Re: [dpdk-dev] RFC: i40e xmit path HW limitation

[Vlad Zolotarov <vladz@cloudius-systems.com>]

On 07/30/15 19:20, Avi Kivity wrote:
>
>
> On 07/30/2015 07:17 PM, Stephen Hemminger wrote:
>> On Thu, 30 Jul 2015 17:57:33 +0300
>> Vlad Zolotarov <vladz@cloudius-systems.com> wrote:
>>
>>> Hi, Konstantin, Helin,
>>> there is a documented limitation of xl710 controllers (i40e driver)
>>> which is not handled in any way by a DPDK driver.
>>>   From the datasheet chapter 8.4.1:
>>>
>>> "• A single transmit packet may span up to 8 buffers (up to 8 data 
>>> descriptors per packet including
>>> both the header and payload buffers).
>>> • The total number of data descriptors for the whole TSO (explained 
>>> later on in this chapter) is
>>> unlimited as long as each segment within the TSO obeys the previous 
>>> rule (up to 8 data descriptors
>>> per segment for both the TSO header and the segment payload buffers)."
>>>
>>> This means that, for instance, long cluster with small fragments has to
>>> be linearized before it may be placed on the HW ring.
>>> In more standard environments like Linux or FreeBSD drivers the 
>>> solution
>>> is straight forward - call skb_linearize()/m_collapse() corresponding.
>>> In the non-conformist environment like DPDK life is not that easy -
>>> there is no easy way to collapse the cluster into a linear buffer from
>>> inside the device driver
>>> since device driver doesn't allocate memory in a fast path and utilizes
>>> the user allocated pools only.
>>>
>>> Here are two proposals for a solution:
>>>
>>>   1. We may provide a callback that would return a user TRUE if a give
>>>      cluster has to be linearized and it should always be called before
>>>      rte_eth_tx_burst(). Alternatively it may be called from inside the
>>>      rte_eth_tx_burst() and rte_eth_tx_burst() is changed to return 
>>> some
>>>      error code for a case when one of the clusters it's given has 
>>> to be
>>>      linearized.
>>>   2. Another option is to allocate a mempool in the driver with the
>>>      elements consuming a single page each (standard 2KB buffers would
>>>      do). Number of elements in the pool should be as Tx ring length
>>>      multiplied by "64KB/(linear data length of the buffer in the pool
>>>      above)". Here I use 64KB as a maximum packet length and not taking
>>>      into an account esoteric things like "Giant" TSO mentioned in the
>>>      spec above. Then we may actually go and linearize the cluster if
>>>      needed on top of the buffers from the pool above, post the buffer
>>>      from the mempool above on the HW ring, link the original 
>>> cluster to
>>>      that new cluster (using the private data) and release it when the
>>>      send is done.
>> Or just silently drop heavily scattered packets (and increment oerrors)
>> with a PMD_TX_LOG debug message.
>>
>> I think a DPDK driver doesn't have to accept all possible mbufs and do
>> extra work. It seems reasonable to expect caller to be well behaved
>> in this restricted ecosystem.
>>
>
> How can the caller know what's well behaved?  It's device dependent.

+1

Stephen, how do you imagine this well-behaved application? Having switch 
case by an underlying device type and then "well-behaving" correspondingly?
Not to mention that to "well-behave" the application writer has to read 
HW specs and understand them, which would limit the amount of DPDK 
developers to a very small amount of people... ;) Not to mention that 
the mentioned above switch-case would be a super ugly thing to be found 
in an application that would raise a big question about the 
justification of a DPDK existence as as SDK providing device drivers 
interface. ;)

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