Zero-copy network transmission with io_uring

Zero-copy network transmission with io_uring

This does also reduce the number of copies when using kTLS. "Zero copy" is a bit of a misnomer - it's only there to eliminate memcpys from user owned memory to kernel owned memory, not all copies.

The point of "zero copy" is that in a normal transfer, data is copied from the user buffer to a kernel buffer, then the network card does DMA from the kernel buffer to its own transmit buffer. "zero copy" reduces that to a copy from the user buffer to the NIC's transmit buffer.

With kTLS, "zero copy" is a win with or without expensive NICs:

1. With expensive NICs, the NIC can do the encryption during DMA from CPU memory to the transmit buffer. You thus avoid copying the data into the kernel, and just have the NIC read and encrypt during DMA.
2. With cheap NICs, the kernel has to do a copy. Without zero copy, it copies plain text from user buffer to kernel buffer and then encrypts from kernel buffer to network buffer. With zero copy, it encrypts from user buffer to network buffer. In either case, the NIC will then DMA the network buffer into the on-chip transmit buffer.