From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@cloudius-systems.com>
Subject: Re: [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1
 for all NICs but 82598
Date: Fri, 11 Sep 2015 17:47:51 +0300
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To: "didier.pallard" <didier.pallard@6wind.com>,
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 "Zhang, Helin" <helin.zhang@intel.com>
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On 09/11/2015 05:25 PM, didier.pallard wrote:
> On 08/25/2015 08:52 PM, Vlad Zolotarov wrote:
>>
>> Helin, the issue has been seen on x540 devices. Pls., see a chapter 
>> 7.2.1.1 of x540 devices spec:
>>
>> A packet (or multiple packets in transmit segmentation) can span any 
>> number of
>> buffers (and their descriptors) up to a limit of 40 minus WTHRESH 
>> minus 2 (see
>> Section 7.2.3.3 for Tx Ring details and section Section 7.2.3.5.1 for 
>> WTHRESH
>> details). For best performance it is recommended to minimize the 
>> number of buffers
>> as possible.
>>
>> Could u, pls., clarify why do u think that the maximum number of data 
>> buffers is limited by 8?
>>
>> thanks,
>> vlad
>
> Hi vlad,
>
> Documentation states that a packet (or multiple packets in transmit 
> segmentation) can span any number of
> buffers (and their descriptors) up to a limit of 40 minus WTHRESH 
> minus 2.
>
> Shouldn't there be a test in transmit function that drops properly the 
> mbufs with a too large number of
> segments, while incrementing a statistic; otherwise transmit function 
> may be locked by the faulty packet without
> notification.
>

What we proposed is that the pmd expose to dpdk, and dpdk expose to the 
application, an mbuf check function.  This way applications that can 
generate complex packets can verify that the device will be able to 
process them, and applications that only generate simple mbufs can avoid 
the overhead by not calling the function.