My dual PPC440 design has an xps_mailbox sitting on two PPC440 PLBs. The mailbox is assigned a different address on each PLB. After I run "Generate Libraries and BSP" in EDK, I find that the address macros in xparameters.h for the mailbox are wrong; it assumes the same address on both PLBs.
There is a manual workaround, which is to make sure the base address of the mailbox is the same in both processors' memory maps, or to edit xparameters.h and update the erroneous XPAR constants with correct values.
This problem is fixed in 11.1.