Re: [PATCH] vhost: Fix last queue index of devices with no cvq

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 29, 2021 at 04:16:08PM +0200, Eugenio Pérez wrote:
> The -1 assumes that all devices with no cvq have an spare vq allocated
> for them, but with no offer of VIRTIO_NET_F_CTRL_VQ. This may not be the
> case, and the device may have a pair number of queues.
> 
> To fix this, just resort to the lower even number of queues.
> 
> Fixes: 049eb15b5fc9 ("vhost: record the last virtqueue index for the virtio device")
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  hw/net/vhost_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index 0d888f29a6..edf56a597f 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -330,7 +330,7 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
>      NetClientState *peer;
>  
>      if (!cvq) {
> -        last_index -= 1;
> +        last_index &= ~1ULL;
>      }
>  
>      if (!k->set_guest_notifiers) {

could this code be made clearer?

> -- 
> 2.27.0