On Wed, Jun 13, 2018 at 12:41:41AM -0700, Christoph Hellwig wrote: > On Mon, Jun 11, 2018 at 01:29:18PM +1000, Benjamin Herrenschmidt wrote: > > At the risk of repeating myself, let's just do the first pass which is > > to switch virtio over to always using the DMA API in the actual data > > flow code, with a hook at initialization time that replaces the DMA ops > > with some home cooked "direct" ops in the case where the IOMMU flag > > isn't set. > > > > This will be equivalent to what we have today but avoids having 2 > > separate code path all over the driver. > > > > Then a second stage, I think, is to replace this "hook" so that the > > architecture gets a say in the matter. > > I don't think we can actually use dma_direct_ops. It still allows > architectures to override parts of the dma setup, which virtio seems > to blindly assume phys == dma and not cache flushing. > > I think the right way forward is to either add a new > VIRTIO_F_IS_PCI_DEVICE (or redefine the existing iommu flag if deemed > possible). Given this is exactly what happens now, this seems possible, but maybe we want a non-PCI specific name. > And then make sure recent qemu always sets it. I don't think that part is going to happen, sorry. Hypervisors can set it when they *actually have* a real PCI device. People emulate systems which have a bunch of overhead in the DMA API which is required for real DMA. Your proposal would double that overhead by first doing it in guest then re-doing it in host. I don't think it's justified when 99% of the world doesn't need it. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
