vhost-user-blk: Check that num-queues is supported by backend

Creating a device with a number of queues that isn't supported by the
backend is pointless, the device won't work properly and the error
messages are rather confusing.

Just fail to create the device if num-queues is higher than what the
backend supports.

Since the relationship between num-queues and the number of virtqueues
depends on the specific device, this is an additional value that needs
to be initialised by the device. For convenience, allow leaving it 0 if
the check should be skipped. This makes sense for vhost-user-net where
separate vhost devices are used for the queues and custom initialisation
code is needed to perform the check.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1935031
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20210429171316.162022-7-kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/block/vhost-user-blk.c

@@ -324,6 +324,7 @@ static int vhost_user_blk_connect(DeviceState *dev, Error **errp)
     }
     s->connected = true;
 
+    s->dev.num_queues = s->num_queues;
     s->dev.nvqs = s->num_queues;
     s->dev.vqs = s->vhost_vqs;
     s->dev.vq_index = 0;

hw/virtio/vhost-user.c

@@ -1909,6 +1909,11 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque)
                 return err;
             }
         }
+        if (dev->num_queues && dev->max_queues < dev->num_queues) {
+            error_report("The maximum number of queues supported by the "
+                         "backend is %" PRIu64, dev->max_queues);
+            return -EINVAL;
+        }
 
         if (virtio_has_feature(features, VIRTIO_F_IOMMU_PLATFORM) &&
                 !(virtio_has_feature(dev->protocol_features,

include/hw/virtio/vhost.h

@@ -74,6 +74,8 @@ struct vhost_dev {
     int nvqs;
     /* the first virtqueue which would be used by this vhost dev */
     int vq_index;
+    /* if non-zero, minimum required value for max_queues */
+    int num_queues;
     uint64_t features;
     uint64_t acked_features;
     uint64_t backend_features;