In particular, Vivado errors on explicit BUFH -> PS7 placement. I've confirmed that Vivado will take the axi_regs_zybo design, and route the design the same way as current master symbiflow-arch-defs. However when Vivado is given the same design with an explicit BUFH, Vivado generates an error because of the BUFH -> PS7 placement. The likely solution is to not emit explicit BUFH instances, but instead emit route through the BUFH with FIXED_ROUTE's. This might be tricky, and is not the most important thing to work on right now.

For now, I recommend we disable the DIFF_FASM test for axi_regs_zybo until this is fixed.