virtio_ring: per virtqueue dma device

This patch introduces a per virtqueue dma device. This will be used
for virtio devices whose virtqueue are backed by different underlayer
devices.

One example is the vDPA that where the control virtqueue could be
implemented through software mediation.

Some of the work are actually done before since the helper like
vring_dma_device(). This work left are:

- Let vring_dma_device() return the per virtqueue dma device instead
  of the vdev's parent.
- Allow passing a dma_device when creating the virtqueue through a new
  helper, old vring creation helper will keep using vdev's parent.
Reviewed-by: Eli Cohen <elic@nvidia.com>
Tested-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230119061525.75068-2-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/linux/virtio_ring.h

@@ -76,6 +76,22 @@ struct virtqueue *vring_create_virtqueue(unsigned int index,
 					 void (*callback)(struct virtqueue *vq),
 					 const char *name);
 
+/*
+ * Creates a virtqueue and allocates the descriptor ring with per
+ * virtqueue DMA device.
+ */
+struct virtqueue *vring_create_virtqueue_dma(unsigned int index,
+					     unsigned int num,
+					     unsigned int vring_align,
+					     struct virtio_device *vdev,
+					     bool weak_barriers,
+					     bool may_reduce_num,
+					     bool ctx,
+					     bool (*notify)(struct virtqueue *vq),
+					     void (*callback)(struct virtqueue *vq),
+					     const char *name,
+					     struct device *dma_dev);
+
 /*
  * Creates a virtqueue with a standard layout but a caller-allocated
  * ring.