netdev - Re: [RFC net-next 6/9] net: dsa: Forward offloading

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Date:   Tue, 4 May 2021 18:21:06 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     davem@...emloft.net, kuba@...nel.org, andrew@...n.ch,
        vivien.didelot@...il.com, f.fainelli@...il.com, roopa@...dia.com,
        nikolay@...dia.com, jiri@...nulli.us, idosch@...sch.org,
        stephen@...workplumber.org, netdev@...r.kernel.org,
        bridge@...ts.linux-foundation.org
Subject: Re: [RFC net-next 6/9] net: dsa: Forward offloading

On Tue, May 04, 2021 at 04:44:31PM +0200, Tobias Waldekranz wrote:
> On Tue, Apr 27, 2021 at 13:17, Vladimir Oltean <olteanv@...il.com> wrote:
> > On Mon, Apr 26, 2021 at 07:04:08PM +0200, Tobias Waldekranz wrote:
> >> Allow DSA drivers to support forward offloading from a bridge by:
> >> 
> >> - Passing calls to .ndo_dfwd_{add,del}_station to the drivers.
> >> 
> >> - Recording the subordinate device of offloaded skbs in the control
> >>   buffer so that the tagger can take the appropriate action.
> >> 
> >> Signed-off-by: Tobias Waldekranz <tobias@...dekranz.com>
> >> ---
> >>  include/net/dsa.h |  7 +++++++
> >>  net/dsa/slave.c   | 36 ++++++++++++++++++++++++++++++++++--
> >>  2 files changed, 41 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/include/net/dsa.h b/include/net/dsa.h
> >> index 1f9ba9889034..77d4df819299 100644
> >> --- a/include/net/dsa.h
> >> +++ b/include/net/dsa.h
> >> @@ -119,6 +119,7 @@ struct dsa_netdevice_ops {
> >>  
> >>  struct dsa_skb_cb {
> >>  	struct sk_buff *clone;
> >> +	struct net_device *sb_dev;
> >>  };
> >>  
> >>  struct __dsa_skb_cb {
> >> @@ -828,6 +829,12 @@ struct dsa_switch_ops {
> >>  					  const struct switchdev_obj_ring_role_mrp *mrp);
> >>  	int	(*port_mrp_del_ring_role)(struct dsa_switch *ds, int port,
> >>  					  const struct switchdev_obj_ring_role_mrp *mrp);
> >> +
> >> +	/* L2 forward offloading */
> >> +	void *	(*dfwd_add_station)(struct dsa_switch *ds, int port,
> >> +				    struct net_device *sb_dev);
> >> +	void	(*dfwd_del_station)(struct dsa_switch *ds, int port,
> >> +				    struct net_device *sb_dev);
> >>  };
> >>  
> >>  #define DSA_DEVLINK_PARAM_DRIVER(_id, _name, _type, _cmodes)		\
> >> diff --git a/net/dsa/slave.c b/net/dsa/slave.c
> >> index 77b33bd161b8..3689ffa2dbb8 100644
> >> --- a/net/dsa/slave.c
> >> +++ b/net/dsa/slave.c
> >> @@ -657,6 +657,13 @@ static netdev_tx_t dsa_slave_xmit(struct sk_buff *skb, struct net_device *dev)
> >>  	return dsa_enqueue_skb(nskb, dev);
> >>  }
> >>  
> >> +static u16 dsa_slave_select_queue(struct net_device *dev, struct sk_buff *skb,
> >> +				  struct net_device *sb_dev)
> >> +{
> >> +	DSA_SKB_CB(skb)->sb_dev = sb_dev;
> >> +	return netdev_pick_tx(dev, skb, sb_dev);
> >> +}
> >> +
> >
> > DSA_SKB_CB is going away:
> > https://patchwork.kernel.org/project/netdevbpf/patch/20210427042203.26258-5-yangbo.lu@nxp.com/
> >
> > Let's either negotiate with Yangbo on keeping it, or make
> > .ndo_select_queue a bypass towards the tagger, where it can use its own
> > SKB_CB structure and be more flexible in general (I think I'm leaning
> > towards the latter).
> 
> Thus far, Yangbo is a tough negotiator, giving me the silent treatment:
> 
> https://lore.kernel.org/netdev/87y2d2noe5.fsf@waldekranz.com/
> 
> :)
> 
> That memset is giving me a hard time. I have just disabled it on my
> branch at the moment. Any ideas on how to get rid of it without breaking
> timestamping?

:)

Is there any guarantee written somewhere that the ownership of skb->cb
belongs to the NIC driver at the time of the ndo_select_queue call?

If there is, then the trivial solution is to just move the memset in
ndo_select_queue.

If there isn't, then we've got bigger issues (such as, for example, the
qdisc layer being able to overwrite your DSA_SKB_CB(skb)->sb_dev).