On Thu, Aug 09, 2018 at 11:57:53AM +1000, Benjamin Herrenschmidt wrote:

Note: We will still need to implement some custom variant of this
for our secure VMs ... 

Basically we'll need to use the existing bounce bufferring as-is but
the condition will be different, it won't be whether the address is
below a certain limit, it will be *always*.

The conditions are in the dma_capable() helper that the architecture
can override (and which powerpc already does override).