[RFC 4/5] virtio_ring: add virtqueue premapped out/in buffers support

["Eugenio Pérez" <eperezma@redhat.com>]

Convenience function to add chains that mixes out and in buffers.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 drivers/virtio/virtio_ring.c | 35 +++++++++++++++++++++++++++++++++++
 include/linux/virtio.h       |  7 +++++++
 2 files changed, 42 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 05729bc5cbb1..e49912fa77c5 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -2385,6 +2385,41 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 					out_sgs, in_sgs, data, ctx, premapped, gfp);
 }
 
+/**
+ * virtqueue_add_sgs_premapped - expose buffers to other end
+ * @_vq: the struct virtqueue we're talking about.
+ * @sgs: array of terminated scatterlists.
+ * @out_sgs: the number of scatterlists readable by other side
+ * @in_sgs: the number of scatterlists which are writable (after readable ones)
+ * @data: the token identifying the buffer.
+ * @gfp: how to do memory allocations (if necessary).
+ *
+ * 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 (ie. ENOSPC, ENOMEM, EIO).
+ */
+int virtqueue_add_sgs_premapped(struct virtqueue *_vq,
+				struct scatterlist *sgs[],
+				unsigned int out_sgs,
+				unsigned int in_sgs,
+				void *data,
+				gfp_t gfp)
+{
+	unsigned int i, total_sg = 0;
+
+	/* Count them first. */
+	for (i = 0; i < out_sgs + in_sgs; i++) {
+		struct scatterlist *sg;
+
+		for (sg = sgs[i]; sg; sg = sg_next(sg))
+			total_sg++;
+	}
+	return virtqueue_add(_vq, sgs, total_sg, out_sgs, in_sgs,
+			     data, NULL, true, gfp);
+}
+EXPORT_SYMBOL_GPL(virtqueue_add_sgs_premapped);
+
 /**
  * virtqueue_add_sgs - expose buffers to other end
  * @_vq: the struct virtqueue we're talking about.
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 28db998d691e..de52a0f6f734 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -84,6 +84,13 @@ int virtqueue_add_sgs(struct virtqueue *vq,
 		      void *data,
 		      gfp_t gfp);
 
+int virtqueue_add_sgs_premapped(struct virtqueue *vq,
+				struct scatterlist *sgs[],
+				unsigned int out_sgs,
+				unsigned int in_sgs,
+				void *data,
+				gfp_t gfp);
+
 struct device *virtqueue_dma_dev(struct virtqueue *vq);
 
 bool virtqueue_kick(struct virtqueue *vq);
-- 
2.48.1