[$] Zero-copy TCP receive

In the performance-conscious world of high-speed networking, anything thatcan be done to avoid copying packet data is welcome. The MSG_ZEROCOPY feature added in 4.14enables zero-copy transmission of data, but does not address the receiveside of the equation. It now appears that the 4.18 kernel will include a zero-copy receive mechanism by Eric Dumazetto close that gap, at least for some relatively specialized applications.