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Date: Fri, 4 Oct 2013 20:28:25 +0300
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From: jigsaw <jigsaw@gmail.com>
To: Stephen Hemminger <stephen@networkplumber.org>
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Subject: Re: [dpdk-dev] Need comment on 82599 TSO
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Hi Stephen,

Thanks for comment. Pls check the other thread that I just posted.

thx &
rgds,
-Qinglai

On Fri, Oct 4, 2013 at 7:41 PM, Stephen Hemminger
<stephen@networkplumber.org> wrote:
> 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.