The CPU cache must be flushed _before_ we start modifying the kernel map ptes,

The CPU cache must be flushed _before_ we start modifying the kernel map ptes,
otherwise data will be missing, which becomes apparent when the kernel evicts
batch buffers which are likely to be written into in the evicted state,
and then rebound to the AGP aperture.
This means we cannot rely on the AGP module to flush the
cache for us.