netdev - Re: [PATCH RFC 2/2] veth: propagate bridge GSO to peer

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Date:   Sun, 26 Nov 2017 23:07:25 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Ahern <dsahern@...il.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH RFC 2/2] veth: propagate bridge GSO to peer

On Sun, 26 Nov 2017 20:13:39 -0700
David Ahern <dsahern@...il.com> wrote:

> On 11/26/17 11:17 AM, Stephen Hemminger wrote:
> > This allows veth device in containers to see the GSO maximum
> > settings of the actual device being used for output.
> > 
> > Signed-off-by: Stephen Hemminger <stephen@...workplumber.org>
> > ---
> >  drivers/net/veth.c | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 72 insertions(+)
> > 
> > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > index f5438d0978ca..0c9ce156943b 100644
> > --- a/drivers/net/veth.c
> > +++ b/drivers/net/veth.c
> > @@ -511,17 +511,89 @@ static struct rtnl_link_ops veth_link_ops = {
> >  	.get_link_net	= veth_get_link_net,
> >  };
> >  
> > +/* When veth device is added to a bridge or other master device
> > + * then reflect the GSO max values from the upper device
> > + * to the other end of veth pair.
> > + */
> > +static void veth_change_upper(struct net_device *dev,
> > +		      const struct netdev_notifier_changeupper_info *info)
> > +{
> > +	struct net_device *upper = info->upper_dev;
> > +	struct net_device *peer;
> > +	struct veth_priv *priv;
> > +
> > +	if (dev->netdev_ops != &veth_netdev_ops)
> > +		return;
> > +
> > +	priv = netdev_priv(dev);
> > +	peer = rtnl_dereference(priv->peer);
> > +	if (!peer)
> > +		return;
> > +
> > +	if (upper) {
> > +		peer->gso_max_segs = upper->gso_max_segs;
> > +		peer->gso_max_size = upper->gso_max_size;
> > +	} else {
> > +		peer->gso_max_segs = GSO_MAX_SEGS;
> > +		peer->gso_max_size = GSO_MAX_SIZE;
> > +	}  
> 
> veth devices can be added to a VRF instead of a bridge, and I do not
> believe the gso propagation works for L3 master devices.
> 
> From a quick grep, team devices do not appear to handle gso changes either.

This code should still work correctly, but no optimization would happen.
The gso_max_size of the VRF or team will
still be GSO_MAX_SIZE so there would be no change. If VRF or Team ever got smart
enough to handle GSO limits, then the algorithm would handle it.