Docs/example of the API for guest <> host communication?

[ardi]

Hi!

I'd like to write a very simple library that behaves as a driver, so
that when a program running in the guest calls a function in this
library, control is passed to the VM code in the host (and it later
returns to the guest program when finished, of course). I suppose this
is how guest drivers are developed, but I couldn't find any simple
demo or docs with an example.

My use would be Linux guests, running in different hosts.

If you know of any example that could put me on the right track, please tell!!

Kind regards and thanks a lot,

ardi