Re: [PATCH RFC net-next 05/19] net: dsa: tag_ar9331: add GRO callbacks

[Florian Fainelli <f.fainelli@gmail.com>]

On 12/30/19 6:30 AM, Alexander Lobakin wrote:
> Add GRO callbacks to the AR9331 tagger so GRO layer can now process
> such frames.
> 
> Signed-off-by: Alexander Lobakin <alobakin@dlink.ru>

This is a good example and we should probably build a tagger abstraction
that is much simpler to fill in callbacks for (although indirect
function calls may end-up killing performance with retpoline and
friends), but let's consider this idea.

> ---
>  net/dsa/tag_ar9331.c | 77 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 77 insertions(+)
> 
> diff --git a/net/dsa/tag_ar9331.c b/net/dsa/tag_ar9331.c
> index c22c1b515e02..99cc7fd92d8e 100644
> --- a/net/dsa/tag_ar9331.c
> +++ b/net/dsa/tag_ar9331.c
> @@ -100,12 +100,89 @@ static void ar9331_tag_flow_dissect(const struct sk_buff *skb, __be16 *proto,
>  	*proto = ar9331_tag_encap_proto(skb->data);
>  }
>  
> +static struct sk_buff *ar9331_tag_gro_receive(struct list_head *head,
> +					      struct sk_buff *skb)
> +{
> +	const struct packet_offload *ptype;
> +	struct sk_buff *p, *pp = NULL;
> +	u32 data_off, data_end;
> +	const u8 *data;
> +	int flush = 1;
> +
> +	data_off = skb_gro_offset(skb);
> +	data_end = data_off + AR9331_HDR_LEN;

AR9331_HDR_LEN is a parameter here which is incidentally
dsa_device_ops::overhead.

> +
> +	data = skb_gro_header_fast(skb, data_off);
> +	if (skb_gro_header_hard(skb, data_end)) {
> +		data = skb_gro_header_slow(skb, data_end, data_off);
> +		if (unlikely(!data))
> +			goto out;
> +	}
> +
> +	/* Data that is to the left from the current position is already
> +	 * pulled to the head
> +	 */
> +	if (unlikely(!ar9331_tag_sanity_check(skb->data + data_off)))
> +		goto out;

This is applicable to all taggers, they need to verify the sanity of the
header they are being handed.

> +
> +	rcu_read_lock();
> +
> +	ptype = gro_find_receive_by_type(ar9331_tag_encap_proto(data));

If there is no encapsulation a tagger can return the frame's protocol
directly, so similarly the tagger can be interrogated for returning that.

> +	if (!ptype)
> +		goto out_unlock;
> +
> +	flush = 0;
> +
> +	list_for_each_entry(p, head, list) {
> +		if (!NAPI_GRO_CB(p)->same_flow)
> +			continue;
> +
> +		if (ar9331_tag_source_port(skb->data + data_off) ^
> +		    ar9331_tag_source_port(p->data + data_off))

Similarly here, the tagger could provide a function whose job is to
return the port number from within its own tag.

So with that being said, what do you think about building a tagger
abstraction which is comprised of:

- header length which is dsa_device_ops::overhead
- validate_tag()
- get_tag_encap_proto()
- get_port_number()

and the rest is just wrapping the general GRO list manipulation?

Also, I am wondering should we somehow expose the DSA master
net_device's napi_struct such that we could have the DSA slave
net_devices call napi_gro_receive() themselves directly such that they
could also perform additional GRO on top of Ethernet frames?
-- 
Florian