vdpa_sim: split vdpasim_virtqueue's iov field in out_iov and in_iov

vringh_getdesc_iotlb() manages 2 iovs for writable and readable
descriptors. This is very useful for the block device, where for
each request we have both types of descriptor.

Let's split the vdpasim_virtqueue's iov field in out_iov and
in_iov to use them with vringh_getdesc_iotlb().

We are using VIRTIO terminology for "out" (readable by the device)
and "in" (writable by the device) descriptors.
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NStefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-18-sgarzare@redhat.comSigned-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -44,7 +44,8 @@ u8 macaddr_buf[ETH_ALEN];
 
 struct vdpasim_virtqueue {
 	struct vringh vring;
-	struct vringh_kiov iov;
+	struct vringh_kiov in_iov;
+	struct vringh_kiov out_iov;
 	unsigned short head;
 	bool ready;
 	u64 desc_addr;
@@ -210,12 +211,12 @@ static void vdpasim_net_work(struct work_struct *work)
 
 	while (true) {
 		total_write = 0;
-		err = vringh_getdesc_iotlb(&txq->vring, &txq->iov, NULL,
+		err = vringh_getdesc_iotlb(&txq->vring, &txq->out_iov, NULL,
 					   &txq->head, GFP_ATOMIC);
 		if (err <= 0)
 			break;
 
-		err = vringh_getdesc_iotlb(&rxq->vring, NULL, &rxq->iov,
+		err = vringh_getdesc_iotlb(&rxq->vring, NULL, &rxq->in_iov,
 					   &rxq->head, GFP_ATOMIC);
 		if (err <= 0) {
 			vringh_complete_iotlb(&txq->vring, txq->head, 0);
@@ -223,13 +224,13 @@ static void vdpasim_net_work(struct work_struct *work)
 		}
 
 		while (true) {
-			read = vringh_iov_pull_iotlb(&txq->vring, &txq->iov,
+			read = vringh_iov_pull_iotlb(&txq->vring, &txq->out_iov,
 						     vdpasim->buffer,
 						     PAGE_SIZE);
 			if (read <= 0)
 				break;
 
-			write = vringh_iov_push_iotlb(&rxq->vring, &rxq->iov,
+			write = vringh_iov_push_iotlb(&rxq->vring, &rxq->in_iov,
 						      vdpasim->buffer, read);
 			if (write <= 0)
 				break;