Basically mapping the device registers into user space and doing exactly what the kernel would do, but without the syscall overhead.

Some researchers made a big splash at OSDI by doing this securely:

http://people.inf.ethz.ch/troscoe/pubs/peter-arrakis-osdi14....

Oracle has long argued the value of bypassing the file system and associated kernel drivers with its raw devices and ASM. It'd be interesting to see such a thing land in other platforms.

For network, the kernel is bypasses through dpdk. For disk, the syscalls are there. But they are always async IO with O_DIRECT. So the OS wont cache and buffer anything. So that is what you bypass