[dpdk-dev] RFC: i40e xmit path HW limitation
Vlad Zolotarov
vladz at cloudius-systems.com
Thu Jul 30 18:50:27 CEST 2015
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 at 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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