When using the XAUI v4.0 core, the MDIO interface of the XAUI core does not properly wait for the "turnaround" in the MDIO frame. As a result, the MDIO interface does not work properly, which leads to incorrect frames being read back. This might also affect MDIO writes.
To resolve this issue with the XAUI v4.0 core, please install the patch that is available in the LogiCORE XAUI v4.0 Release Notes (Xilinx Answer 18575).
This issue will also be resolved in the next release of the core.