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