Hi Oliver, > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Olivier MATZ > Sent: Friday, November 07, 2014 5:16 PM > To: Yong Wang; Liu, Jijiang > Cc: dev at dpdk.org > Subject: Re: [dpdk-dev] [PATCH v8 10/10] app/testpmd:test VxLAN Tx checksum > offload > > Hello Yong, > > On 11/07/2014 01:43 AM, Yong Wang wrote: > >>> As to HW TX checksum offload, do you have special requirement for > >>> implementing TSO? > > > >> Yes. TSO implies TX TCP and IP checksum offload. > > > > Is this a general requirement or something specific to ixgbe/i40e? FWIW, > > vmxnet3 device does not support tx IP checksum offload but doe support > > TSO. In that case, we cannot leave IP checksum field as 0 (the correct > > checksum needs to be filled in the header) before passing it the the NIC > > when TSO is enabled. > > This is a good question because we need to define the proper API that > will work on other PMDs in the future. > > Indeed, there is a hardware specificity in ixgbe: when TSO is enabled, > the IP checksum flag must also be passed to the driver if it's IPv4. > From 82599 datasheets (7.2.3.2.4 Advanced Transmit Data Descriptor): > > IXSM (bit 0) - Insert IP Checksum: This field indicates that IP > checksum must be inserted. In IPv6 mode, it must be reset to 0b. > If DCMD.TSE and TUCMD.IPV4 are set, IXSM must be set as well. > If this bit is set, the packet should at least contain an > IP header. > > If we allow the user to give the TSO flag without the IP checksum > flag in mbuf flags, the ixgbe driver would have to set the IP checksum > flag in hardware descriptors if the packet is IPv4. The driver would > have to parse the IP header: this is not a problem as we already need > it for TCP checksum. > > To summarize, I think we have 3 options when transmitting a packet to be > segmented using TSO: > > - set IP checksum to 0 in the application: in this case, it would > require additional work in virtual drivers if the peer expects > to receive a packet with a valid IP checksum. But I'm wondering > what is the need for calculating a checksum when transmitting on > a virtual device (the peer receiving the packet knows that the > packet is not corrupted as it comes from memory). Moreover, if the > device advertise TSO, I assume it can also advertise IP checksum > offload. > > - calculate the IP checksum in the application. It would take additional > cycles although it may not be needed as the driver probably knows > how to calculate it. > > - if the driver supports both TSO and IP checksum, the 2 flags MUST > be given to the driver and the IP checksum must be set to 0 and the > checksum cannot be calculated in software. If the driver only > supports TSO, the checksum has to be calculated in software. > > Currently, I choosen the first solution, but I'm open to change the > design. Maybe the 3rd one is also a good solution. > > By the way, we had the same kind of discussion with Konstantin [1] > about what to do with the TCP checksum. My feeling is that setting it > to the pseudo-header checksum is the best we can do: > - linux does that > - many hardware requires that (this is not the case for ixgbe, which > need a pshdr checksum without the IP len) > - it can be reused if received by a virtual device and sent to a > physical device supporting TSO
Yes, I remember that discussion. I still think we better avoid any read/write access of the packet data inside PMD TX routine. (packet header parsing and/or pseudo-header checksum calculations). As I said before - if different HW have different requirements of what have to be recalculated for HW TX offloads - why not introduce a new function dev_prep_tx(portid, queueid, mbuf[], num)? PMD developer can put all necessary calculations/updates of the packet data and related mbuf fields inside that function. It would be then a PMD responsibility to provide that function and it would be an app layer responsibility to call it for mbufs with TX offload flags before calling tx_burst(). Konstantin > > Best regards, > Olivier > > > [1] http://dpdk.org/ml/archives/dev/2014-May/002766.html
