refactor(virtio-net): avoid copy on rx

resources/seccomp/aarch64-unknown-linux-musl.json

@@ -25,6 +25,10 @@
             {
                 "syscall": "read"
             },
+            {
+                "syscall": "readv",
+                "comment": "Used by the VirtIO net device to read from tap"
+            },
             {
                 "syscall": "write"
             },

resources/seccomp/x86_64-unknown-linux-musl.json

@@ -25,6 +25,10 @@
             {
                 "syscall": "read"
             },
+            {
+                "syscall": "readv",
+                "comment": "Used by the VirtIO net device to read from tap"
+            },
             {
                 "syscall": "write"
             },

src/vmm/src/devices/virtio/iovec.rs

@@ -285,7 +285,7 @@
                 .checked_add(desc.len)
                 .ok_or(IoVecError::OverflowedDescriptor)?;
             if matches!(max_size, Some(max) if self.len >= max) {
                 break;
             }
             if desc.load_next_descriptor().is_none() {
                 break;
@@ -327,6 +327,23 @@
         self.len = 0u32;
     }
 
+    /// Push an iovec into the `IoVecBufferMut`.
+    /// # Safety
+    /// The iovec must refer to a valid memory slice.
+    pub unsafe fn push(&mut self, iovec: iovec) -> Result<(), IoVecError> {
+        let iov_len = iovec
+            .iov_len
+            .try_into()
+            .map_err(|_| IoVecError::OverflowedDescriptor)?;
+
+        self.vecs.push(iovec);
+        self.len = self
+            .len
+            .checked_add(iov_len)
+            .ok_or(IoVecError::OverflowedDescriptor)?;
+        Ok(())
+    }
+
     /// Writes a number of bytes into the `IoVecBufferMut` starting at a given offset.
     ///
     /// This will try to fill `IoVecBufferMut` writing bytes from the `buf` starting from