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
