[Qemu-devel] [Bug 1335444] Re: qemu loses serial console data on EAGAIN

[Andreas Gustafsson]

A separate bug report has now been filed for the sparc case as
requested: bug #1399943.

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https://bugs.launchpad.net/bugs/1335444

Title:
  qemu loses serial console data on EAGAIN

Status in QEMU:
  Fix Released

Bug description:
  When running a guest OS with a serial console under "qemu-system-i386
  -nographic", parts of the serial console output are sometimes lost.

  This happens when a write() to standard output by qemu returns EAGAIN,
  as may be the case when the guest is generating console output faster
  than the tty (or pty/pipe/socket, etc.) connected to qemu's standard
  output accepts it.  The bug affects all releases of qemu since 1.5,
  which was the first version to set stdout to O_NONBLOCK mode.  Version
  1.4.2 and earlier work correctly.

  To reproduce the bug, you will need a guest OS configured with a
  serial console, and a host with a slow tty.  Two different methods
  of setting this up are outlined below.

  
  Method 1

  This fully automated test uses the "pexpect" Python module to run qemu
  under a pty, with an Aboriginal Linux guest.  A "seq" command is sent
  to the guest to generate 100,000 lines of output containing sequential
  integers, and the output is checked for gaps.  The script limits the
  tty output rate by occasionally sleeping for 1/10 of a second.

  Run the following commands in a Bourne shell:

  wget 
http://landley.net/aboriginal/downloads/binaries/system-image-i686.tar.bz2
  bunzip2 <system-image-i686.tar.bz2 | tar xf -
  cd system-image-i686
  cat <<\END >test.py
  #!/usr/bin/python
  import sys
  import time
  import pexpect
  n = 100000
  child = pexpect.spawn('./run-emulator.sh', logfile = sys.stderr)
  child.expect("/home #")
  child.send("seq -f '<%%08.0f>' 0 %d\r" % (n - 1))
  for i in range(n):
      child.expect("<([0-9]+)>", timeout = 5)
      got = int(child.match.group(1))
      if got != i:
          print >>sys.stderr, "\nFAIL: expected %d, got %d" % (i, got)
          sys.exit(1)
      if i % 100 == 0:
          time.sleep(0.1)
  child.send("exit")
  print >>sys.stderr, "\nPASS"
  sys.exit(0)
  END
  python test.py

  This will output PASS if the console output contains the 100,000
  sequential integers as expected, or FAIL if parts of the output
  are missing due to the bug.

  
  Method 2

  This method does not require Python or pexpect.  Instead, the qemu source
  is modified to simulate a simulate a slow tty by forcing an EAGAIN return
  from every other write().  If qemu were working correctly, this
  change would not cause any data loss, because the writes would be
  retried, but in actuality, they are not retried, and the end result is
  that every other character in the guest output will be missing.

  Apply the following patch to the qemu source (this is against 2.0.0):

  --- ../qemu-2.0.0.orig/qemu-char.c      2014-04-17 16:44:45.000000000 +0300
  +++ ./qemu-char.c       2014-06-20 16:47:18.000000000 +0300
  @@ -779,6 +779,17 @@
       size_t offset = 0;
       GIOStatus status = G_IO_STATUS_NORMAL;

  +    /*
  +     * Simulate a tty with a limited output buffer by returning
  +     * EAGAIN on every second call.
  +     */
  +    static unsigned int toggle = 0;
  +    toggle++;
  +    if (toggle & 1) {
  +       errno = EAGAIN;
  +       return -1;
  +    }
  +
       while (offset < len && status == G_IO_STATUS_NORMAL) {
           gsize bytes_written = 0;

  Build and install qemu.

  Run any serial console guest.  You could use the same Aboriginal Linux image
  as in Method 1, or for example the PLD RescueCD:

    wget http://rescuecd.pld-linux.org/download/2011-02-12/x86/RCDx86_11_02.iso
    qemu-system-i386 -nographic -cdrom RCDx86_11_02.iso

  If this command is run with an unmodified qemu, a set of boot messages
  will appear, starting with:

    ISOLINUX 4.03 2010-10-22  Copyright (C) 1994-2010 H. Peter Anvin et
  al

  When qemu has been patched to simulate EAGAIN returns, every other
  character in the boot messages will be missing, so that the first line
  of output will instead read:

    SLNX40 001-2 oyih C 9421 .PtrAvne l

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