On Thu, Jan 21, 2016 at 8:56 PM, David Miller [off-list ref] wrote:

From: Or Gerlitz <redacted>
Date: Thu, 21 Jan 2016 14:49:25 +0200

quoted

On Thu, Jan 21, 2016 at 1:27 PM, Jesper Dangaard Brouer
[off-list ref] wrote:

quoted

On Wed, 20 Jan 2016 15:27:38 -0800 Tom Herbert [off-list ref] wrote:

quoted

eth_type_trans touches headers

True, the eth_type_trans() call in the driver is a major bottleneck,
because it touch the packet header and happens very early in the driver.

In my experiments, where I extract several packet before calling
napi_gro_receive(), and I also delay calling eth_type_trans().  Most of
my speedup comes from this trick, as the prefetch() now that enough
time.

 while ((skb = __skb_dequeue(&rx_skb_list)) != NULL) {
        skb->protocol = eth_type_trans(skb, rq->netdev);
        napi_gro_receive(cq->napi, skb);
 }

What is the HW could provide the info we need in the descriptor?!?


eth_type_trans() does two things:

1) determine skb->protocol
2) setup skb->pkt_type = PACKET_{BROADCAST,MULTICAST,OTHERHOST}

Could the HW descriptor deliver the "proto", or perhaps just some bits
on the most common proto's?

The skb->pkt_type don't need many bits.  And I bet the HW already have
the information.  The BROADCAST and MULTICAST indication are easy.  The
PACKET_OTHERHOST, can be turned around, by instead set a PACKET_HOST
indication, if the eth->h_dest match the devices dev->dev_addr (else a
SW compare is required).

Is that doable in hardware?

As I wrote earlier, for determination of the eth-type HWs can do what you ask
here and more.

Protocol being IP or not (and only then you look in the data) you could
get I guess from many NICs, e.g if the NIC sets PKT_HASH_TYPE_L4
or PKT_HASH_TYPE_L3 then we know it's an IP packets and only if
we don't see this indication we look into the data.

This doesn't differentiate ipv4 vs. ipv6 which is critical here, so this
mechanism is not sufficient.

Dave, at least in the ConnectX4 (mlx5e driver), as I commented earlier
on this thread, we can use programmed tags reported by the HW on the
completion of packets  whether the ethtype is ipv4 or ipv6 or
something else, and let the kernel
branch look into the packet memory on in the last case.

We must know the exact ETH_P_* value.