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 microVM with larger buffers to a
microVM with smaller ones, some guest workloads might not work due to
the lack of the buffer spaces. The issue is not a problem of the
snapshot feature and should also occur on a purely booted microVM (not
restored from a snapshot). Thus, removes those fields from snapshots.

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

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

Author: zulinx86

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(