hwrng: virtio - don't waste entropy

if we don't use all the entropy available in the buffer, keep it
and use it later.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Link: https://lore.kernel.org/r/20211028101111.128049-4-lvivier@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/char/hw_random/virtio-rng.c

@@ -26,6 +26,7 @@ struct virtrng_info {
 	/* data transfer */
 	struct completion have_data;
 	unsigned int data_avail;
+	unsigned int data_idx;
 	/* minimal size returned by rng_buffer_size() */
 #if SMP_CACHE_BYTES < 32
 	u8 data[32];
@@ -42,6 +43,9 @@ static void random_recv_done(struct virtqueue *vq)
 	if (!virtqueue_get_buf(vi->vq, &vi->data_avail))
 		return;
 
+	vi->data_idx = 0;
+	vi->busy = false;
+
 	complete(&vi->have_data);
 }
 
@@ -58,6 +62,16 @@ static void register_buffer(struct virtrng_info *vi)
 	virtqueue_kick(vi->vq);
 }
 
+static unsigned int copy_data(struct virtrng_info *vi, void *buf,
+			      unsigned int size)
+{
+	size = min_t(unsigned int, size, vi->data_avail);
+	memcpy(buf, vi->data + vi->data_idx, size);
+	vi->data_idx += size;
+	vi->data_avail -= size;
+	return size;
+}
+
 static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 {
 	int ret;
@@ -68,17 +82,29 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 	if (vi->hwrng_removed)
 		return -ENODEV;
 
-	if (!vi->busy) {
-		vi->busy = true;
-		reinit_completion(&vi->have_data);
-		register_buffer(vi);
+	read = 0;
+
+	/* copy available data */
+	if (vi->data_avail) {
+		chunk = copy_data(vi, buf, size);
+		size -= chunk;
+		read += chunk;
 	}
 
 	if (!wait)
-		return 0;
+		return read;
 
-	read = 0;
+	/* We have already copied available entropy,
+	 * so either size is 0 or data_avail is 0
+	 */
 	while (size != 0) {
+		/* data_avail is 0 */
+		if (!vi->busy) {
+			/* no pending request, ask for more */
+			vi->busy = true;
+			reinit_completion(&vi->have_data);
+			register_buffer(vi);
+		}
 		ret = wait_for_completion_killable(&vi->have_data);
 		if (ret < 0)
 			return ret;
@@ -88,20 +114,11 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 		if (vi->data_avail == 0)
 			return read;
 
-		chunk = min_t(unsigned int, size, vi->data_avail);
-		memcpy(buf + read, vi->data, chunk);
-		read += chunk;
+		chunk = copy_data(vi, buf + read, size);
 		size -= chunk;
-		vi->data_avail = 0;
-
-		if (size != 0) {
-			reinit_completion(&vi->have_data);
-			register_buffer(vi);
-		}
+		read += chunk;
 	}
 
-	vi->busy = false;
-
 	return read;
 }
 
@@ -161,6 +178,7 @@ static void remove_common(struct virtio_device *vdev)
 
 	vi->hwrng_removed = true;
 	vi->data_avail = 0;
+	vi->data_idx = 0;
 	complete(&vi->have_data);
 	vdev->config->reset(vdev);
 	vi->busy = false;