vmm: Perform sparse initial memory transfer for live migration

Add sparse region support to remote live migration as it will save some
of the network bandwidth and therefore speed up the migration to the
destination VM.

Instead of sending the entire guest RAM over the network, only send
ranges which have been populated during the first iteration. Pages full
of holes are simply skipped.

Signed-off-by: Sebastien Boeuf <sboeuf@meta.com>
Assisted-by: Claude:claude-opus-4-8

vmm/src/lib.rs

@@ -56,7 +56,7 @@ use crate::config::{MemoryRestoreMode, RestoreConfig, add_to_config};
 use crate::coredump::GuestDebuggable;
 use crate::device_manager::DeviceManager;
 use crate::landlock::Landlock;
-use crate::memory_manager::MemoryManager;
+use crate::memory_manager::{MemoryManager, MemoryRangePolicy};
 #[cfg(all(feature = "kvm", target_arch = "x86_64"))]
 use crate::migration::get_vm_snapshot;
 use crate::migration::transport::{
@@ -1112,7 +1112,10 @@ impl Vmm {
                     ))
                 })?;
             let mm = config_data.memory_manager.clone();
-            let saved_regions = mm.lock().unwrap().memory_range_table(false)?;
+            let saved_regions = mm
+                .lock()
+                .unwrap()
+                .memory_range_table(MemoryRangePolicy::Full)?;
             mm.lock()
                 .unwrap()
                 .start_postcopy_serving(
@@ -1372,7 +1375,7 @@ impl Vmm {
             let iteration_begin = Instant::now();
 
             let iteration_table = if ctx.iteration == 0 {
-                vm.memory_range_table()?
+                vm.memory_range_table(MemoryRangePolicy::Sparse)?
             } else {
                 // TODO do this in a thread #7816
                 vm.dirty_log()?