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

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