On Wed, Jan 11, 2012 at 8:46 PM, Benjamin Herrenschmidt
[off-list ref] wrote:

On Wed, 2012-01-11 at 14:28 +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.

Is this just an academic exercise or is there any actual value in doing
this ?

It's a corner case, the value is small.  I also hit this with virtio
on SPARC which is made difficult by the fact that the Solaris kernel
assumes there is an IOMMU for scatter-gather and doesn't provide
functions for allocating physically contiguous memory in drivers.
It's another instance where this shortcut comes up against problems
and behaving like a real PCI device would work fine.

Using an iommu is going to slaugher your performance, so at the very
least it should be kept an option.

That's a good idea.  By default it can continue to use physical addresses.

I guess there's no point in worrying about it until we have a real user.

Stefan