netdev - Re: Optimizing instruction-cache, more packets at each stage

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Date:	Thu, 21 Jan 2016 10:54:01 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	brouer@...hat.com
Cc:	tom@...bertland.com, eric.dumazet@...il.com, gerlitz.or@...il.com,
	edumazet@...gle.com, netdev@...r.kernel.org,
	alexander.duyck@...il.com, alexei.starovoitov@...il.com,
	borkmann@...earbox.net, marek@...udflare.com,
	hannes@...essinduktion.org, fw@...len.de, pabeni@...hat.com,
	john.r.fastabend@...el.com, amirva@...il.com
Subject: Re: Optimizing instruction-cache, more packets at each stage

From: Jesper Dangaard Brouer <brouer@...hat.com>
Date: Thu, 21 Jan 2016 12:27:30 +0100

> 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?

I feel like we've had this discussion before several years ago.

I think having just the protocol value would be enough.

skb->pkt_type we could deal with by using always an accessor and
evaluating it lazily.  Nothing needs it until we hit ip_rcv() or
similar.