refactor: drop offset field from GuestMemoryRegionState

In praxis, the way we wrote our snapshot files has always been just
writing all regions in-order. This mean that the offset of a region is
simply the sum of the sizes of the preceding regions. The new
`GuestMemoryMmap::create` code already computes the offsets for mapping
the memory file this way, so drop the explicit calculation at snapshot
creation time (as the calculated value isnt used by the restoration
anymore).

Bumps the snapshot version number.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/vmm/src/persist.rs

@@ -161,7 +161,7 @@ pub enum CreateSnapshotError {
 }
 
 /// Snapshot version
-pub const SNAPSHOT_VERSION: Version = Version::new(5, 0, 0);
+pub const SNAPSHOT_VERSION: Version = Version::new(6, 0, 0);
 
 /// Creates a Microvm snapshot.
 pub fn create_snapshot(
@@ -604,13 +604,15 @@ fn create_guest_memory(
         track_dirty_pages,
     )?;
     let mut backend_mappings = Vec::with_capacity(guest_memory.num_regions());
-    for (mem_region, state_region) in guest_memory.iter().zip(mem_state.regions.iter()) {
+    let mut offset = 0;
+    for mem_region in guest_memory.iter() {
         backend_mappings.push(GuestRegionUffdMapping {
             base_host_virt_addr: mem_region.as_ptr() as u64,
             size: mem_region.size(),
-            offset: state_region.offset,
+            offset,
             page_size_kib: huge_pages.page_size_kib(),
         });
+        offset += mem_region.size() as u64;
     }
 
     Ok((guest_memory, backend_mappings))
@@ -790,7 +792,6 @@ mod tests {
             regions: vec![GuestMemoryRegionState {
                 base_address: 0,
                 size: 0x20000,
-                offset: 0x10000,
             }],
         };
 
@@ -799,7 +800,7 @@ mod tests {
 
         assert_eq!(uffd_regions.len(), 1);
         assert_eq!(uffd_regions[0].size, 0x20000);
-        assert_eq!(uffd_regions[0].offset, 0x10000);
+        assert_eq!(uffd_regions[0].offset, 0);
         assert_eq!(
             uffd_regions[0].page_size_kib,
             HugePageConfig::None.page_size_kib()

src/vmm/src/vstate/memory.rs

@@ -129,8 +129,6 @@ pub struct GuestMemoryRegionState {
     pub base_address: u64,
     /// Region size.
     pub size: usize,
-    /// Offset in file/buffer where the region is saved.
-    pub offset: u64,
 }
 
 /// Describes guest memory regions and their snapshot file mappings.
@@ -184,15 +182,11 @@ impl GuestMemoryExtension for GuestMemoryMmap {
     /// Describes GuestMemoryMmap through a GuestMemoryState struct.
     fn describe(&self) -> GuestMemoryState {
         let mut guest_memory_state = GuestMemoryState::default();
-        let mut offset = 0;
         self.iter().for_each(|region| {
             guest_memory_state.regions.push(GuestMemoryRegionState {
                 base_address: region.start_addr().0,
                 size: u64_to_usize(region.len()),
-                offset,
             });
-
-            offset += region.len();
         });
         guest_memory_state
     }
@@ -567,12 +561,10 @@ mod tests {
                 GuestMemoryRegionState {
                     base_address: 0,
                     size: page_size,
-                    offset: 0,
                 },
                 GuestMemoryRegionState {
                     base_address: page_size as u64 * 2,
                     size: page_size,
-                    offset: page_size as u64,
                 },
             ],
         };
@@ -597,12 +589,10 @@ mod tests {
                 GuestMemoryRegionState {
                     base_address: 0,
                     size: page_size * 3,
-                    offset: 0,
                 },
                 GuestMemoryRegionState {
                     base_address: page_size as u64 * 4,
                     size: page_size * 3,
-                    offset: page_size as u64 * 3,
                 },
             ],
         };