Re: how can I modify the config at runtime?

Hi Keqin,

When I read the source code in os/src/init/main.cc I noticed that

Genode could respond to config changes at runtime. I wonder how I can modify the config at runtime? Are there any interfaces to do that?

in order to change the configuration of init (or any other process that is able to respond to dynamic reconfigurations) you will need to let this process fetch its configuration from a ROM service that allows updates of ROM modules at runtime. This is not the case for core's ROM service. The ROM modules provided by core are just the boot modules, which will stay the same during the whole runtime.

This is where the 'fs_rom' service (see 'os/src/server/fs_rom') comes into play. It provides a ROM service by fetching data from a file system. By combining 'fs_rom' with the RAM file system (see 'os/src/server/ram_fs'), arbitrary files stored in a RAM file system can be used as ROM modules. Naturally, files stored in a 'ram_fs' can be changed dynamically (e.g., by mounting the 'ram_fs' in an instance of Noux and modifying a file using vim). The 'fs_rom' service is able to detect file modifications on the used file system. If such a modification is detected for a file that is provided as ROM module, the ROM-session client will receive a signal. Hence, if init obtained its configuration from 'fs_rom', it will reload its configuration and replace the currently running scenario by the one described in the new configuration.

To see how to route the request for a config file to a dedicated ROM service, you may find the example at 'os/run/dynamic_config.run' useful. In this example, the 'test-dynamic_config' program fetches its configuration from a ROM service provided by 'test-dynamic_config_server'.

Another way to use Genode's dynamic configuration feature is by the use of the 'Slave' API. In this scenario, the parent process updates the configuration of a child process using the runtime of the child. We use this for the 'qt_avplay' program to update the audio volume during the playback of a video. Our backend of libSDL will pick up such configuration changes and modify the audio parameters of avplay accordingly. A simpler example can be found at 'os/run/dynamic_config_slave.run'.

Cheers Norman