vm-virtio: net: Expose network counters through VirtioDevice

Through the counters() function on the trait expose the accumulated
counters.

TEST=Observe that the counters from the VM match those from the tap on
the host (RX-TX inverted) and inside the guest (non inverted.)

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-virtio/src/net.rs

@@ -20,6 +20,7 @@ use libc::EAGAIN;
 use libc::EFD_NONBLOCK;
 use net_util::{MacAddr, Tap};
 use std::cmp;
+use std::collections::HashMap;
 use std::fs::File;
 use std::io::Read;
 use std::io::{self, Write};
@@ -739,6 +740,29 @@ impl VirtioDevice for Net {
             self.queue_evts.take().unwrap(),
         ))
     }
+
+    fn counters(&self) -> Option<HashMap<&'static str, Wrapping<u64>>> {
+        let mut counters = HashMap::new();
+
+        counters.insert(
+            "rx_bytes",
+            Wrapping(self.counters.rx_bytes.load(Ordering::Acquire)),
+        );
+        counters.insert(
+            "rx_frames",
+            Wrapping(self.counters.rx_frames.load(Ordering::Acquire)),
+        );
+        counters.insert(
+            "tx_bytes",
+            Wrapping(self.counters.tx_bytes.load(Ordering::Acquire)),
+        );
+        counters.insert(
+            "tx_frames",
+            Wrapping(self.counters.tx_frames.load(Ordering::Acquire)),
+        );
+
+        Some(counters)
+    }
 }
 
 virtio_ctrl_q_pausable!(Net);