vhost-net: cleanup macros for DMA status tracking

Better document macros for DMA tracking. Add an
explicit one for DMA in progress instead of
relying on user supplying len != 1.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/net.c

@@ -237,7 +237,8 @@ static void handle_tx(struct vhost_net *net)
 			} else {
 				struct ubuf_info *ubuf = &vq->ubuf_info[head];
 
-				vq->heads[vq->upend_idx].len = len;
+				vq->heads[vq->upend_idx].len =
+					VHOST_DMA_IN_PROGRESS;
 				ubuf->callback = vhost_zerocopy_callback;
 				ubuf->ctx = vq->ubufs;
 				ubuf->desc = vq->upend_idx;

drivers/vhost/vhost.c

@@ -1606,7 +1606,7 @@ void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool status)
 	struct vhost_virtqueue *vq = ubufs->vq;
 
 	vhost_poll_queue(&vq->poll);
-	/* set len = 1 to mark this desc buffers done DMA */
+	/* set len to mark this desc buffers done DMA */
 	vq->heads[ubuf->desc].len = VHOST_DMA_DONE_LEN;
 	kref_put(&ubufs->kref, vhost_zerocopy_done_signal);
 }

drivers/vhost/vhost.h

@@ -13,9 +13,15 @@
 #include <linux/virtio_ring.h>
 #include <linux/atomic.h>
 
-/* This is for zerocopy, used buffer len is set to 1 when lower device DMA
- * done */
-#define VHOST_DMA_DONE_LEN	1
+/*
+ * For transmit, used buffer len is unused; we override it to track buffer
+ * status internally; used for zerocopy tx only.
+ */
+/* Lower device DMA done */
+#define VHOST_DMA_DONE_LEN	2
+/* Lower device DMA in progress */
+#define VHOST_DMA_IN_PROGRESS	1
+/* Buffer unused */
 #define VHOST_DMA_CLEAR_LEN	0
 
 struct vhost_device;