[MirageOS-devel] NTP client integration with MirageOS / Mini-OS

[Kia <sadieperkins@xxxxxxxxxx>]

I have a working NTP client written in ocaml and that runs in mirageos. I still
need to do cleanup work, write and verify code that properly handles leap
second events, write .mli files, and write tests -- but the client code works.
I have a unikernel which periodically queries an NTP server to generate data
that lets other code in the unikernel convert the current value of RDTSC [0] to
an absolute time:

https://github.com/matildah/mirage-ntp/blob/a5d06d05cc20bb9af680238437aeea5835fc54fa/unikernel/unikernel.ml#L58-L63

The algorithms used to convert timestamped (with RDTSC when they are
sent/received) NTP packets to offset/rate information that can be used to
convert the current RDTSC value to a time (or a pair of counter values to a
difference of time) are a reimplementation of RADclock's (source at
https://github.com/synclab/radclock), papers at http://www.synclab.org/docs/).
RADclock has been extensively tested with months of data (and shown to give
accuracy an order of magnitude better than the reference NTP implementation in
the same conditions) -- http://www.synclab.org/testbed/ has details and my post
https://matildah.github.io/posts/2016-05-23-ntp-status.html concisely explains
the advantages of RADclock's feed-forward clock synchronization and its
suitability for a unikernel environment.


The user-facing parts of the client code are:
A type representing the current state of the NTP client, "state":
https://github.com/matildah/mirage-ntp/blob/a5d06d05cc20bb9af680238437aeea5835fc54fa/lib/client/types.ml#L196-L202
A function that generates a new NTP query, "new_query":
https://github.com/matildah/mirage-ntp/blob/a5d06d05cc20bb9af680238437aeea5835fc54fa/lib/client/ntp_client.ml#L66-L69
A function that updates the current state with the NTP server's reply, 
"add_sample":
https://github.com/matildah/mirage-ntp/blob/a5d06d05cc20bb9af680238437aeea5835fc54fa/lib/client/ntp_client.ml#L118-L121
A type that contains current rate/offset estimates, "output":
https://github.com/matildah/mirage-ntp/blob/a5d06d05cc20bb9af680238437aeea5835fc54fa/lib/client/types.ml#L73-L80
A function that takes the current NTP client's state and extracts the current 
rate/offset estimates, "output_of_state":
https://github.com/matildah/mirage-ntp/blob/a5d06d05cc20bb9af680238437aeea5835fc54fa/lib/client/ntp_client.ml#L123-L136

and you can see them called in the demonstration unikernel code at
https://github.com/matildah/mirage-ntp/blob/a5d06d05cc20bb9af680238437aeea5835fc54fa/unikernel/unikernel.ml#L51-L63

The client code can take the value returned by output_of_state and the current 
value of RDTSC
and use 
https://github.com/matildah/mirage-ntp/blob/a5d06d05cc20bb9af680238437aeea5835fc54fa/lib/clocklib/diffabs.ml#L3-L10
to generate an absolute time. (I haven't written the difference time function 
yet).

I have a few questions about how to properly integrate my client code into a
module that can be used like https://github.com/mirage/mirage-clock by any
unikernel that needs the time (but doesn't depend on time information from
dom0).

1. How can I make my NTP client module depend on the mirage RANDOM module and
call int32 from it? (The new_query function needs to be able to get some
randomness to put in the packet as a nonce)

2. Is there way to have a module that starts a background thread (in my case,
that queries the NTP server, updates its estimates of time counter rate/offset,
and then sleeps until the next poll interval) but also enable other threads in
the unikernel to get the current value of a variable in its context?

Failing that, what way can I have a background thread that continually runs and
always makes available the latest calculated value of "output" to other threads
(so they can compute the current time based on the current RDTSC value)?

Morally, my NTP client thread needs to let other threads have access to its
latest rate/offset information so they can calculate the current time (much how
Xen makes available vcpu_time_info in a shared page), but I do not know how
to set this up in mirage.

-- 
Kia

[0] everything that i say about RDTSC applies to the ARM equivalent
https://github.com/mirage/mini-os/blob/89268f00b0b0215057cb74edd94e866536a02489/arch/arm/time.c#L59-L64


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