Subject: Re: [RFC PATCH 1/3] veth: implement support for set_channel ethtool op

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 09 Jul 2021 13:36:22 +0200

Paolo Abeni <[email-masked]> writes:

> Hello,
>
> On Fri, 2021-07-09 at 12:15 +0200, Toke Høiland-Jørgensen wrote:
>> > @@ -224,10 +226,49 @@ static void veth_get_channels(struct net_device *dev,
>> >  {
>> >  	channels->tx_count = dev->real_num_tx_queues;
>> >  	channels->rx_count = dev->real_num_rx_queues;
>> > -	channels->max_tx = dev->real_num_tx_queues;
>> > -	channels->max_rx = dev->real_num_rx_queues;
>> > +	channels->max_tx = dev->num_tx_queues;
>> > +	channels->max_rx = dev->num_rx_queues;
>> >  	channels->combined_count = min(dev->real_num_rx_queues, dev->real_num_tx_queues);
>> > -	channels->max_combined = min(dev->real_num_rx_queues, dev->real_num_tx_queues);
>> > +	channels->max_combined = min(dev->num_rx_queues, dev->num_tx_queues);
>> > +}
>> > +
>> > +static int veth_close(struct net_device *dev);
>> > +static int veth_open(struct net_device *dev);
>> > +
>> > +static int veth_set_channels(struct net_device *dev,
>> > +			     struct ethtool_channels *ch)
>> > +{
>> > +	struct veth_priv *priv = netdev_priv(dev);
>> > +	struct veth_priv *peer_priv;
>> > +
>> > +	/* accept changes only on rx/tx */
>> > +	if (ch->combined_count != min(dev->real_num_rx_queues, dev->real_num_tx_queues))
>> > +		return -EINVAL;
>> > +
>> > +	/* respect contraint posed at device creation time */
>> > +	if (ch->rx_count > dev->num_rx_queues || ch->tx_count > dev->num_tx_queues)
>> > +		return -EINVAL;
>> > +
>> > +	if (!ch->rx_count || !ch->tx_count)
>> > +		return -EINVAL;
>> > +
>> > +	/* avoid braking XDP, if that is enabled */
>> > +	if (priv->_xdp_prog && ch->rx_count < priv->peer->real_num_tx_queues)
>> > +		return -EINVAL;
>> > +
>> > +	peer_priv = netdev_priv(priv->peer);
>> > +	if (peer_priv->_xdp_prog && ch->tx_count > priv->peer->real_num_rx_queues)
>> > +		return -EINVAL;
>> 
>> An XDP program could be loaded later, so I think it's better to enforce
>> this constraint unconditionally.
>
> The relevant check is already present in veth_xdp_set(), the load will
> be rejected with an appropriate extack message.

Ah, right, of course; silly me, that's fine then... :)

> I enforcing the above contraint uncontiditionally will make impossible
> changing the number of real queues at runtime, as we must update dev
> and peer in different moments.
>
>> (What happens on the regular xmit path if the number of TX queues is out
>> of sync with the peer RX queues, BTW?)
>
> if tx < rx, the higly number of rx queue will not be used, if rx < tx,
> XDP/gro can't take place: the normal veth path is used.

Right, OK, that's probably also fine then :)

-Toke