netdev - Re: [PATCH v3 net-next 2/2] tc: add 'needs

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Date:	Wed, 08 Apr 2015 22:44:04 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ast@...mgrid.com
Cc:	daniel@...earbox.net, tgraf@...g.ch, jiri@...nulli.us,
	jhs@...atatu.com, netdev@...r.kernel.org
Subject: Re: [PATCH v3 net-next 2/2] tc: add 'needs_l2' flag to ingress
 qdisc

From: Alexei Starovoitov <ast@...mgrid.com>
Date: Wed,  8 Apr 2015 16:26:15 -0700

> index 4cd5cf1aedf8..887033fbdfa6 100644
> --- a/net/sched/act_csum.c
> +++ b/net/sched/act_csum.c
> @@ -565,6 +565,8 @@ static struct tc_action_ops act_csum_ops = {
>  	.act		= tcf_csum,
>  	.dump		= tcf_csum_dump,
>  	.init		= tcf_csum_init,
> +	/* todo: fix pskb_may_pull in tcf_csum to not assume L2 */
> +	.flags		= TCA_NEEDS_L2,
>  };
>  
>  MODULE_DESCRIPTION("Checksum updating actions");


All this module really cares about is where skb_network_header() is
for data accesses.  And a simple offset would allow it to calculate
the pskb_may_pull() length argument correctly.

And it would therefore work without all of these expensive SKB share
check calls etc.

I really don't like your approach, and I'd rather you propagate the
offset into the BPF programs you generate.  I bet you can do it in
an efficient way, and just haven't put in the effort to discover
how.
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