netdev - Re: [RFC PATCH 1/3] veth: implement support for set

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Date:   Fri, 09 Jul 2021 16:38:02 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [RFC PATCH 1/3] veth: implement support for set_channel ethtool
 op

On Fri, 2021-07-09 at 12:49 +0200, Paolo Abeni wrote:
> On Fri, 2021-07-09 at 12:15 +0200, Toke Høiland-Jørgensen wrote:
> > > +	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 ;)

Here there is a possible problem: if the latter
netif_set_real_num_*_queues() fails, we should not change the current
configuration, so we should revert the
first netif_set_real_num_*_queues() change.

Even that additional revert operation could fail. If/when that happen
set_channel() will leave the device in a different state from both the
old one and the new one, possibly with an XDP-incompatible number of
queues.

Keeping the  netif_set_real_num_*_queues() calls in veth_open() avoid
the above issue: if the queue creation is problematic, the device will
stay down.

I think the additional fields are worthy, WDYT? 

Cheers,
Paolo