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.
