[Bug 195395] New: serial console as qvm-kemu guest stops responding freezes on duplex I/O

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=195395

            Bug ID: 195395
           Summary: serial console as qvm-kemu guest stops responding
                    freezes on duplex I/O
           Product: Base System
           Version: 10.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: bkabe at vega.pgw.jp

Overview:
When FreeBSD as a guest under qvm-kemu, and guest setup to use serial console,
simultaneous I/O on serial console will lock up the serial console.

That is, on console, you could "ls -lR /" and press any key during the
scrolling, 
and console freezes instantly.
Ditto for Copy-Pasting a long string on the serial console.


Reproduce:
Host OS: CentOS 6.5 + kernel-2.6.32-431.el6.x86_64 +
qemu-kvm-0.12.1.2-2.415.el6_5.10.x86_64
Guest OS: FreeBSD 10.1 release (amd64)

* Boot FreeBSD media on host as Linux qemu-kvm guest.
Example: 
host# virt-install -n fbtest -r 128 --vcpus=1 \
    --nodisks \
    --cdrom /home/libvirt/images/FreeBSD-10.0-RELEASE-amd64-dvd1.iso \
    --network=network=default,model=virtio \
    --video=vga --graphics none


* Loader prompt will appear on serial; press [Esc] to get OK prompt and type
OK set console=comconsole
OK boot -s
to get single user prompt.

* # ls -lR /
During scrolling, press any key.

Actual Results:
Console stops responding.

Expected results:
At least it should respond for buffer overflow or something.

Additional Information:
Console freeze will still occur on HDD(-image) installed, multiuser
FreeBSD guest with serial console configured as in Hnadbook.
When ps-ed from other tty (such as ssh), "ls -lR / >/dev/console" is stuck on
"ttyout".

  0 632    1   0  20  0 14488 1932 ttyin    Ss+  u0  0:00.02 /usr/libexec/getty
3wire.9600 ttyu0
  0 595    1   0  52  0 14488 1932 ttyin    Is+  v0  0:00.01 /usr/libexec/getty
Pc ttyv0
  0 621  620   0  21  0 47716 2672 wait     Is    0  0:00.06 su
  0 624  621   0  20  0 23572 3292 pause    S     0  0:00.03 _su (csh)
  0 636  624   0  52  0 16852 2932 ttyout   S     0  0:00.39 ls -lR /
  0 643  624   0  20  0 18736 2100 -        R+    0  0:00.00 ps axlww

SIGHUP or SIGKILL-ing process holding /dev/ttyu0 doesn't help.
The only way to unlock the console is to reboot. (single user is not enough)

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