are currently two problems with the use of 1GB page mappings by the direct
map. First, at least one device driver uses pmap_extract() rather than
DMAP_TO_PHYS() to translate a direct map address to a physical address.
Unfortunately, neither pmap_extract() nor pmap_kextract() yet support 1GB
page mappings. Second, pmap_bootstrap() needs to interrogate the MTRRs to
ensure that a 1GB page mapping doesn't span two MTRRs of different types.
Reported and tested by: Daniel O'Connor
MFC after: 3 days
102b592620