# Cannon Fault Proof Virtual Machine Specification

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**Table of Contents**

- [Overview](#overview)
- [State](#state)
  - [State Hash](#state-hash)
- [Memory](#memory)
  - [Heap](#heap)
- [Delay Slots](#delay-slots)
- [Syscalls](#syscalls)
- [I/O](#io)
  - [Standard Streams](#standard-streams)
  - [Hint Communication](#hint-communication)
  - [Pre-image Communication](#pre-image-communication)
    - [Pre-image I/O Alignment](#pre-image-io-alignment)
- [Exceptions](#exceptions)

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## Overview

This is a description of the Cannon Fault Proof Virtual Machine (FPVM). The Cannon FPVM emulates
a minimal Linux-based system running on big-endian 32-bit MIPS32 architecture.
Alot of its behaviors are copied from Linux/MIPS with a few tweaks made for fault proofs.
For the rest of this doc, we refer to the Cannon FPVM as simply the FPVM.

Operationally, the FPVM is a state transition function. This state transition is referred to as a *Step*,
that executes a single instruction. We say the VM is a function $f$, given an input state $S_{pre}$, steps on a
single instruction encoded in the state to produce a new state $S_{post}$.
$$f(S_{pre}) \rightarrow S_{post}$$

Thus, the trace of a program executed by the FPVM is an ordered set of VM states.

## State

The virtual machine state highlights the effects of running a Fault Proof Program on the VM.
It consists of the following fields:

1. `memRoot` - A `bytes32` value representing the merkle root of VM memory.
2. `preimageKey` - `bytes32` value of the last requested pre-image key.
3. `preimageOffset` - The 32-bit value of the last requested pre-image offset.
4. `pc` - 32-bit program counter.
5. `nextPC` - 32-bit next program counter. Note that this value may not always be $pc+4$
 when executing a branch/jump delay slot.
6. `lo` - 32-bit MIPS LO special register.
7. `hi` - 32-bit MIPS HI special register.
8. `heap` - 32-bit base address of the most recent memory allocation via mmap.
9. `exitCode` - 8-bit exit code.
10. `exited` - 1-bit indicator that the VM has exited.
11. `registers` - General-purpose MIPS32 registers. Each register is a 32-bit value.

The state is represented by packing the above fields, in order, into a 226-byte buffer.

### State Hash

The state hash is computed by hashing the 226-byte state buffer with the Keccak256 hash function
and then setting the high-order byte to the respective VM status.

The VM status can be derived from the state's `exited` and `exitCode` fields.

```rs
enum VmStatus {
    Valid = 0,
    Invalid = 1,
    Panic = 2,
    Unfinished = 3,
}

fn vm_status(exit_code: u8, exited: bool) -> u8 {
    if exited {
        match exit_code {
            0 => VmStatus::Valid,
            1 => VmStatus::Invalid,
            _ => VmStatus::Panic,
        }
    } else {
        VmStatus::Unfinished
    }
}
```

## Memory

Memory is represented as a binary merkle tree.
The tree has a fixed-depth of 27 levels, with leaf values of 32 bytes each.
This spans the full 32-bit address space, where each leaf contains the memory at that part of the tree.
The state `memRoot` represents the merkle root of the tree, reflecting the effects of memory writes.
As a result of this memory representation, all memory operations are 4-byte aligned.
Memory access doesn't require any privileges. An instruction step can access any memory
location as the entire address space is unprotected.

### Heap

FPVM state contains a `heap` that tracks the base address of the most recent memory allocation.
Heap pages are bump allocated at the page boundary, per `mmap` syscall.
mmap-ing is purely to satisfy program runtimes that need the memory-pointer
result of the syscall to locate free memory. The page size is 4096.

The FPVM has a fixed program break at `0x40000000`. However, the FPVM is permitted to extend the
heap beyond this limit via mmap syscalls.
For simplicity, there are no memory protections against "heap overruns" against other memory segments.
Such VM steps are still considered valid state transitions.

Specification of memory mappings is outside the scope of this document as it is irrelevant to
the VM state. FPVM implementers may refer to the Linux/MIPS kernel for inspiration.

## Delay Slots

The post-state of a step updates the `nextPC`, indicating the instruction following the `pc`.
However, in the case of where a branch instruction is being stepped, the `nextPC` post-state is
set to the branch target. And the `pc` post-state set to the branch delay slot as usual.

A VM state transition is invalid whenever the current instruction is a delay slot that is filled
with jump or branch type instruction.
That is, where $nextPC \neq pc + 4$ while stepping on a jump/branch instruction.
Otherwise, there would be two consecutive delay slots. While this is considered "undefined"
behavior in typical MIPS implementations, FPVM must raise an exception when stepping on such states.

