The osmesa
context makes use of the OSMesa offscreen rendering library in order to provide a headless OpenGL context that does not require X or even a GPU running. Yes, it's a software renderer. The video contents can be read from an UNIX socket.
To use the osmesa
context, you must first recompile your RetroArch passing the --enable-osmesa
switch to the configure
script. If you don't see a line similar to the one bellow, it means that your Mesa install does not include OSMesa.