You may experience the above warning when running a dual rank DDR3 simulation.
This warning occurs during the memory initilialization and calibration process. Uninitialized reads occur as dummy reads are performed by the sequencer in order to switch between shadow registers. The dummy read operations no longer occur after the memory calibration process is complete.
These warnings are harmless and should be ignored. The warnings don't occur in single rank cases because shadow registers are disabled in those designs.