snapshot: Remove max_connections and max_pending_resets fields

`TcpIPv4Handler` for MMDS network stack preallocates several buffers
whose sizes are saved into a snapshot as `max_connections` and
`max_pending_resets` in `MmdsNetworkStackState`. But they are always the
same constant hardcoded values (`DEFAULT_MAX_CONNECTIONS` and
`DEFAULT_MAX_PENDING_RESETS`) as of today, which means there is no need
to save them into a snapshot. Even if we change the hardcoded sizes
across Firecracker versions, that should not be a problem. This is
because the snapshot feature does not support migration of network
connections and those buffers are initialized with empty on snapshot
restoration. When migrating from a Firecracker version with larger
buffers to another version with smaller ones, guest workloads that
worked previously might start to fail due to the less buffer spaces.
However, the issue is not a problem of the snapshot feature and it
should also occur even on a purely booted microVM (not restored from a
snapshot). Thus, it is fine to remove those fields from a snapshot.

Since this is a breaking change of the snapshot format, bumps the major
version.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to
 
 ### Changed
 
+- [#4913](https://github.com/firecracker-microvm/firecracker/pull/4913): Removed
+  unnecessary fields (`max_connections` and `max_pending_resets`) from the
+  snapshot format, bumping the snapshot version to 5.0.0.
+
 ### Deprecated
 
 ### Removed

src/vmm/src/mmds/ns.rs

@@ -81,8 +81,6 @@ impl MmdsNetworkStack {
         mac_addr: MacAddr,
         ipv4_addr: Ipv4Addr,
         tcp_port: u16,
-        max_connections: NonZeroUsize,
-        max_pending_resets: NonZeroUsize,
         mmds: Arc<Mutex<Mmds>>,
     ) -> Self {
         MmdsNetworkStack {
@@ -93,8 +91,8 @@ impl MmdsNetworkStack {
             tcp_handler: TcpIPv4Handler::new(
                 ipv4_addr,
                 tcp_port,
-                max_connections,
-                max_pending_resets,
+                NonZeroUsize::new(DEFAULT_MAX_CONNECTIONS).unwrap(),
+                NonZeroUsize::new(DEFAULT_MAX_PENDING_RESETS).unwrap(),
             ),
             mmds,
         }
@@ -105,14 +103,7 @@ impl MmdsNetworkStack {
         let ipv4_addr = mmds_ipv4_addr.unwrap_or_else(|| Ipv4Addr::from(DEFAULT_IPV4_ADDR));
 
         // The unwrap()s are safe because the given literals are greater than 0.
-        Self::new(
-            mac_addr,
-            ipv4_addr,
-            DEFAULT_TCP_PORT,
-            NonZeroUsize::new(DEFAULT_MAX_CONNECTIONS).unwrap(),
-            NonZeroUsize::new(DEFAULT_MAX_PENDING_RESETS).unwrap(),
-            mmds,
-        )
+        Self::new(mac_addr, ipv4_addr, DEFAULT_TCP_PORT, mmds)
     }
 
     pub fn set_ipv4_addr(&mut self, ipv4_addr: Ipv4Addr) {

src/vmm/src/mmds/persist.rs

@@ -4,7 +4,6 @@
 //! Defines the structures needed for saving/restoring MmdsNetworkStack.
 
 use std::net::Ipv4Addr;
-use std::num::NonZeroUsize;
 use std::sync::{Arc, Mutex};
 
 use serde::{Deserialize, Serialize};
@@ -20,8 +19,6 @@ pub struct MmdsNetworkStackState {
     mac_addr: [u8; MAC_ADDR_LEN as usize],
     ipv4_addr: u32,
     tcp_port: u16,
-    max_connections: NonZeroUsize,
-    max_pending_resets: NonZeroUsize,
 }
 
 impl Persist<'_> for MmdsNetworkStack {
@@ -37,8 +34,6 @@ impl Persist<'_> for MmdsNetworkStack {
             mac_addr,
             ipv4_addr: self.ipv4_addr.into(),
             tcp_port: self.tcp_handler.local_port(),
-            max_connections: self.tcp_handler.max_connections(),
-            max_pending_resets: self.tcp_handler.max_pending_resets(),
         }
     }
 
@@ -50,8 +45,6 @@ impl Persist<'_> for MmdsNetworkStack {
             MacAddr::from_bytes_unchecked(&state.mac_addr),
             Ipv4Addr::from(state.ipv4_addr),
             state.tcp_port,
-            state.max_connections,
-            state.max_pending_resets,
             mmds,
         ))
     }
@@ -83,13 +76,5 @@ mod tests {
             restored_ns.tcp_handler.local_port(),
             ns.tcp_handler.local_port()
         );
-        assert_eq!(
-            restored_ns.tcp_handler.max_connections(),
-            ns.tcp_handler.max_connections()
-        );
-        assert_eq!(
-            restored_ns.tcp_handler.max_pending_resets(),
-            ns.tcp_handler.max_pending_resets()
-        );
     }
 }

src/vmm/src/persist.rs

@@ -157,7 +157,7 @@ pub enum CreateSnapshotError {
 }
 
 /// Snapshot version
-pub const SNAPSHOT_VERSION: Version = Version::new(4, 0, 0);
+pub const SNAPSHOT_VERSION: Version = Version::new(5, 0, 0);
 
 /// Creates a Microvm snapshot.
 pub fn create_snapshot(