Change IoVecBuffer[Mut] len to u32

This commit changes the iovec len primitive to match descriptor chain's
(u32). This removes some ugly casting and potential overflow problems,
and allows us to upcast when needed in a non-lossy manor.

Signed-off-by: Brandon Pike <bpike@amazon.com>

src/vmm/src/devices/virtio/net/device.rs

@@ -472,7 +472,7 @@ impl Net {
                 METRICS.mmds.rx_accepted.inc();
 
                 // MMDS frames are not accounted by the rate limiter.
-                Self::rate_limiter_replenish_op(rate_limiter, frame_iovec.len() as u64);
+                Self::rate_limiter_replenish_op(rate_limiter, u64::from(frame_iovec.len()));
 
                 // MMDS consumed the frame.
                 return Ok(true);
@@ -493,7 +493,7 @@ impl Net {
         let _metric = net_metrics.tap_write_agg.record_latency_metrics();
         match Self::write_tap(tap, frame_iovec) {
             Ok(_) => {
-                let len = frame_iovec.len() as u64;
+                let len = u64::from(frame_iovec.len());
                 net_metrics.tx_bytes_count.add(len);
                 net_metrics.tx_packets_count.inc();
                 net_metrics.tx_count.inc();
@@ -618,7 +618,7 @@ impl Net {
                 continue;
             }
 
-            if !Self::rate_limiter_consume_op(&mut self.tx_rate_limiter, buffer.len() as u64) {
+            if !Self::rate_limiter_consume_op(&mut self.tx_rate_limiter, u64::from(buffer.len())) {
                 tx_queue.undo_pop();
                 self.metrics.tx_rate_limiter_throttled.inc();
                 break;

src/vmm/src/devices/virtio/rng/device.rs

@@ -120,7 +120,7 @@ impl Entropy {
 
         // It is ok to unwrap here. We are writing `iovec.len()` bytes at offset 0.
         iovec.write_all_volatile_at(&rand_bytes, 0).unwrap();
-        Ok(iovec.len().try_into().unwrap())
+        Ok(iovec.len())
     }
 
     fn process_entropy_queue(&mut self) {
@@ -142,7 +142,7 @@ impl Entropy {
                     // Check for available rate limiting budget.
                     // If not enough budget is available, leave the request descriptor in the queue
                     // to handle once we do have budget.
-                    if !Self::rate_limit_request(&mut self.rate_limiter, iovec.len() as u64) {
+                    if !Self::rate_limit_request(&mut self.rate_limiter, u64::from(iovec.len())) {
                         debug!("entropy: throttling entropy queue");
                         METRICS.entropy_rate_limiter_throttled.inc();
                         self.queues[RNG_QUEUE].undo_pop();