[PATCH vhost v10 02/10] virtio_ring: introduce virtqueue_set_premapped()

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

This helper allows the driver change the dma mode to premapped mode.
Under the premapped mode, the virtio core do not do dma mapping
internally.

This just work when the use_dma_api is true. If the use_dma_api is false,
the dma options is not through the DMA APIs, that is not the standard
way of the linux kernel.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio_ring.c | 40 ++++++++++++++++++++++++++++++++++++
 include/linux/virtio.h       |  2 ++
 2 files changed, 42 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 72ed07a604d4..2afdfb9e3e30 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -172,6 +172,9 @@ struct vring_virtqueue {
 	/* Host publishes avail event idx */
 	bool event;
 
+	/* Do DMA mapping by driver */
+	bool premapped;
+
 	/* Head of free buffer list. */
 	unsigned int free_head;
 	/* Number we've added since last sync. */
@@ -2059,6 +2062,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
 	vq->packed_ring = true;
 	vq->dma_dev = dma_dev;
 	vq->use_dma_api = vring_use_dma_api(vdev);
+	vq->premapped = false;
 
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
 		!context;
@@ -2548,6 +2552,7 @@ static struct virtqueue *__vring_new_virtqueue(unsigned int index,
 #endif
 	vq->dma_dev = dma_dev;
 	vq->use_dma_api = vring_use_dma_api(vdev);
+	vq->premapped = false;
 
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
 		!context;
@@ -2691,6 +2696,41 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
 }
 EXPORT_SYMBOL_GPL(virtqueue_resize);
 
+/**
+ * virtqueue_set_premapped - set the vring premapped mode
+ * @_vq: the struct virtqueue we're talking about.
+ *
+ * Enable the premapped mode of the vq.
+ *
+ * The vring in premapped mode does not do dma internally, so the driver must
+ * do dma mapping in advance. The driver must pass the dma_address through
+ * dma_address of scatterlist. When the driver got a used buffer from
+ * the vring, it has to unmap the dma address. So the driver must call
+ * virtqueue_get_buf_premapped()/virtqueue_detach_unused_buf_premapped().
+ *
+ * This must be called before adding any buf to vring.
+ * So this should be called immediately after init vq or vq reset.
+ *
+ * Caller must ensure we don't call this with other virtqueue operations
+ * at the same time (except where noted).
+ *
+ * Returns zero or a negative error.
+ * 0: success.
+ * -EINVAL: vring does not use the dma api, so we can not enable premapped mode.
+ */
+int virtqueue_set_premapped(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
+	if (!vq->use_dma_api)
+		return -EINVAL;
+
+	vq->premapped = true;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(virtqueue_set_premapped);
+
 /* Only available for split ring */
 struct virtqueue *vring_new_virtqueue(unsigned int index,
 				      unsigned int num,
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index b93238db94e3..1fc0e1023bd4 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -78,6 +78,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
 
 unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
 
+int virtqueue_set_premapped(struct virtqueue *_vq);
+
 bool virtqueue_poll(struct virtqueue *vq, unsigned);
 
 bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
-- 
2.32.0.3.g01195cf9f