AR# 33837 Virtex-6 FPGA Integrated Block Wrapper v1.4 for PCI Express - x8 GEN2 Operation is not supported in Virtex-6 HXT devices for the v1.4 release
For the 11.4 v1.4 release, x8 GEN2 operation is not supported for the Virtex-6 HXT family.
Solution
This is scheduled to be added in a later release. Support for GEN1 operation was added for all HXT devices, except 6VHX380T-FF1154, 6VHX380T-FF1923, and 6VHX255T-FF1923.