On Wed, Jan 11, 2012 at 3:39 PM, Michael S. Tsirkin [off-list ref] wrote:

On Wed, Jan 11, 2012 at 02:28:48PM +0000, Stefan Hajnoczi wrote:

quoted

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

quoted

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

It won't be hard to show siginificant performance regression if
we do this. Hard to justify for something as niche as nested virt.

For x86 this should be mostly a nop.  For ppc and SPARC architectures
maybe you're right.  I still think it's a design flaw because if
virtio v2 doesn't use bus addresses then it will simply not be
possible to do passthrough for nested virt and other cases we haven't
hit yet.

Stefan