I think most of the work is in the fact that there often isn't an "equivalent call", and it can be quite a lot of code to make it work. One funny thing is the whole esync-fsync-ntsync issue, where synchronization is done differently on Linux and on windows, and translating it was a big performance hit, and difficult to do accurately. If I understood correctly, esync, fsync and ntsync were a series of kernel patches implementing additional synchronization code in the kernel, with ntsync actually replicating the windows style.