > > use the DMA ops (which we do, because our emulation platforms have an IOMMU > > for all virtio devices). > > >From all that I've gather so far: no you do not want that. We really > need to figure out virtio "dma" interacts with the host / device. > > If you look at the current iommu spec it does talk of physical address > with a little careveout for VIRTIO_F_IOMMU_PLATFORM. > > So between that and our discussion in this thread and its previous > iterations I think we need to stick to the current always physical, > bypass system dma ops mode of virtio operation as the default. > > We just need to figure out how to deal with devices that deviate > from the default. One things is that VIRTIO_F_IOMMU_PLATFORM really > should become VIRTIO_F_PLATFORM_DMA to cover the cases of non-iommu > dma tweaks (offsets, cache flushing), which seems well in spirit of > the original design. Well I wouldn't say that. VIRTIO_F_IOMMU_PLATFORM is for guest programmable protection which is designed for things like userspace drivers but still very much which a CPU doing the accesses. I think VIRTIO_F_IO_BARRIER needs to be extended to VIRTIO_F_PLATFORM_DMA. > The other issue is VIRTIO_F_IO_BARRIER > which is very vaguely defined, and which needs a better definition. > And last but not least we'll need some text explaining the challenges > of hardware devices - I think VIRTIO_F_PLATFORM_DMA + VIRTIO_F_IO_BARRIER > is what would basically cover them, but a good description including > an explanation of why these matter. I think VIRTIO_F_IOMMU_PLATFORM + VIRTIO_F_PLATFORM_DMA but yea. -- MST