why tx_queue_size must be 256 for non-vhost-user tap nic

[yue]

Hi, all:


  I tried to test the effort of nic queue's ring-size, but  nic' tx vring-size 
is always 256.  finally i found  the code.   
could you tell why ?
thanks
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n->net_conf.tx_queue_size = MIN(virtio_net_max_tx_queue_size(n),
                                    n->net_conf.tx_queue_size);
n->net_conf.tx_queue_size is 256 if peer->info->type != 
NET_CLIENT_DRIVER_VHOST_USER.