## Syscalls

Syscalls work similar to [Linux/MIPS](https://www.linux-mips.org/wiki/Syscall), including the
syscall calling conventions and general syscall handling behavior.
However, the FPVM supports a subset of Linux/MIPS syscalls with slightly different behaviors.
The following table list summarizes the supported syscalls and their behaviors.

| $v0 | system call | $a0 | $a1 | $a2 | Effect |
| -- | -- | -- | -- | -- | -- |
| 4090 | mmap | uint32 addr | uint32 len | | Allocates a page from the heap. See [heap](#heap) for details. |
| 4045 | brk | | | | Returns a fixed address for the program break at `0x40000000` |
| 4120 | clone | | | | Returns 1 |
| 4246 | exit_group | uint8 exit_code | | | Sets the Exited and ExitCode states to `true` and `$a0` respectively. |
| 4003 | read | uint32 fd | char *buf | uint32 count | Similar behavior as Linux/MIPS with support for unaligned reads. See [I/O](#io) for more details. |
| 4004 | write | uint32 fd | char *buf | uint32 count | Similar behavior as Linux/MIPS with support for unaligned writes. See [I/O](#io) for more details. |
| 4055 | fcntl | uint32 fd | int32 cmd | | Similar behavior as Linux/MIPS. Only the `F_GETFL` (3) cmd is supported. Sets errno to `0x16` for all other commands |

For all of the above syscalls, an error is indicated by setting the return
register (`$v0`) to `0xFFFFFFFF` (-1) and `errno` (`$a3`) is set accordingly.
The VM must not modify any register other than `$v0` and `$a3` during syscall handling.
For unsupported syscalls, the VM must do nothing except to zero out the syscall return (`$v0`)
and errno (`$a3`) registers.

Note that the above syscalls have identical syscall numbers and ABIs as Linux/MIPS.

## I/O

The VM does not support Linux open(2). However, the VM can read from and write to a predefined set of file descriptors.
| Name | File descriptor | Description |
| ---- | --------------- | ----------- |
| stdin | 0 | read-only standard input stream. |
| stdout | 1 | write-only standaard output stream. |
| stderr | 2 | write-only standard error stream. |
| hint response | 3 | read-only. Used to read the status of [pre-image hinting](./fault-proof.md#hinting). |
| hint request | 4 | write-only. Used to provide [pre-image hints](./fault-proof.md#hinting) |
| pre-image response | 5 | read-only. Used to [read pre-images](./fault-proof.md#pre-image-communication). |
| pre-image request | 6 | write-only. Used to [request pre-images](./fault-proof.md#pre-image-communication). |

Syscalls referencing unnkown file descriptors fail with an `EBADF` errno as done on Linux.

Writing to and reading from standard output, input and error streams have no effect on the FPVM state.
FPVM implementations may use them for debugging purposes as long as I/O is stateless.

All I/O operations are restricted to a maximum of 4 bytes per operation.
Any read or write syscall request exceeding this limit will be truncated to 4 bytes.
Consequently, the return value of read/write syscalls is at most 4, indicating the actual number of bytes read/written.

### Standard Streams

Writing to stderr/stdout standard stream always succeeds with the write count input returned,
effectively continuing execution without writing work.
Reading from stdin has no effect other than to return zero and errno set to 0, signalling that there is no input.

### Hint Communication

Hint requests and responses have no effect on the VM state other than setting the `$v0` return
register to the requested read/write count.
VM implementations may utilize hints to setup subsequent pre-image requests.

### Pre-image Communication

The `preimageKey` and `preimageOffset` state are updated via read/write syscalls to the pre-image
read and write file descriptors (see [I/O](#io)).
The `preimageKey` buffers the stream of bytes written to the pre-image write fd.
The `preimageKey` buffer is shifted to accomodate new bytes written to the end of it.
A write also resets the `preimageOffset` to 0, indicating the intent to read a new pre-image.

When handling pre-image reads, the `preimageKey` is used to lookup the pre-image data from an Oracle.
A max 4-byte chunk of the pre-image at the `preimageOffset` is read to the specified address.
Each read operation increases the `preimageOffset` by the number of bytes requested
(truncated to 4 bytes and subject to alignment contraints).

#### Pre-image I/O Alignment

As mentioned earlier in [memory](#memory), all memory operations are 4-byte aligned.
Since pre-image I/O occurs on memory, all pre-image I/O operations must strictly adhere to alignment boundaries.
This means the start and end of a read/write operation must fall within the same alignment boundary.
If an operation were to violate this, the input `count` of the read/write syscall must be
truncated such that the effective address of the last byte read/written matches the input effective address.

The VM must read/write the maximum amount of bytes possible without crossing the input address alignment boundary.
For example, the effect of a write request for a 3-byte aligned buffer must be exactly 3 bytes.
If the buffer is misaligned, then the VM may write less than 3 bytes depending on the size of the misalignment.

## Exceptions

The FPVM may raise an exception rather than output a post-state to signal an invalid state
transition. Nominally, the FPVM must raise an exception in at least the following cases:

- Invalid instruction (either via an invalid opcode or an instruction referencing registers
outside the general purpose registers).
- Pre-image read at an offset larger than the size of the pre-image.
- Delay slot contains branch/jump instruction types.

VM implementations may raise an exception in other cases that is specific to the implementation.
For example, an on-chain FPVM that relies on pre-supplied merkle proofs for memory access may
raise an exception if the supplied merkle proof does not match the pre-state `memRoot`.
