(In reply to lijin from comment #0)
> Description of problem:
> hot plug virtio serial port to existing chardev already been used,qemu
> prompted warning "Property 'virtserialport.chardev' can't take value
> 'channel1', it's in use",but hotplug serialport with the same cmd again,the
> hotplug can success.
> The new port can be seen in guest and qtree.
> 
> Version-Release number of selected component (if applicable):
>     qemu-kvm-rhev-0.12.1.2-2.415.el6_5.3.x86_64
>     kernel-2.6.32-433.el6.x86_64
>     seabios-0.6.1.2-28.el6.x86_64
>     virtio-win-prewhql-74
> 
> How reproducible:
> 100%
> 
> Steps to Reproduce:
> 1. Start guest w/ virtio-serial-pci and virtio-serial-port
> CLI:/usr/libexec/qemu-kvm -m 4G -smp 4 -usb -device usb-tablet,id=tablet0
> -drive
> file=/home/win7-64.qcow2,if=none,id=drive-virtio0-0-0,format=qcow2,
> werror=stop,rerror=stop,cache=none -device
> ide-drive,drive=drive-virtio0-0-0,id=virti0-0-0,bootindex=1 -netdev
> tap,id=hostnet0,vhost=on,script=/etc/qemu-ifup -device
> virtio-net-pci,netdev=hostnet0,id=net0,mac=00:12:10:94:a3:f8 -uuid
> 8677bf9b-3cf6-47a1-b3e9-94213b801c87 -monitor stdio -spice
> id=on,disable-ticketing,port=5911 -vga qxl -bios
> /usr/share/seabios/bios-pm.bin -device
> virtio-serial-pci,id=virtio-serial0,max_ports=16 -chardev
> socket,id=channel1,path=/tmp/helloworld1,server,nowait -device
> virtserialport,chardev=channel1,name=com.redhat.rhevm.vdsm1,bus=virtio-
> serial0.0,id=port1
> 
> 2. hot add virtioserialport to the chardev channel1
> (qemu)device_add
> virtserialport,chardev=channel1,name=com.redhat.rhevm.vdsm2,bus=virtio-
> serial0.0,id=port2
> 
> 3.hot add virtioserialport to the chardev channel1 again
> virtserialport,chardev=channel1,name=com.redhat.rhevm.vdsm2,bus=virtio-
> serial0.0,id=port2
> 
> Actual results:
> after step2:qemu prompt "Property 'virtserialport.chardev' can't take value
> 'channel1', it's in use"
> after step3,no warning,and the new can be seen in guest and in qemu qtree.
> 
> Expected results:
> after step 3,qemu should also prompt a warning message as step2
> 
> Additional info:
> after the hotplug,both the original and the new port can not work
> normally.No date can be transferred.

in this case,I tested with win2012R2 guest instead of win7-64

Isn't that a kvm-qemu's bug? I assume that you get this error even without a guest OS running.

(In reply to Gal Hammer from comment #2)
> Isn't that a kvm-qemu's bug? I assume that you get this error even without a
> guest OS running.

yes,it is a qemu-kvm bug,change component to qemu-kvm.

win8.1-32 guest hit the similar issue when hot plug a virtserialport to an used chardev,qemu can always prompt "Property 'virtserialport.chardev' can't take value 'chardev1', it's in use",but I still can see the pluged port in "info qtree".

package info:
kernel-3.10.0-110.el7.x86_64
qemu-kvm-rhev-1.5.3-53.el7.x86_64
seabios-1.7.2.2-12.el7.x86_64
virtio-win-prewhql-75

still hit this isssue.

qemu-kvm-rhev-0.12.1.2-2.430.el6.x86_64
kernel-2.6.32-460.el6.x86_64
seabios-0.6.1.2-28.el6.x86_64
virtio-win-prewhql-86

steps:
1.boot win2k8r2 guest with:
/usr/libexec/qemu-kvm -drive file=win2k8R2.qcow2,if=none,cache=none,media=disk,format=qcow2,id=drive-ide0-0-1 -device ide-drive,id=ide0-0-1,drive=drive-ide0-0-1,bootindex=0 -global PIIX4_PM.disable_s3=0 -global PIIX4_PM.disable_s4=0 -vnc :0 -vga cirrus -usb -device usb-tablet -netdev tap,id=hostnet0 -device e1000,netdev=hostnet0,id=net0,mac=52:54:00:7f:f9:F6,bus=pci.0 -monitor unix:/tmp/tt,server,nowait -chardev file,path=/root/console.log,id=serial1 -device isa-serial,chardev=serial1,id=s1 -cpu SandyBridge -smp 4,maxcpus=4,cores=2,threads=2,sockets=1 -m 4G -enable-kvm -qmp tcp:0:4444,server,nowait -device virtio-serial-pci,id=virtio-serial0,max_ports=16 -chardev socket,id=channel0,path=/tmp/tt0,server,nowait -device virtserialport,nr=2,chardev=channel0,name=com.redhat.rhevm.vdsm,bus=virtio-serial0.0,id=port0 -device virtio-serial-pci,id=virtio-serial1,max_ports=31 -chardev socket,id=channel1,path=/tmp/tt1,server,nowait -device virtserialport,nr=2,chardev=channel1,name=com.redhat.rhevm.vdsm1,bus=virtio-serial1.0,id=port1

2.hot add virtioserialport to the already been used chardev:channel1
{"execute":"device_add","arguments":{"driver":"virtserialport","name":"com.redhat.rhevm.vdsm2","chardev":"channel1","bus":"virtio-serial0.0","id":"port2"}}
{"timestamp": {"seconds": 1406013479, "microseconds": 820443}, "event": "DEVICE_DELETED", "data": {"device": "port2"}}
{"error": {"class": "PropertyValueInUse", "desc": "Property 'virtserialport.chardev' can't take value 'channel1', it's in use", "data": 

3.hot add again:
{"device": "virtserialport", "property": "chardev", "value": "channel1"}}}
{"execute":"device_add","arguments":{"driver":"virtserialport","name":"com.redhat.rhevm.vdsm2","chardev":"channel1","bus":"virtio-serial0.0","id":"port2"}}
{"return": {}}

all three ports can be used normally

expected result:
hot add to an already used chardev should failed