volatile_memory: Only use slow path when needed

Where small objects are those objects that are less then the native data
width for the platform. This ensure that volatile and alignment safe
read/writes are used when updating structures that are sensitive to this
such as virtio devices where the spec requires writes to be atomic.

Fixes: cloud-hypervisor/cloud-hypervisor#1258
Fixes: #100

Signed-off-by: Rob Bradford <[email protected]>

Author: rbradford

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 85.4,
+  "coverage_score": 85.5,
   "exclude_path": "mmap_windows.rs",
   "crate_features": "backend-mmap,backend-atomic"
 }

src/volatile_memory.rs

@@ -1082,7 +1082,7 @@ mod copy_slice_impl {
         }
     }
 
-    pub(super) fn copy_slice(dst: &mut [u8], src: &[u8]) -> usize {
+    fn copy_slice_volatile(dst: &mut [u8], src: &[u8]) -> usize {
         let total = min(src.len(), dst.len());
         let mut left = total;
 
@@ -1110,6 +1110,17 @@ mod copy_slice_impl {
 
         total
     }
+
+    pub(super) fn copy_slice(dst: &mut [u8], src: &[u8]) -> usize {
+        let total = min(src.len(), dst.len());
+        if total <= size_of::<usize>() {
+            copy_slice_volatile(dst, src);
+        } else {
+            dst[..total].copy_from_slice(&src[..total]);
+        }
+
+        total
+    }
 }
 
 #[cfg(test)]