What is a Proof-producing Virtual Machine (PVM)?

Question

[copied from Slack]

Can someone explain what is Proof-producing Virtual Machine (PVM) introduced with SCORUs? Any guide or documentation will also work.

Answer 1

Copied from file src/proto_alpha/lib_protocol/sc_rollup_PVM_sig.ml, which introduces the expected signature from a PVM:

This module introduces the semantics of Proof-generating Virtual Machines.

A PVM defines an operational semantics for some computational model. The
specificity of PVMs, in comparison with standard virtual machines, is their
ability to generate and to validate a *compact* proof that a given atomic
execution step turned a given state into another one.

In the smart-contract rollups, PVMs are used for two purposes:

- They allow for the externalization of rollup execution by completely
  specifying the operational semantics of a given rollup. This
  standardization of the semantics gives a unique and executable source of
  truth about the interpretation of smart-contract rollup inboxes, seen as a
  transformation of a rollup state.

- They allow for the validation or refutation of a claim that the processing
  of some messages led to a given new rollup state (given an actual source
  of truth about the nature of these messages).