vm-migration: add protocol versioning

Add protocol-side support for migration protocol versioning.

Use the existing 6-byte Start command header, which was
previously zero padding, to carry the sender's migration
protocol version without changing the wire layout.

Store the version as a little-endian u16 in the first two
bytes and ignore the remaining four bytes. A zeroed command
header continues to mean a legacy v0 sender.

This keeps the message flow unchanged for rollout:
Start is still followed by plain OK or Error (Aborted), and no new
command is needed.

On-behalf-of: SAP leander.kohler@sap.com
Signed-off-by: Leander Kohler <leander.kohler@cyberus-technology.de>

vm-migration/src/protocol.rs

@@ -73,8 +73,22 @@
 //!    Source->>Destination: Complete
 //!    Destination-->>Source: OK
 //! ```
+//!
+//! ## Protocol Versioning
+//!
+//! `Start` carries the sender's migration protocol version.
+//! A zeroed version field is treated as legacy protocol `v0`.
+//!
+//! The destination validates that version and replies with a plain `OK` or
+//! `Error`.
+//!
+//! Only the current and immediately previous protocol versions are
+//! supported. Compatibility is one-way, from older protocol versions
+//! to newer ones.
 
 use std::io::{Read, Write};
+use std::mem::size_of;
+use std::ops::RangeInclusive;
 use std::{mem, slice};
 
 use itertools::Itertools;
@@ -127,18 +141,33 @@ pub enum Command {
     CompletePaused = 8,
 }
 
+/// Newest migration protocol version sent by this implementation.
+pub const CURRENT_PROTOCOL_VERSION: u16 = 0;
+
+/// Returns the current migration protocol version and the previous version, if any.
+pub fn supported_protocol_versions() -> RangeInclusive<u16> {
+    CURRENT_PROTOCOL_VERSION.saturating_sub(1)..=CURRENT_PROTOCOL_VERSION
+}
+
 #[repr(C)]
 #[derive(Default, Copy, Clone)]
 pub struct Request {
     command: Command,
-    padding: [u8; 6],
-    length: u64, // Length of payload for command excluding the Request struct
+    command_headers: [u8; 6],
+    /// Length of payload for command excluding the Request struct
+    length: u64,
 }
 
 // SAFETY: Request contains a series of integers with no implicit padding
 unsafe impl ByteValued for Request {}
 
 impl Request {
+    fn encode_sender_version(version: u16) -> [u8; 6] {
+        let mut command_headers = [0; 6];
+        command_headers[..size_of::<u16>()].copy_from_slice(&version.to_le_bytes());
+        command_headers
+    }
+
     pub fn new(command: Command, length: u64) -> Self {
         Self {
             command,
@@ -148,7 +177,11 @@ impl Request {
     }
 
     pub fn start() -> Self {
-        Self::new(Command::Start, 0)
+        Self {
+            command: Command::Start,
+            command_headers: Self::encode_sender_version(CURRENT_PROTOCOL_VERSION),
+            length: 0,
+        }
     }
 
     pub fn state(length: u64) -> Self {
@@ -189,6 +222,10 @@ impl Request {
         self.length
     }
 
+    pub fn command_headers(&self) -> &[u8; 6] {
+        &self.command_headers
+    }
+
     pub fn read_from(fd: &mut dyn Read) -> Result<Request, MigratableError> {
         let mut request = Request::default();
         fd.read_exact(Self::as_mut_slice(&mut request))
@@ -485,7 +522,21 @@ impl MemoryRangeTable {
 
 #[cfg(test)]
 mod unit_tests {
-    use crate::protocol::{MemoryRange, MemoryRangeTable};
+    use crate::protocol::{Command, MemoryRange, MemoryRangeTable, Request};
+
+    #[test]
+    fn test_start_request_ignores_residual_command_headers_bytes() {
+        let request = Request {
+            command: Command::Start,
+            command_headers: [1, 0, 0xaa, 0xbb, 0xcc, 0xdd],
+            length: 0,
+        };
+
+        assert_eq!(
+            u16::from_le_bytes([request.command_headers()[0], request.command_headers()[1]]),
+            1
+        );
+    }
 
     #[test]
     fn test_memory_range_table_from_dirty_ranges_iter() {