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

[Stephen Hemminger]

On Thu, 30 Jul 2015 19:50:27 +0300
Vlad Zolotarov <vladz at cloudius-systems.com> wrote:

> 
> 
> 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. ;)

Either have a RTE_MAX_MBUF_SEGMENTS that is global or
a mbuf_linearize function?  Driver already can stash the
mbuf pool used for Rx and reuse it for the transient Tx buffers.