qemu spinning on serial port writes

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?

david

I sent this right before the holidays, and I have not seen any response. Has anyone had success connecting guest serial port to host serial port directly with guest boot output streaming out?

david