vhost-scsi: Add better resource allocation failure handling

If we can't allocate mem to map in data for a request or can't find
a tag for a command, we currently drop the command. This leads to the
error handler running to clean it up. Instead of dropping the command
this has us return an error telling the initiator that it queued more
commands than we can handle. The initiator will then reduce how many
commands it will send us and retry later.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20241203191705.19431-4-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

drivers/vhost/scsi.c

@@ -629,7 +629,7 @@ vhost_scsi_get_cmd(struct vhost_virtqueue *vq, struct vhost_scsi_tpg *tpg,
 
 	tag = sbitmap_get(&svq->scsi_tags);
 	if (tag < 0) {
-		pr_err("Unable to obtain tag for vhost_scsi_cmd\n");
+		pr_warn_once("Guest sent too many cmds. Returning TASK_SET_FULL.\n");
 		return ERR_PTR(-ENOMEM);
 	}
 
@@ -928,6 +928,24 @@ static void vhost_scsi_target_queue_cmd(struct vhost_scsi_cmd *cmd)
 	target_submit(se_cmd);
 }
 
+static void
+vhost_scsi_send_status(struct vhost_scsi *vs, struct vhost_virtqueue *vq,
+		       int head, unsigned int out, u8 status)
+{
+	struct virtio_scsi_cmd_resp __user *resp;
+	struct virtio_scsi_cmd_resp rsp;
+	int ret;
+
+	memset(&rsp, 0, sizeof(rsp));
+	rsp.status = status;
+	resp = vq->iov[out].iov_base;
+	ret = __copy_to_user(resp, &rsp, sizeof(rsp));
+	if (!ret)
+		vhost_add_used_and_signal(&vs->dev, vq, head, 0);
+	else
+		pr_err("Faulted on virtio_scsi_cmd_resp\n");
+}
+
 static void
 vhost_scsi_send_bad_target(struct vhost_scsi *vs,
 			   struct vhost_virtqueue *vq,
@@ -1215,8 +1233,8 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
 					 exp_data_len + prot_bytes,
 					 data_direction);
 		if (IS_ERR(cmd)) {
-			vq_err(vq, "vhost_scsi_get_cmd failed %ld\n",
-			       PTR_ERR(cmd));
+			ret = PTR_ERR(cmd);
+			vq_err(vq, "vhost_scsi_get_tag failed %dd\n", ret);
 			goto err;
 		}
 		cmd->tvc_vhost = vs;
@@ -1253,11 +1271,15 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
 		 * EINVAL: Invalid response buffer, drop the request
 		 * EIO:    Respond with bad target
 		 * EAGAIN: Pending request
+		 * ENOMEM: Could not allocate resources for request
 		 */
 		if (ret == -ENXIO)
 			break;
 		else if (ret == -EIO)
 			vhost_scsi_send_bad_target(vs, vq, vc.head, vc.out);
+		else if (ret == -ENOMEM)
+			vhost_scsi_send_status(vs, vq, vc.head, vc.out,
+					       SAM_STAT_TASK_SET_FULL);
 	} while (likely(!vhost_exceeds_weight(vq, ++c, 0)));
 out:
 	mutex_unlock(&vq->mutex);