dev->max_queues was never initialised for backends that don't support
VHOST_USER_PROTOCOL_F_MQ, so it would use 0 as the maximum number of
queues to check against and consequently fail for any such backend.

Set it to 1 if the backend doesn't have multiqueue support.

Fixes: c90bd505a3e8210c23d69fecab9ee6f56ec4a161
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 hw/virtio/vhost-user.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 1ac4a2ebec..29ea2b4fce 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -1913,7 +1913,10 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
             if (err < 0) {
                 return -EPROTO;
             }
+        } else {
+            dev->max_queues = 1;
         }
+
         if (dev->num_queues && dev->max_queues < dev->num_queues) {
             error_setg(errp, "The maximum number of queues supported by the "
                        "backend is %" PRIu64, dev->max_queues);
-- 
2.31.1

On Mon, Jul 05 2021, Kevin Wolf <kwolf@redhat.com> wrote:

> dev->max_queues was never initialised for backends that don't support
> VHOST_USER_PROTOCOL_F_MQ, so it would use 0 as the maximum number of
> queues to check against and consequently fail for any such backend.
>
> Set it to 1 if the backend doesn't have multiqueue support.
>
> Fixes: c90bd505a3e8210c23d69fecab9ee6f56ec4a161
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  hw/virtio/vhost-user.c | 3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>

On Mon, Jul 05, 2021 at 07:14:29PM +0200, Kevin Wolf wrote:
> dev->max_queues was never initialised for backends that don't support
> VHOST_USER_PROTOCOL_F_MQ, so it would use 0 as the maximum number of
> queues to check against and consequently fail for any such backend.
> 
> Set it to 1 if the backend doesn't have multiqueue support.
> 
> Fixes: c90bd505a3e8210c23d69fecab9ee6f56ec4a161
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>

Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>

> ---
>  hw/virtio/vhost-user.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 1ac4a2ebec..29ea2b4fce 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1913,7 +1913,10 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
>              if (err < 0) {
>                  return -EPROTO;
>              }
> +        } else {
> +            dev->max_queues = 1;
>          }
> +
>          if (dev->num_queues && dev->max_queues < dev->num_queues) {
>              error_setg(errp, "The maximum number of queues supported by the "
>                         "backend is %" PRIu64, dev->max_queues);
> -- 
> 2.31.1
>