disk_layered: populate read caches (#683)

Populate read cache layers in the read path.

Cargo.lock

@@ -1242,8 +1242,11 @@ dependencies = [
  "futures",
  "guestmem",
  "inspect",
+ "pal_async",
+ "parking_lot",
  "scsi_buffers",
  "thiserror 2.0.0",
+ "tracelimit",
  "vm_resource",
 ]
 

vm/devices/storage/disk_layered/Cargo.toml

@@ -14,11 +14,17 @@ scsi_buffers.workspace = true
 guestmem.workspace = true
 vm_resource.workspace = true
 inspect = { workspace = true, features = ["std"] }
+tracelimit.workspace = true
 
 anyhow.workspace = true
 async-trait.workspace = true
 futures.workspace = true
 thiserror.workspace = true
 
+[dev-dependencies]
+pal_async.workspace = true
+
+parking_lot.workspace = true
+
 [lints]
 workspace = true

vm/devices/storage/disk_layered/src/bitmap.rs

@@ -77,6 +77,19 @@ impl SectorBitmapRange<'_> {
     pub fn set_count(&self) -> usize {
         self.set_count
     }
+
+    pub(crate) fn unset_iter(&self) -> impl '_ + Iterator<Item = Range<u64>> {
+        let mut n = self.start_sector;
+        self.bits.chunk_by(|&a, &b| a == b).filter_map(move |bits| {
+            let start = n;
+            n += bits.len() as u64;
+            if bits.first().is_some_and(|&x| !x) {
+                Some(start..n)
+            } else {
+                None
+            }
+        })
+    }
 }
 
 /// A type to mark sectors that have been read by a layer as part of a