Device init is failing, and there's not enough info why.  I can't see how this is related to virtio-serial; looks like the failure is happening in the qdev layer itself.  I suspect it could be due to some typo somewhere but it's not really easy to follow.

Can you try a few things:

1. How easily is this reproducible?  You've mentioned 5 times, but does it happen each time you try, or does it happen 5 out of 100 times?
2. Can you try via the hmp?

(In reply to Amit Shah from comment #1)
> Device init is failing, and there's not enough info why.  I can't see how
> this is related to virtio-serial; looks like the failure is happening in the
> qdev layer itself.  I suspect it could be due to some typo somewhere but
> it's not really easy to follow.
> 
> Can you try a few things:
> 
> 1. How easily is this reproducible?  You've mentioned 5 times, but does it
> happen each time you try, or does it happen 5 out of 100 times?
> 2. Can you try via the hmp?

Hi Amit,

   Could you please go through my testing results,thank you very much.

For 1,100% reproducible if use my steps
For 2,QE got the following error message from HMP

1.#chardev-add backend=pty,id=chardev-pty

2.#device_add  virtconsole,bus=virtio-serial0.0,chardev=chardev-pty,name=org.linux-kvm.port.0,id=port0

3.#device_del  port0

4.#chardev-remove  chardev-pty

5.#chardev-add backend=pty,id=chardev-two

6.#device_add virtserialport,bus=virtio-serial0.0,chardev=chardev-two,name=org.linux-kvm.port.1,id=port1 
(This was original issue in the bug)

7.#device_add virtserialport,bus=virtio-serial0.0,chardev=chardev-two,name=org.linux-kvm.port.1,id=port1,nr=0 (Step 7's results was expected as it was reserved for virtconsole)

For do step6 without "nr" or step7 with "nr=0" get the following info from qemu
Port number 0 on virtio-serial devices reserved for virtconsole devices for backward compatibility.
Device initialization failed.
Device 'virtserialport' could not be initialized

8.Change "nr=1" QE still failed to add port.I think it should be successfully here but it didn't.
#device_add virtserialport,bus=virtio-serial0.0,chardev=chardev-two,name=org.linux-kvm.port.1,id=port1,nr=1
The message was following as below and still failed to add port
virtio-serial-bus: A port already exists at id 1
Device initialization failed.
Device 'virtserialport' could not be initialized

9.After changing "nr=2" successfully added port1 from HMP.
#device_add virtserialport,bus=virtio-serial0.0,chardev=chardev-two,name=org.linux-kvm.port.1,id=port1,nr=2

Amit,any issues please let me know,thanks.

Best regards
Min Deng

Reduced testcase:

$ ./x86_64-softmmu/qemu-system-x86_64 -m 512 -cpu host -enable-kvm -device virtio-serial-pci -monitor stdio  -vnc :1
QEMU 2.0.91 monitor - type 'help' for more information
(qemu) device_add virtconsole,id=p1
(qemu) device_del p1
(qemu) device_add virtserialport,id=p1
Port number 0 on virtio-serial devices reserved for virtconsole devices for backward compatibility.
Device 'virtserialport' could not be initialized
(qemu) quit

What's happening is hot-unplug of virtconsole removes the 'reserved' tag for port 0.

Reproduce this bug as follow version:
Host:
# uname -r
3.10.0-144.el7.x86_64
# rpm -q qemu-kvm-rhev
qemu-kvm-rhev-1.5.3-30.el7.x86_64

Guest :rhel7

Steps:
1.Boot guest 
...-device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x5...

2.Hotadd chardev and virtio console
...
{"execute": "chardev-add", "arguments": {"id": "chardev-pty", "backend": {"data": {}, "type": "pty"}}}
{"return": {"pty": "/dev/pts/1"}}

{"execute":"device_add","arguments":{"driver":"virtconsole","bus":"virtio-serial0.0","chardev":"chardev-pty","name":"org.linux-kvm.port.0","id":"port0"}}
{"return": {}}
...

3.Hotunplug  virtio console and chardev
...
{"execute": "device_del", "arguments": {"id": "port0"}} 
{"timestamp": {"seconds": 1409538647, "microseconds": 669236}, "event": "DEVICE_DELETED", "data": {"device": "port0", "path": "/machine/peripheral/port0"}}
{"return": {}}

{"execute": "chardev-remove", "arguments": {"id": "chardev-pty"}}
{"return": {}}

4.Hotadd chardev and virtio console

After step 4
..
{"execute": "chardev-add", "arguments": {"id": "chardev-two", "backend": {"data": {}, "type": "pty"}}}
{"return": {"pty": "/dev/pts/1"}}

{"execute":"device_add","arguments":{"driver":"virtserialport","bus":"virtio-serial0.0","chardev":"chardev-two","name":"org.linux-kvm.port.1","id":"port1"}}
{"error": {"class": "GenericError", "desc": "Device initialization failed."}}


Verify this bug as follow version:
# uname -r
3.10.0-144.el7.x86_64
# rpm -q qemu-kvm-rhev
qemu-kvm-rhev-2.1.0-3.el7.x86_64

Guest :rhel7

Steps as same as reproduce

Resutls:

After step 4
...
{"execute": "chardev-add", "arguments": {"id": "chardev-two", "backend": {"data": {}, "type": "pty"}}}
{"return": {"pty": "/dev/pts/3"}}
{"execute":"device_add","arguments":{"driver":"virtserialport","bus":"virtio-serial0.0","chardev":"chardev-two","name":"org.linux-kvm.port.1","id":"port1"}}
{"return": {}}


According to above test ,this bug fixed.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-0624.html