Joymappings
For a long time the SDL port has had a joystick mappings file. There are 2 sample ones: data/C64/sdl_joymap_thec64.vjm and data/C64/sdl_joymap_ps3.vjm.
These files contain associations between host joystick actions and emulator actions. Possible actions include a corresponding emulated joystick action, a shortcut to an emulator menu action etc.