Virtual machines should be invisible

Current VM designs prioritise implementor freedom and performance, at the expense of other concerns of the end programmer. We motivate an alternative approach to VM design aiming to be unobtrusive in general, and prioritising two key concerns specifically: foreign function interfacing and support for runtime analysis tools (such as debuggers, profilers etc.). We describe our experiences building a Python VM in this manner, and identify some simple constraints that help enable low-overhead foreign function interfacing and direct use of native tools. We then discuss how to extend this towards a higher-performance VM suitable for Java or similar languages.