[Bug 255571] vmx mtu 9000 performance issues

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255571

            Bug ID: 255571
           Summary: vmx mtu 9000 performance issues
           Product: Base System
           Version: 12.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: jcaplan at blackberry.com

Overview
--------

Setting mtu=9000 for a vmx interface on vmware VM and running iperf2,
performance goes down to 272 kb/s from 943 Mb/s with default 1500 mtu.


Steps to Reproduce
------------------

1) On host, change interface to 9000

# ifconfig vmnet20 mtu 9000

2) On target do the same:

root at freebsd: # ifconfig vmx0 mtu 9000

3) Run iperf

root at freebsd: # iperf -s

Actual Results
--------------

On freebsd target:
# ./src/iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 64.0 KByte (default)
------------------------------------------------------------
(null)local 172.16.129.205 port 5001 connected with 172.16.129.1 port 34798


On host:
iperf -c 172.16.129.205
------------------------------------------------------------
Client connecting to 172.16.129.205, TCP port 5001
TCP window size:  325 KByte (default)
------------------------------------------------------------
[  3] local 172.16.129.1 port 34798 connected with 172.16.129.205 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.1 sec   335 KBytes   272 Kbits/sec


Expected Results
----------------
iperf shouldn't hang on target and performance should be similar to 1500 mtu
case.

Build Date & Hardware:
----------------------

FreeBSD freebsd 12.2-RELEASE FreeBSD 12.2-RELEASE r366954 GENERIC  amd64

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