Re: [RFC 0/4] Virtio uses DMA API for all devices

[Christoph Hellwig]

On Sun, Aug 05, 2018 at 11:10:15AM +1000, Benjamin Herrenschmidt wrote:
>  - One you have rejected, which is to have a way for "no-iommu" virtio
> (which still doesn't use an iommu on the qemu side and doesn't need
> to), to be forced to use some custom DMA ops on the VM side.
> 
>  - One, which sadly has more overhead and will require modifying more
> pieces of the puzzle, which is to make qemu uses an emulated iommu.
> Once we make qemu do that, we can then layer swiotlb on top of the
> emulated iommu on the guest side, and pass that as dma_ops to virtio.

Or number three:  have a a virtio feature bit that tells the VM
to use whatever dma ops the platform thinks are appropinquate for
the bus it pretends to be on.  Then set a dma-range that is limited
to your secure memory range (if you really need it to be runtime
enabled only after a device reset that rescans) and use the normal
dma mapping code to bounce buffer.