[Bug 268910] ixgbe(4): rxcsum register setting can cause TCP connection hangs

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=268910

            Bug ID: 268910
           Summary: ixgbe(4): rxcsum register setting can cause TCP
                    connection hangs
           Product: Base System
           Version: 12.3-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: brian90013@gmail.com

Hello,

I started diagnosing a FreeBSD-12.3 system with a X550-T2 10GbE controller that
was having trouble making outgoing TCP connections (ssh for example). I used
tcpdump and could see a response returning to the host but the client software
didn't seem to receive the packet and move forward.

The port was configured for 1 queue per direction
(dev.ix.0.iflib.override_nrxqs=1 and dev.ix.0.iflib.override_ntxqs=1) and all
offloads were disabled. I discovered that enabling rxcsum restored stable
networking. I could complete ssh connections as well as generic TCP/UDP
connections using nc. As soon as I removed the rxcsum offload, the initial
behavior returned. I also discovered setting the number of queues > 1 was
stable while queues=1 hung connections.

I looked at ixgbe_initialize_receive_units() in if_ix.c where the RXCSUM
register is set. Here's the relevant section:

    rxcsum = IXGBE_READ_REG(hw, IXGBE_RXCSUM);

    ixgbe_initialize_rss_mapping(sc);

    if (sc->num_rx_queues > 1) {
        /* RSS and RX IPP Checksum are mutually exclusive */
        rxcsum |= IXGBE_RXCSUM_PCSD;
    }

    if (ifp->if_capenable & IFCAP_RXCSUM)
        rxcsum |= IXGBE_RXCSUM_PCSD;

    /* This is useful for calculating UDP/IP fragment checksums */
    if (!(rxcsum & IXGBE_RXCSUM_PCSD))
        rxcsum |= IXGBE_RXCSUM_IPPCSE;

    IXGBE_WRITE_REG(hw, IXGBE_RXCSUM, rxcsum);

Not hard to see that if either queues>1 OR rxcsum is enabled the PCSD bit is
set and IPPCSE is not set. In the case where queues==1 AND !rxcsum, PCSD is not
set and IPPCSE is set (and my TCP connections hang).

I believe there is a problem here but I'm not sure which part. A few notes:
* The linux driver unconditionally sets PCSD and doesn't touch IPPCSE in
ixgbe_set_mrqc().
* The e1000 FreeBSD driver sets PCSD if rx_queues > 1. It only calls
xxx_initialize_rss_mapping() if rx_queues > 1 is true. ixgbe calls
xxx_initialize_rss_mapping outside of the conditional so it is always run. Does
this mean PCSD must always be set?
* I couldn't find anywhere in the driver where the UDP/IP fragment checksum was
used. Is there any benefit to setting IPPCSE?

I modified the above code to always set PCSD and never set IPPCSE. In my
testing on the X550-T2 and an optical X520 82599ES, this change eliminated the
hangs even with queues=1 and -rxcsum.

I'm hoping this is enough detail for someone to identity the true root cause
and best solution. Please let me know if you need additional information.

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