On Tue, Apr 11, 2023 at 11:31 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > 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, Xuan, is it possible to set up the DMA mapping inside the virtio driver itself? I vaguely remember at least the RX buffer mapping is done by the driver. If this is true, we can avoid exposing DMA details to the upper layer. > 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. Ok, so if there's no DMA at all we should avoid using the DMA API completely. This means we should check dma_dev against NULL in virtio_has_dma_quirk(). Thanks > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
