What is the maximum allowable skew between RXN and RXP?
According to the Rocket I/O Transceiver User Guide (see following link), the maximum tolerable differential input skew is 75 ps.
Note that 75 ps is the maximum skew allowable for data to be correctly interpreted by the receiver. Large skew can negatively impact the data eye at the receive end. To maintain good margin on the receive side, there should be a maximum of 9 ps of skew between RXN and RXP at 3.125 Gbps (as described in the High Speed Serial Trace Design section of the RocketIO User Guide). This number is a percentage of bit time (9 ps/320 ps = 2.8%), so it scales with the bit time.
You should adhere to the following guidelines for the specified data rates:
Maximum skew between RXN and RXP at receiver:
3.125 Gbps: 0.028 * 320 = 9 ps
2.5 Gbps: 0.028 * 400 ps = 11 ps
1.0 Gbps: 0.028 * 1000ps = 28 ps
A delay of 9 ps translates to roughly 50 mils of difference between trace lengths, which is easily achievable with most automated routing tools.
NOTE: Due to these specifications, Xilinx recommends using twisted differential cable because the negative and positive line lengths are closely matched. If you must use two separate cables, the lengths must be matched to within 2.8% of the bit time.