From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Date: Fri, 3 Aug 2018 22:17:32 +0300 Message-ID: <20180803221634-mutt-send-email-mst@kernel.org> References: <20180801081637.GA14438@arm.com> <20180801083639.GF26378@infradead.org> <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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20180803070507.GA1344@infradead.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Christoph Hellwig Cc: robh@kernel.org, srikar@linux.vnet.ibm.com, Benjamin Herrenschmidt , Will Deacon , linux-kernel@vger.kernel.org, linuxram@us.ibm.com, virtualization@lists.linux-foundation.org, paulus@samba.org, marc.zyngier@arm.com, mpe@ellerman.id.au, joe@perches.com, robin.murphy@arm.com, david@gibson.dropbear.id.au, linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net, haren@linux.vnet.ibm.com, Anshuman Khandual List-Id: virtualization@lists.linuxfoundation.org On Fri, Aug 03, 2018 at 12:05:07AM -0700, Christoph Hellwig wrote: > On Thu, Aug 02, 2018 at 04:13:09PM -0500, Benjamin Herrenschmidt wrote: > > So let's differenciate the two problems of having an IOMMU (real or > > emulated) which indeeds adds overhead etc... and using the DMA API. > > > > At the moment, virtio does this all over the place: > > > > if (use_dma_api) > > dma_map/alloc_something(...) > > else > > use_pa > > > > The idea of the patch set is to do two, somewhat orthogonal, changes > > that together achieve what we want. Let me know where you think there > > is "a bunch of issues" because I'm missing it: > > > > 1- Replace the above if/else constructs with just calling the DMA API, > > and have virtio, at initialization, hookup its own dma_ops that just > > "return pa" (roughly) when the IOMMU stuff isn't used. > > > > This adds an indirect function call to the path that previously didn't > > have one (the else case above). Is that a significant/measurable > > overhead ? > > If you call it often enough it does: > > https://www.spinics.net/lists/netdev/msg495413.html > > > 2- Make virtio use the DMA API with our custom platform-provided > > swiotlb callbacks when needed, that is when not using IOMMU *and* > > running on a secure VM in our case. > > And total NAK the customer platform-provided part of this. We need > a flag passed in from the hypervisor that the device needs all bus > specific dma api treatment, and then just use the normal plaform > dma mapping setup. To get swiotlb you'll need to then use the DT/ACPI > dma-range property to limit the addressable range, and a swiotlb > capable plaform will use swiotlb automatically. 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. -- MST