feat: add read_iovec to the tap

Add method to use readv syscall to read
from the tap into iov.

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

Author: ShadowCurse

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

@@ -185,6 +185,20 @@ impl Tap {
         Ok(())
     }
 
+    /// Read into slice of iovec from the tap
+    pub(crate) fn read_iovec(&mut self, buffers: &[libc::iovec]) -> Result<usize, IoError> {
+        let iovcnt = i32::try_from(buffers.len()).unwrap();
+        let iov = buffers.as_ptr();
+
+        // SAFETY: `writev` is safe. Called with a valid tap fd, the iovec pointer and length
+        // is provide by the `IoVecBuffer` implementation and we check the return value.
+        let ret = unsafe { libc::readv(self.tap_file.as_raw_fd(), iov, iovcnt) };
+        if ret == -1 {
+            return Err(IoError::last_os_error());
+        }
+        Ok(usize::try_from(ret).unwrap())
+    }
+
     /// Write an `IoVecBuffer` to tap
     pub(crate) fn write_iovec(&mut self, buffer: &IoVecBuffer) -> Result<usize, IoError> {
         let iovcnt = i32::try_from(buffer.iovec_count()).unwrap();