feat: fix retransmission issues with MRG_RXBUF

Fix retransmission issues introduced by enabling
MRG_RXBUF. Do this by tracking total capacity of the
`RxBuffer` struct and only return an iov slice if
it contains at least 64K bytes.

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse

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

@@ -650,7 +650,10 @@ impl Net {
 
     fn read_tap(&mut self) -> std::io::Result<usize> {
         if self.has_feature(u64::from(VIRTIO_NET_F_MRG_RXBUF)) {
-            self.tap.read_iovec(self.rx_buffer.all_chains_mut_slice())
+            let Some(s) = self.rx_buffer.all_chains_mut_slice() else {
+                return Err(std::io::Error::from_raw_os_error(EAGAIN));
+            };
+            self.tap.read_iovec(s)
         } else {
             self.tap.read_iovec(self.rx_buffer.one_chain_mut_slice())
         }

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

@@ -61,6 +61,8 @@ pub struct RxBuffer {
     pub iovecs: IovRingBuffer,
     // Ring buffer of meta data about descriptor chains stored in the `iov_ring`.
     pub chain_infos: RingBuffer<ChainInfo>,
+    // Total capacity this buffer holds.
+    pub total_capacity: u32,
     // Number of descriptor chains we have used to process packets.
     pub used_descriptors: u16,
 }
@@ -72,6 +74,7 @@ impl RxBuffer {
             iovecs: IovRingBuffer::new()?,
             chain_infos: RingBuffer::new_with_size(u32::from(FIRECRACKER_MAX_QUEUE_SIZE)),
             used_descriptors: 0,
+            total_capacity: 0,
         })
     }
 
@@ -93,8 +96,12 @@ impl RxBuffer {
 
     /// Returns a slice of underlying iovec for the all chains
     /// in the buffer.
-    pub fn all_chains_mut_slice(&mut self) -> &mut [iovec] {
-        self.iovecs.as_mut_slice()
+    pub fn all_chains_mut_slice(&mut self) -> Option<&mut [iovec]> {
+        if self.total_capacity < u32::from(u16::MAX) {
+            None
+        } else {
+            Some(self.iovecs.as_mut_slice())
+        }
     }
 
     /// Add a new `DescriptorChain` that we received from the RX queue in the buffer.
@@ -151,6 +158,7 @@ impl RxBuffer {
             chain_len,
             chain_capacity,
         });
+        self.total_capacity += chain_capacity;
 
         Ok(())
     }
@@ -210,6 +218,7 @@ impl RxBuffer {
                 .pop_front()
                 .expect("This should never happen if write to the buffer succeded.");
             self.iovecs.pop_front(usize::from(iov_info.chain_len));
+            self.total_capacity -= iov_info.chain_capacity;
 
             if bytes_written <= iov_info.chain_capacity {
                 write_used(iov_info.head_index, bytes_written, rx_queue);