Subject: Re: [RFC PATCH 2/2] ixgbe: add af_packet direct copy support

From: Alexei Starovoitov <[email-masked]>
Date: Mon, 30 Jan 2017 18:53:05 -0800

On 1/27/17 1:34 PM, John Fastabend wrote:
> +	h2 = page_address(rx_buffer->page) + rx_buffer->page_offset - hdrlen;
> +	eth = page_address(rx_buffer->page) + rx_buffer->page_offset,

I don't think it compiles ;)

> +	/* This indicates a bug in ixgbe leaving for testing purposes */
> +	WARN_ON(TP_STATUS_USER & h2->tp_status);
> +	len = le16_to_cpu(rx_desc->wb.upper.length);
> +	h2->tp_len = len;
> +	h2->tp_snaplen = len;
> +	h2->tp_mac = ALIGN(TPACKET_ALIGN(TPACKET2_HDRLEN), L1_CACHE_BYTES);
> +	h2->tp_net = h2->tp_mac + ETH_HLEN;
> +	h2->tp_sec = div_s64_rem(ns, NSEC_PER_SEC, &rem);
> +	h2->tp_nsec = rem;
> +
> +	sll = (void *)h2 + TPACKET_ALIGN(sizeof(struct tpacket2_hdr));
> +	sll->sll_halen = ETH_HLEN;
> +	memcpy(sll->sll_addr, eth->h_source, ETH_ALEN);
> +	sll->sll_family = AF_PACKET;
> +	sll->sll_hatype = rx_ring->netdev->type;
> +	sll->sll_protocol = eth->h_proto;
> +	sll->sll_pkttype = PACKET_HOST;
> +	sll->sll_ifindex = rx_ring->netdev->ifindex;

performance wise it looks very expensive to do all these header copies
and integer divide for every packet.
I think unless we move to new dumb and simple header format
performance of this approach is not going to be satisfactory.