virtio-devices: Optimize add_used() usage

Now that we rely on pop_descriptor_chain() rather than iter() to iterate
over a queue, there's no more borrow on the queue itself, meaning we can
invoke add_used() directly for the iteration loop. This simplifies the
processing of the queues for each virtio device, and bring some possible
performance improvement given we don't have to iterate twice over the
list of descriptors to invoke add_used().

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

net_util/src/ctrl_queue.rs

@@ -62,7 +62,6 @@ impl CtrlQueue {
         queue: &mut Queue,
         access_platform: Option<&Arc<dyn AccessPlatform>>,
     ) -> Result<()> {
-        let mut used_desc_heads = Vec::new();
         while let Some(mut desc_chain) = queue.pop_descriptor_chain(mem) {
             let ctrl_desc = desc_chain.next().ok_or(Error::NoControlHeaderDescriptor)?;
 
@@ -139,7 +138,10 @@ impl CtrlQueue {
                 )
                 .map_err(Error::GuestMemory)?;
             let len = ctrl_desc.len() + data_desc.len() + status_desc.len();
-            used_desc_heads.push((desc_chain.head_index(), len));
+
+            queue
+                .add_used(desc_chain.memory(), desc_chain.head_index(), len)
+                .map_err(Error::QueueAddUsed)?;
 
             if !queue
                 .enable_notification(mem)
@@ -149,12 +151,6 @@ impl CtrlQueue {
             }
         }
 
-        for (desc_index, len) in used_desc_heads.iter() {
-            queue
-                .add_used(mem, *desc_index, *len)
-                .map_err(Error::QueueAddUsed)?;
-        }
-
         Ok(())
     }
 }