I'm assuming that the real objection to an interface like sendfile() is that you shouldn't need to copy_from_user() just to construct an SKB if the user address being referred to is pointing at a file backed VMA. If we could do a zero-copy send() from userspace, there would be no need for sendfile() in the first place.

IOW, { data = mmap(fd, ...); ...; send(fd, data + offset, length); } could be just as fast as sendfile() if we were smarter.

We're starting to build infrastructure to do zero-copy networking in userspace specifically for KVM. So far, it's not at the socket level but instead at the macvtap level but that's arguably another problem with the networking stack--macvtap/tap should just be an address family :-)