On Fri, Apr 06, 2018 at 06:37:18PM +1000, Benjamin Herrenschmidt wrote:

quoted

quoted

implemented as DMA API which the virtio core understands. There is no
need for an IOMMU to be involved for the device representation in this
case IMHO.

This whole virtio translation issue is a mess.  I think we need to
switch it to the dma API, and then quirk the legacy case to always
use the direct mapping inside the dma API.

Fine with using a dma API always on the Linux side, but we do want to
special case virtio still at the arch and qemu side to have a "direct
mapping" mode. Not sure how (special flags on PCI devices) to avoid
actually going through an emulated IOMMU on the qemu side, because that
slows things down, esp. with vhost.

IE, we can't I think just treat it the same as a physical device.

We should have treated it like a physical device from the start, but
that device has unfortunately sailed.

But yes, we'll need a per-device quirk that says 'don't attach an
iommu'.