Best transport for audio IPC on modern (embedded) Linux

[wsnark at tuta.io]

Hi,

Continuing discussing [1], but at different angle - probably worth a separate thread.

To clarify a bit: I'm building a voice control & communication system for embedded Linux (part of yet another IoT system, based on Yocto Linux). Of course on top of GStreamer.

At the PoC stage we're using unix pipes everywhere, but I want to consider other transports if they're better, especially in my case of limited CPU (~500MHz signle-core armv7 currently).

So, I'm considering pipes vs POSIX shared memory vs memfds:
(i) Security aspect. 
- With anonymous pipes, I'm planning on passing fd from privileged process to a sandboxed one - so it can probably have no filesystem access at all. 

- With shm, does it require more privileges? Also does shm require both parties to cooperate - i.e. if a client is compromised, there is no security control preventing it from writing to shm instead of reading only? Also is there a way to withdraw client's access to a shared memory region? (like, one session with audio data was intended for one client, another data set - for different one). If that's not the case, a copy of shared memory is required per each untrusted client then... Looking at PulseAudio's state, they're using shm only for per-user setup [2].

- Looks like memfds address the drawbacks of shm, and also get their way into pulseaudio [2]. But how to use them for audio IPC properly is a bit unclear - each audio packet = memfd file (sealed)?

(ii) Performance - does shm/memfds provide noticeable (or rather, worth the trouble) benefits over pipes? Especially given that we need to use a separate socket for signaling for shm (and even if we replace it with eventfd..)? Also, there is a way to speed up pipes (splice [3], O_DIRECT flag [4] - packet mode for pipes actually)... 

Any insights would be much appreciated.

[1] https://lists.freedesktop.org/archives/gstreamer-devel/2016-December/062103.html
[2] https://www.freedesktop.org/wiki/Software/PulseAudio/Notes/9.0
[3] https://linux.die.net/man/2/splice
[4] http://man7.org/linux/man-pages/man2/pipe.2.html

Thanks,
Wire Snark
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