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Date: Fri, 4 Oct 2013 09:41:22 -0700
From: Stephen Hemminger <stephen@networkplumber.org>
To: jigsaw <jigsaw@gmail.com>
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Cc: "dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] Need comment on 82599 TSO
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On Fri, 4 Oct 2013 15:44:19 +0300
jigsaw <jigsaw@gmail.com> wrote:

> Hi,
> 
> I'm working on TSO for 82599, and encounter a problem: nowhere to store MSS.
> 
> TSO must be aware of MSS, or gso in skb of kernel.
> But MSS nees 16 bits per mbuf. And we have no spare 16 bits in
> rte_mbuf or rte_pktmbuf.
> If we add 16 bit field in rte_pktmbuf, the size of rte_mbuf will be
> doubled, coz currently the size is at the edge of cacheline(32 byte).
> 
> I have two solutions here:
> 
> 1. Store MSS in struct rte_eth_conf.
> This is actually a very bad idea, coz MSS is not bound to device.
> 
> 2. Turn on and off TSO with rte_ctrlmbuf.
> I found that rte_ctrlmbuf is not used at all. So it could be the first
> use case of it.
> With rte_ctrlmbuf we have enough space to store MSS.
> 
> Looking forward to your comments.
> 
> thx &
> rgds,
> -Qinglai

The mbuf needs to grow to 2 cache lines. There are other things that need
to be added to mbuf eventually as well. For example the QoS bitfield is
too small when crammed into 32 bits. Ideally the normal small packet
stuff would be in the first cacheline; and the other part of the struct
would have things less likely to be used.