There is a provision to have two boot devices in the Zynq UltraScale+ MPSoC architecture. The primary boot mode is the boot mode used by bootROM to load FSBL and optionally PMU FW. The secondary boot mode is the boot device used by FSBL to load all the other partitions. The supported secondary boot modes are QSPI24, QSPI32, SD0, eMMC, SD1, SD1-LS, NAND and USB.
When using PS- PCIe® on ZU+ in Endpoint mode, running FSBL is enough to set up the block for endpoint mode operation. FSBL should be able to program the PS/PS- PCIe® and GTR within 100 ms. However, this doesn’t include PL-bitstream programming as including that would make this greater than 100 ms.
See What is Secondary Boot Mode in FSBL wiki page for more information.