[Qemu-devel] [Bug 778032] Re: qemu spinning on serial port writes

[Thomas Huth]

Which version of QEMU have you been using here? Can you still reproduce
this problem with the latest version of QEMU (currently version 2.9)?

** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/778032

Title:
  qemu spinning on serial port writes

Status in QEMU:
  Incomplete

Bug description:
  As originally found at http://www.mail-
  archive.com/address@hidden/msg08745.html from 3 years ago!

  Basically qemu seizes up in the event that the file descriptor for its
  emulated serial port has a full buffer, i.e. write() returns EAGAIN.
  For me, this happened when the serial port was being directed through
  a UNIX socket, with a default-sized 4KB buffer.  Just the normal
  output from a Linux kernel boot caused it to seize up, and stop the
  main emulation / select loop.

  My suggestion is to remove the detection of EAGAIN in qemu-char.c:521,
  so that if the buffer is full, KVM discards the byte(s) it was trying
  to write.  This is a surely better outcome than the process spinning
  forever.

  I will submit a separate patch to control the buffer sizes when
  creating UNIX sockets, which will help allow slow-reading processes to
  tune things so that they don't miss any output.

  Additionally, in the context of a hosted environment, if the -serial
  option is used, this could be a small security issue. An untrusted
  user of a guest system, knowing their serial output is going via a
  small buffer, could spew output to their /dev/ttyS0 at a rate fast
  enough to trigger this bug and eat a CPU core on the host.

  To quote David S. Ahern's original bug report (mine was the same, only
  with the latest version from git, so line numbers may have changed -
  my suggested fix above is accurate though):

  I am trying to redirect a guest's boot output through the host's serial
  port.  Shortly after launching qemu, the main thread is spinning on:

  write(9, "0", 1)   = -1 EAGAIN (Resource temporarily unavailable)

  fd 9 is the serial port, ttyS0.

  
  The backtrace for the thread is:

  #0  0x00002ac3433f8c0b in write () from /lib64/libpthread.so.0
  #1  0x0000000000475df9 in send_all (fd=9, buf=<value optimized out>,
  len1=1) at qemu-char.c:477
  #2  0x000000000043a102 in serial_xmit (opaque=<value optimized out>) at
  /root/kvm-81/qemu/hw/serial.c:311
  #3  0x000000000043a591 in serial_ioport_write (opaque=0x14971790,
  addr=<value optimized out>, val=48)
      at /root/kvm-81/qemu/hw/serial.c:366
  #4  0x00000000410eeedc in ?? ()
  #5  0x0000000000129000 in ?? ()
  #6  0x0000000014821fa0 in ?? ()
  #7  0x0000000000000007 in ?? ()
  #8  0x00000000004a54c5 in tlb_set_page_exec (env=0x10ab4,
  vaddr=46912496956816, paddr=1, prot=-1, mmu_idx=0, is_softmmu=1)
      at /root/kvm-81/qemu/exec.c:388
  #9  0x0000000000512f3b in tlb_fill (addr=345446292, is_write=1,
  mmu_idx=-1, retaddr=0x0)
      at /root/kvm-81/qemu/target-i386/op_helper.c:4690
  #10 0x00000000004a6bd2 in __ldb_cmmu (addr=9, mmu_idx=0) at
  /root/kvm-81/qemu/softmmu_template.h:135
  #11 0x00000000004a879b in cpu_x86_exec (env1=<value optimized out>) at
  /root/kvm-81/qemu/cpu-exec.c:628
  #12 0x000000000040ba29 in main (argc=12, argv=0x7fff67f7a398) at
  /root/kvm-81/qemu/vl.c:3816

  send_all() invokes unix_write() which by design is not breaking out on
  EAGAIN.

  The following command is enough to show the problem:

  qemu-system-x86_64 -m 256 -smp 1 -no-kvm \
      -drivefile=/dev/cciss/c0d0,if=scsi,cache=off,boot=on \
      -vnc :1 -serial /dev/ttyS0

  
  The guest is running RHEL3 with the parameter 'console=ttyS0' added to
  grub.conf; the problem appears to be with qemu, so I would expect it to
  show with any linux guest. This particular host is running RHEL5.2 with
  kvm-81, but I have also seen the problem with Fedora-9 as the host OS.

  Yes, the serial port of the server is connected to another system via a
  null modem. If I change the serial argument to '-serial udp::4555' and
  use  'nc -u -l  localhost 4555  > /dev/ttyS0' I see the guest's boot
  output show up on the second system as expected. I'd prefer to be able
  to use the serial port connection directly without nc as a proxy.
  Suggestions?

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