Subject: Re: [PATCH V2 3/4] net-next: dsa: fix flow dissection

From: Andrew Lunn <[email-masked]>
Date: Wed, 26 Jul 2017 17:10:13 +0200

On Fri, Jul 21, 2017 at 10:58:12AM +0200, John Crispin wrote:
> RPS and probably other kernel features are currently broken on some if not
> all DSA devices. The root cause of this is that skb_hash will call the
> flow_dissector. At this point the skb still contains the magic switch header
> and the skb->protocol field is not set up to the correct 802.3 value yet.
> By the time the tag specific code is called, removing the header and
> properly setting the protocol an invalid hash is already set. In the case
> of the mt7530 this will result in all flows always having the same hash.
> 
> This patch makes the flow dissector honour the nh and protocol offset
> defined by the dsa tag driver thus fixing dissection, hashing and RPS.
> 
> Signed-off-by: John Crispin <[email-masked]>
> ---
>  net/core/flow_dissector.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index fc5fc4594c90..1268ae75c3b3 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -4,6 +4,7 @@
>  #include <linux/ip.h>
>  #include <linux/ipv6.h>
>  #include <linux/if_vlan.h>
> +#include <net/dsa.h>
>  #include <net/ip.h>
>  #include <net/ipv6.h>
>  #include <net/gre.h>
> @@ -440,6 +441,17 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
>  			 skb->vlan_proto : skb->protocol;
>  		nhoff = skb_network_offset(skb);
>  		hlen = skb_headlen(skb);
> +
> +		if (unlikely(netdev_uses_dsa(skb->dev))) {
> +			const struct dsa_device_ops *ops;
> +			u8 *p = (u8 *)data;
> +
> +			ops = skb->dev->dsa_ptr->tag_ops;
> +			if (ops->hash_proto_off)
> +				proto = (u16)p[ops->hash_proto_off];

Hi John

Unfortunately, this is not generic enough to work for DSA and EDSA
tagging. With these tagging protocols, the size of the tag depends on
the presence or not of a VLAN header.

To make this work for all tagging protocols, we are going to need to
add an a new op to tag_ops.

    Andrew