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

[Paolo Abeni <pabeni@redhat.com>]

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.

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.

> > +	if (netif_running(dev))
> > +		veth_close(dev);
> > +
> > +	priv->num_tx_queues = ch->tx_count;
> > +	priv->num_rx_queues = ch->rx_count;
> 
> Why can't just you use netif_set_real_num_*_queues() here directly (and
> get rid of the priv members as above)?

Uhm... I haven't thought about it. Let me try ;)

Thanks!

/P