You need a few syscalls: to grow your heap size (brk) or to exit your program (exit). I took a quick look at their code and here are the syscalls and arguments implemented: https://github.com/varnish/tinykvm/blob/master/src/functions...

It’s a bit more than running a program under seccomp strict mode, but conceptually similar, so running anything too complicated likely won't work. You certainly won’t be able to sandbox chromium for taking website snapshots for example.