Due to historical reason, some legacy shipped system doesn't follow
OpRegion 2.1 spec but still stick to OpRegion 2.0, in which the extended
VBT is not contiguous after OpRegion in physical address, but any
location pointed by RVDA via absolute address. Also although current
OpRegion 2.1+ systems appears that the extended VBT follows OpRegion,
RVDA is the relative address to OpRegion head, the extended VBT location
may change to non-contiguous to OpRegion. In both cases, it's impossible
to map a contiguous range to hold both OpRegion and the extended VBT and
expose via one vfio region.

The only difference between OpRegion 2.0 and 2.1 is where extended
VBT is stored: For 2.0, RVDA is the absolute address of extended VBT
while for 2.1, RVDA is the relative address of extended VBT to OpRegion
baes, and there is no other difference between OpRegion 2.0 and 2.1.
To support the non-contiguous region case as described, the updated read
op will patch OpRegion version and RVDA on-the-fly accordingly. So that
from vfio igd OpRegion view, only 2.1+ with contiguous extended VBT
after OpRegion is exposed, regardless the underneath host OpRegion is
2.0 or 2.1+. The mechanism makes it possible to support legacy OpRegion
2.0 extended VBT systems with on the market, and support OpRegion 2.1+
where the extended VBT isn't contiguous after OpRegion.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Hang Yuan <hang.yuan@linux.intel.com>
Cc: Swee Yee Fonn <swee.yee.fonn@intel.com>
Cc: Fred Gao <fred.gao@intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Link: https://lore.kernel.org/r/20211012124855.52463-1-colin.xu@gmail.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>