You could do worse than Mach ports

[Terry Lambert]

First: congradulations on the new project!

I know you said you didn't want to go all the way to Mach ports for
the messaging, but... you could do a lot worse.

Mach ports each take up only 64 bytes.  In addition, they allow for
the idea of passing pages ranges and/or entire address spaces between
participants.  I think this type of thing is going to be critical for
you to realize your user space VFS design goals, unless you are going
to copy all your data in and out of the kernel.

BTW: that's 64 bytes on a 64 bit machine, so it's the equivalent of
8 pointers worth, which is not much, considering.

-- Terry