[Bug 229188] Bhyve console hang on serial, works on stdio

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229188

            Bug ID: 229188
           Summary: Bhyve console hang on serial, works on stdio
           Product: Base System
           Version: CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: dgilbert at eicat.ca

I have a bulldozer (AMD 9590) running -CURRENT (a few days ago) that exhibits
some strange behavior lighting up a linux VM that we have transferred over from
Digital Ocean.

If it boots with console as stdio (-f argument), it runs.  If it boots without
-f (console is a null-modem serial), then I get the following on the console
(and the boot never finishes)

[   54.548000] serial8250: too much work for irq4
[   57.412000] serial8250: too much work for irq4
[   61.040000] [sched_delayed] sched: RT throttling activated
[   61.044000] serial8250: too much work for irq4
[   63.892000] serial8250: too much work for irq4
[   66.760000] serial8250: too much work for irq4
[   69.656000] serial8250: too much work for irq4

Now I've done some research on this message.  According to old posts
(2009-2012-ish) Other virtual machines got this message from linux when they
were either delivering interrupts on the serial when the serial was empty _or_
they were emptying the serial buffer too quickly and interrupting for more
output.

On that latter point, apparently this is a legitimate failure mode of some
physical chips, so the was much pushback on the linux side that the VMs need be
fixed.

I don't see any published work arounds and I'd rather not leave screens running
the console of machines ... so ...

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