Add DmaBuf for DMA-capable memory areas

kernel/comps/virtio/src/dma_buf.rs

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use aster_frame::vm::{DmaCoherent, DmaStream, HasDaddr};
+
+/// A DMA-capable buffer.
+///
+/// Any type implements this trait should also implements `HasDaddr` trait,
+/// and provides the exact length of DMA area.
+#[allow(clippy::len_without_is_empty)]
+pub trait DmaBuf: HasDaddr {
+    /// The length of Dma area, in bytes
+    fn len(&self) -> usize;
+}
+
+impl DmaBuf for DmaStream {
+    fn len(&self) -> usize {
+        self.nbytes()
+    }
+}
+
+impl DmaBuf for DmaCoherent {
+    fn len(&self) -> usize {
+        self.nbytes()
+    }
+}