On Tue, Apr 11, 2023 at 10:09:40AM +0800, Jason Wang wrote: > We want to support AF_XDP for virtio-net. It means AF_XDP needs to > know the dma device to perform DMA mapping. So we introduce a helper > to expose the dma dev of the virtio device. The whole virtio architecture is based around the core code doing the DMA mapping. I can't see how you can just export a helper to expose the dma device. You'd have to complete rework the layering of the virtio code if you want to do it in the upper level drivers. And why would you want to do this? The low-level code is the only piece that can actually know if you need to do a dma mapping. All the kernel subsystems that don't do it inside the low-level drivers or helpers closely associtated are a giant and hard to fix map (see usb for the prime exhibit). So the first question is: why do you want this for XF_ADP, and the next question will be how to do that without making a complete mess. > This works fine as long as VIRTIO_F_ACCESS_PLATFORM is negotiated. But > when it is not, the virtio driver needs to use a physical address so > we want to expose the virtio device without dma_ops in the hope that > it will go for direct mapping where the physical address is used. But > it may not work on some specific setups (arches that assume an IOMMU > or have arch dma ops). The DMA device for virtio_pci is the underlying PCI device, always. !VIRTIO_F_ACCESS_PLATFORM means there is no dma device at all. Because of all these things you can't just expose a pointer to the dma_device as that is just a completely wrong way of thinking about the problem. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
