Hmm. After a bit of research, it seems that some VM's handle this by using pthreads a few times (presumably one per core), then having each pthread spawn several green threads.
This seems a good solution if we can arrange to spawn a new pthread also for blocking I/O, maybe by catching I/O functions and putting into a pthread the I/O operation caller. This hybrid would make the compiler code not very clean, because it should handle both green threads and pthreads.
