snapshot: avoid duplicating pages mapped multiple times

Previously, if a sandbox mapped one physical page to multiple virtual
addresses, taking a snapshot would duplicate the page. This commit
changes the logic around snapshots to keep track of the physical pages
that have been encountered during the virtual memory traversal so far,
and ensure that the same page is not copied multiple times.  Note that
this does not (yet) track the contents of pages to allow
de-duplicating them---it merely ensures that two mappings of the same
physical address do not result in needless duplication.

Signed-off-by: Lucy Menon <168595099+syntactically@users.noreply.github.com>

Author: syntactically

src/hyperlight_host/src/sandbox/snapshot.rs

@@ -447,6 +447,8 @@ impl Snapshot {
         sregs: CommonSpecialRegisters,
         entrypoint: NextAction,
     ) -> Result<Self> {
+        use std::collections::HashMap;
+        let mut phys_seen = HashMap::<u64, usize>::new();
         let memory = shared_mem.with_contents(|snap_c| {
             scratch_mem.with_contents(|scratch_c| {
                 // Pass 1: count how many pages need to live
@@ -458,9 +460,6 @@ impl Snapshot {
                 let pt_buf = GuestPageTableBuffer::new(layout.get_pt_base_gpa() as usize);
                 let mut snapshot_memory: Vec<u8> = Vec::new();
                 for (mapping, contents) in live_pages {
-                    let new_offset = snapshot_memory.len();
-                    snapshot_memory.extend(contents);
-                    let new_gpa = new_offset + SandboxMemoryLayout::BASE_ADDRESS;
                     let kind = match mapping.kind {
                         MappingKind::Cow(cm) => MappingKind::Cow(cm),
                         MappingKind::Basic(bm) if bm.writable => MappingKind::Cow(CowMapping {
@@ -473,8 +472,13 @@ impl Snapshot {
                             executable: bm.executable,
                         }),
                     };
+                    let new_gpa = phys_seen.entry(mapping.phys_base).or_insert_with(|| {
+                        let new_offset = snapshot_memory.len();
+                        snapshot_memory.extend(contents);
+                        new_offset + SandboxMemoryLayout::BASE_ADDRESS
+                    });
                     let mapping = Mapping {
-                        phys_base: new_gpa as u64,
+                        phys_base: *new_gpa as u64,
                         virt_base: mapping.virt_base,
                         len: PAGE_SIZE as u64,
                         kind,