On Wed, Jan 11, 2012 at 9:10 AM, Benjamin Herrenschmidt
[off-list ref] wrote:

On Wed, 2012-01-11 at 08:47 +0000, Stefan Hajnoczi wrote:

quoted

This is also an opportunity to stop using CPU physical addresses in
the ring and instead perform DMA like a normal PCI device (use bus
addresses).

Euh why ?

Because it's a paravirt hack that ends up hitting corner cases.  It's
not possible to do virtio-pci passthrough under nested virtualization
unless we use an IOMMU.  Imagine passing virtio-net from L0 into the
L2 guest (i.e. PCI-passthrough).  If virtio-pci is really "PCI" this
should be possible but it's not when we use physical addresses instead
of bus addresses.

Stefan