为什么Linux内核中没有零拷贝网络？

I'd like to know the reasons for why the linux kernel (or any other mainstream OS) does not have a feature for zero copy networking ? By zero copy I mean, that an packet/datastream does not get copied for passing to an application in userspace but e.g. uses a memory-pool type of allocator to share the memory between kernel and userspace. I've came up with 3 theory's on my own:I'd like to know the reasons for why the linux