in kernel-2.6.18-238.4.1.el5

linux-2.6-char-virtio-wake-console-outvq-on-host-notifications.patch

Verified on qemu-kvm-0.12.1.2-2.144.el6.
guest kernel : kernel-2.6.18-238.4.1.el5

steps:
1.start VM with virtio-serial-port w/o -M parameter.
2.open the socket file on the host and not read it
eg:#cat open-socket 
#!/usr/bin/python
import os
import sys
import socket
import time

#fd = os.open(sys.argv[1], os.O_RDONLY)

s = socket.socket(socket.AF_UNIX)
s.connect(sys.argv[1])

while 1:
        # do nothing
        time.sleep(1)

#python open-socket /tmp/vport0
3.transfer a file whose size > 2G via virtio-serial
eg :#cat /tt > /dev/vport0p1

Actual Results:
qemu-kvm process does not freeze.

Based on above ,this issue has been fixed.
Change status to VERIFIED.

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHSA-2011-0303.html

    Technical note added. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    New Contents:
Using a virtio serial port from an application, filling it until the write command returns -EAGAIN and then executing a select command for the write command caused the select command to not return any values, when using the virtio serial port in a non-blocking mode. When used in a blocking mode, the write command waited until the host indicated it used up the buffers. This was due to the fact that the poll operation waited for the port->waitqueue pointer, however, nothing woke the waitqueue when there was room again in the queue. With this update, the queue is woken via host notifications so that buffers consumed by the host can be reclaimed, the queue freed, and the application write operations may proceed again.