> We will focus on in-kernel solutions for now. Indeed, user-space tools have a fundamental limitation: if they need to re-inject packets onto the network, they must again pay the expensive cost of crossing the kernel barrier

This is glossing a little quickly over user-space solutions.

If you are handling packets at the rate of which you thus need to put a kernel bypass in place, then there is chance that you can't afford the kernel to handle them on their way back onto the network.

The obvious solution is to kernel bypass this transmit side as well. All user-space tools offer this.

There are some control packets that will need to get back to the kernel, but those do not require that kind of speed.