[DPDK/other Bug 1898] net/iavf: random packet reception delays with RTE_ETH_RX_OFFLOAD_TIMESTAMP

[DPDK/other Bug 1898] net/iavf: random packet reception delays with RTE_ETH_RX_OFFLOAD_TIMESTAMP

[bugzilla]

http://bugs.dpdk.org/show_bug.cgi?id=1898

            Bug ID: 1898
           Summary: net/iavf: random packet reception delays with
                    RTE_ETH_RX_OFFLOAD_TIMESTAMP
           Product: DPDK
           Version: unspecified
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: other
          Assignee: dev@dpdk.org
          Reporter: david.marchand@redhat.com
  Target Milestone: ---

Random latency for some packets has been observed when polling from some IAVF
device.
The PF driver is a RHEL9.6 kernel ice driver.

This issue was observed after a kernel upgrade and bisecting points at kernel
commit "7c1178a9df58 ("ice: support Rx timestamp on flex descriptor")".

Our understanding is that following this change, the PF announces the
VIRTCHNL_VF_CAP_PTP capability, which then allows using
RTE_ETH_RX_OFFLOAD_TIMESTAMP on the DPDK driver side.

Disabling RTE_ETH_RX_OFFLOAD_TIMESTAMP on the application side made the random
latency issue disappear.

Looking at the Rx path for the iavf driver, a (frequent) call to
iavf_get_phc_time() can be noticed which is likely the cause of the random
slowdowns.

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