pkg/48071: qemu 1.5.1 serial console output lossage

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         48071
>Category:       pkg
>Synopsis:       qemu 1.5.1 serial console output lossage
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jul 21 15:05:00 +0000 2013
>Originator:     Andreas Gustafsson
>Release:        NetBSD 6.0.1
>Organization:
>Environment:
System: NetBSD gusto.gson.org 6.0.1
Architecture: amd64
Machine: amd64
>Description:

When attempting to install a NetBSD/i386 or NetBSD/amd64 guest in a
qemu virtual machine on a NetBSD 6 host using qemu 1.5.1 from pkgsrc,
parts of the serial console output are missing.  In practice, this
makes it impossible to complete an installation, as much of the
information needed to correctly respond to sysinst's prompts is
never displayed.

A typical behavior is that you get as far as the following screen in
sysinst:

   You have chosen to install NetBSD on your hard disk.  This will change       
  
   information on your hard disk.  You should have made a full backup before    
  
   this procedure!  This procedure will do the following things:                
  
           a) Partition your disk                                               
  
           b) Create new BSD file systems                                       
  
           c) Load and install distribution sets                                
  
           d) Some initial system configuration                                 
  

   (After you enter the partition information but before your disk is changed,  
  
   you will have the opportunity to quit this procedure.)                       
  

   Shall we continue?                                                           
  
                                 ---------------
                                |  yes or no?   |

but the screen fails to display in its entirety.  Sometimes the box
containing the "yes or no?" prompt is missing entirely, and sometimes
it is displayed only partially as shown above.

When this happens, sysinst still responds to a control-L by redrawing
the screen as expected, but the redrawn screen is again incomplete
(though it usually ends in a different place than before).  Typing "b"
and ENTER (if you happen to known that "b" is the right answer even
though the part of the screen telling you that is missing) has the
expected effect of continuing to the next screen, but that screen is
also displayed only in part.

Typing control-C drops to a shell prompt as expected, but typing
"ls -l /bin" shows only the beginning of the expected output.

This behavior all seems consistent with serial console output somehow
getting discarded whenever more than a certain amount is output at once.

Doing a similar installation using qemu-0.15.1nb5 works with no problems.

>How-To-Repeat:

On a NetBSD 6.x system, install qemu 1.5.1 from pkgsrc.
Then run:

  ftp 
http://ftp.netbsd.org/pub/NetBSD/NetBSD-6.1/amd64/installation/cdrom/boot-com.iso
  dd if=/dev/zero of=wd0.img bs=1m count=1024
  qemu-system-x86_64 -cdrom boot-com.iso -hda wd0.img -nographic

Follow the prompts and attempt to install NetBSD in the virtual
machine.  At some point during the installation (not necessarily the
exact screen shown above), notice how a sysinst dialog fails to
display fully.

>Fix: