The problem with the Signal service implementation.

[ivan.bludov]

Hi, All.

We were investigating problem of sending big amount of data via network 
in Genode on Fiasco.OC. And we have found out some significant bug of 
the Genode Signal service implementation and we also could provide the 
simple stable solution. As know Packet_stream_rx and Packet_stream_tx 
implementations (os/include/packet_stream_*) are based on a Signal 
service (signal.lib) and use RPC submit(Signal_context_capability 
context) of Genode::Signal_session 
(base/include/signla_session/signal_session.h). And this submit RPC is 
invoked all the time while RX – TX packet transferring (as the packet 
acknowledge). So the problem is that the submit is RPC and according to 
RPC implementation all parameters, which are capabilities, are passed by 
special way know as capability marshalling/unmarshalling 
(base-foc/include/base/ipc.h). And while capability unmarshalling the 
new l4_cap_sel is allocated and capabilities are mapped once again all 
the time. In the limit all capability table of the l4 task will be 
filled completely; especially while intensive packet transferring. And 
it damages the Genode Core foremost and provides other troubles. As we 
investigated the decision of using the capability in a sense of signal 
context is redundant. Signal_session_component::submit uses 
Siganl_context_capability only for searching it in the context 
entrypoint (base/src/core/signal_session_component.cc). But the same 
effect could be achieved by searching signal context by a 
badge(local_name()) and then there would not be necessity of capability 
unmarshalling. The decision is quote simple to change the 
Signal_context_capability typedef from Capability<Signal_context> to 
simple int or long type (base/include/signla_session/signal_session.h) 
and then to fix all errors generated by compiler. So we think that this 
problem of Signal service is significant and has to be fixed as quick as 
possible. But of course we'll glad to hear any your remarks about the 
decision to use the capability is a sense of Signal_context.

Best,
Ivan Bludov.