Re: error loading xdp program on virtio nic

[David Ahern <dsahern@gmail.com>]

On 11/21/19 12:02 AM, Jason Wang wrote:
> By specifying queues property like:
> 
> <devices>
> 
>   <interface type='network'>
>     <source network='default'/>
>     <target dev='vnet1'/>
>     <model type='virtio'/>
>     <driver name='vhost' txmode='iothread' ioeventfd='on'
> event_idx='off' queues='5' rx_queue_size='256' tx_queue_size='256'>

I can not check this because the 3.0 version of libvirt does not support
tx_queue_size. It is multiqueue (queues=5 in the example) setting that
needs to be set to 2*Nvcpus for XDP, correct?

>       <host csum='off' gso='off' tso4='off' tso6='off' ecn='off'
> ufo='off' mrg_rxbuf='off'/>
>       <guest csum='off' tso4='off' tso6='off' ecn='off' ufo='off'/>
>     </driver>
>     </interface>
> </devices>
> 
> 

The virtio_net driver suggests the queues are needed for XDP_TX:

       /* XDP requires extra queues for XDP_TX */
        if (curr_qp + xdp_qp > vi->max_queue_pairs) {
                NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings
available");
                netdev_warn(dev, "request %i queues but max is %i\n",
                            curr_qp + xdp_qp, vi->max_queue_pairs);
                return -ENOMEM;
        }

Doubling the number of queues for each tap device adds overhead to the
hypervisor if you only want to allow XDP_DROP or XDP_DIRECT. Am I
understanding that correctly?