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 41hDtF6jyJzF2DC for ; Fri, 3 Aug 2018 01:41:25 +1000 (AEST) Date: Thu, 2 Aug 2018 18:41:15 +0300 From: "Michael S. Tsirkin" To: Benjamin Herrenschmidt Cc: Christoph Hellwig , 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: <20180802182959-mutt-send-email-mst@kernel.org> References: <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> <26c1d3d50d8e081eed44fe9940fbefed34598cbd.camel@kernel.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <26c1d3d50d8e081eed44fe9940fbefed34598cbd.camel@kernel.crashing.org> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Aug 02, 2018 at 10:24:57AM -0500, Benjamin Herrenschmidt wrote: > On Wed, 2018-08-01 at 01:36 -0700, Christoph Hellwig wrote: > > 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. > > I don't completely agree: > > 1 - VIRTIO_F_IOMMU_PLATFORM is a property of the "other side", ie qemu > for example. It indicates that the peer bypasses the normal platform > iommu. The platform code in the guest has no real way to know that this > is the case, this is a specific "feature" of the qemu implementation. > > 2 - VIRTIO_F_PLATFORM_DMA (or whatever you want to call it), is a > property of the guest platform itself (not qemu), there's no way the > "peer" can advertize it via the virtio negociated flags. At least for > us. I don't know for sure whether that would be workable for the ARM > case. In our case, qemu has no idea at VM creation time that the VM > will turn itself into a secure VM and thus will require bounce > buffering for IOs (including virtio). > > So unless we have another hook for the arch code to set > VIRTIO_F_PLATFORM_DMA on selected (or all) virtio devices from the > guest itself, I don't see that as a way to deal with it. > > > 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. > > Ben. > So is it true that from qemu point of view there is nothing special going on? You pass in a PA, host writes there. -- MST