[Qemu-devel] qemu spinning on serial port writes

[David S. Ahern]

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