On Thu, 2013-08-15 at 12:10 +0100, Stefano Stabellini wrote:

At the moment always rely on swiotlb-xen, but when Xen starts supporting
hardware IOMMUs we'll be able to avoid it conditionally on the presence
of an IOMMU on the platform.

Do we have any idea how we are going to do this?

It's extra complicated if you consider that on some systems on some of
the devices are behind an IOMMU :-/

I wonder if we can enumerate which devices have an IOMMU at boot time
and force a ludicrous dma mask (such as 0) if one isn't present in order
to force us to always take the exchange_and_pin path?

Ian.