virtio-pci physical addresses vs bus addresses

[Stefan Hajnoczi <stefanha@redhat.com>]

The vring has traditionally used physical addresses since the hypervisor
has access to guest RAM.

As virtio continues to be deployed in new ways, including as real
hardware, I wonder if physical addresses will become a hindrance.

Nested virtualization also poses a problem because virtio devices cannot
be passed through to L2 or deeper guests.  The L2 guest has no concept
of the physical address that the L0 hypervisor expects.

Given the other PCI changes that are being proposed for the virtio
standard, this might be the right opportunity to make virtio-pci behave
like a real PCI adapter with respect to DMA and bus addresses.

Of course this may add overhead to existing virtio implementations since
emulating real DMA on modern systems can mean an extra layer of
indirection (IO-MMU).

At the same time, we need to be aware of the limitations of using
physical addresses directly.

Thoughts?

Stefan