[RFC V2] virtio: Add platform specific DMA API translation for virito devices

[Christoph Hellwig]

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).  And then make sure recent qemu always sets it.