Flips tracking of IPC

[Martin Pohlack]

After some local discussions I want to make some concluding remarks to
this issue:

I see two possible solutions to the problem of missing parallelity in
L4VFS / flips.

1. Solve it as it is done in "term_server", i.e., use one distributor
thread receiving and replying to all request in flips.  This thread
simply forwards all work to a set of worker threads.  This feature is
also supported by our IDL compiler DICE.  The distributor thread would
then never block, only accept request and send answers.

2. L4VFS supports a kind of session management (the connection
interface): If a task contacts a server for the first time it may ask
the server for a new session and use the answer (a thread id) in the
following.  This can be used for load balancing etc.  Currently no
server implements this (as far as I know) and the client-side is not
active (it is commented out in the source code).  You could fix this at:
l4vfs/lib/libc_backends/file_table/volumes.c:vol_resolve_thread_for_volume_id().
For your problem this would not yet be sufficient as you need several
sessions per task, that is, you need one per local thread of your
client.  This means you would have to extend this L4VFS mechanism to
setup a new connection for each local thread, store this information
locally, and use it for each remote function call.  I would start with
vol_resolve_thread_for_volume_id() and its equivalent from the
socket_io-backend.

I personally would prefer to use the first attempt as I think it is more
future-proof and maybe elegant, because the exposure of server internal
thread structures is not the future.

Regards,
Martin