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core: Support memfd transport; bump protocol version

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Now that all layers in the stack support memfd blocks, add memfd
support for the daemon's global core mempool. Also introduce
"enable-memfd=" daemon argument and configuration option.

For now, memfd support is an opt-in feature to be activated only
when daemon's enable-memfd= is set to yes.

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
This commit is contained in:
Ahmed S. Darwish 2016-04-15 23:07:36 +02:00 • committed by Arun Raghavan
parent b1d47d60fc
commit d2a6afcab3
13 changed files with 122 additions and 24 deletions
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37
PROTOCOL
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@ -371,6 +371,43 @@ PA_COMMAND_DISABLE_SRBCHANNEL
Tells the client to stop listening on the additional SHM ringbuffer channel.
Acked by client by sending PA_COMMAND_DISABLE_SRBCHANNEL back.
## v31, implemented by >= 9.0
Memfd shared-memory support is now added to PulseAudio as an opt-in feature.
Add 'enable-memfd=yes' to daemon's configuration to use memfds, instead of
POSIX shm, by default.
Memfd is a simple memory sharing mechanism, added by the systemd/kdbus
developers, to share pages between processes in an anonymous, no global
registry needed, no mount-point required, relatively secure, manner.
PulseAudio memfd support builds the necessary (but not yet sufficient)
groundwork for a better integration with per-app containers (e.g. xdg-app)
For further details on memfds in general, please check:
https://dvdhrm.wordpress.com/2014/06/10/memfd_create2/
Archived at: http://www.webcitation.org/6gnHTy9Kr
Moreover, for both client and server, the second most-significant bit of
the version tag is now used to flag memfd SHM support. On the way forward,
the two most-significant _bytes_ of the version tag are now also reserved
for flags.
PA_COMMAND_REGISTER_MEMFD_SHMID
New command that can be sent both ways, from client to server and vice versa.
This is needed to transfer a memfd pool's blocks without passing its fd every
time, thus minimizing overhead and avoiding fd leaks.
The registration command above sends a packet with the pool's memfd fd as
ancillary data. Such packet has an ID that uniquely identifies the pool's
memfd memory area. Upon arrival, the other end (client or server) creates a
permanent ID<->memfd mapping.
By doing so, there's need to reference the pool's memfd file descriptor any
further -- just its ID. Thus both endpoints can then quickly and safely
close their memfd file descriptors.
#### If you just changed the protocol, read this
## module-tunnel depends on the sink/source/sink-input/source-input protocol
## internals, so if you changed these, you might have broken module-tunnel.