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. ;) > >
