I test it in rhel6.4 also hit this issue.

host info:
kernel-2.6.32-356.el6.x86_64
qemu-kvm-0.12.1.2-2.355.el6.x86_64
seabios-0.6.1.2-26.el6.x86_64
guest info:
kernel-2.6.32-356.el6.x86_64

Best Regards.
sluo

patches posted.

Steps to reproduce and verify this bug:
1. launch a guest with serial specified in -drive option:
/usr/libexec/qemu-kvm -M rhel6.4.0 -enable-kvm -m 2048 -smp 2,sockets=1,cores=2,threads=1 -device usb-ehci,id=ehci_1 -drive file=/home/rhel6.4.bak.qcow2,if=none,id=drive-virtio-disk0,format=qcow2,cache=none,werror=stop,rerror=stop,aio=native -device usb-storage,bus=ehci_1.0,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,serial=abcdefgh-520c-469b-90c2-663277e90438  -net none -nodefaults -spice port=8000,disable-ticketing -k en-us -vga qxl -global qxl-vga.vram_size=67108864 -monitor stdio -device virtio-scsi-pci,id=scsi_1 -drive file=/home/test.qcow2,if=none,id=drive-virtio-disk1,format=qcow2,cache=none,werror=stop,rerror=stop,aio=native -device scsi-hd,bus=scsi_1.0,drive=drive-virtio-disk1,id=virtio-disk1,serial=qwertui-520c-469b-90c2-663277e90448
2. observe the output of info qtree as well as in guest through scsi_id
3. hot plug a disk into guest with serial specified and repeat step 2.

Reproduction:
------------
Reproduced with 2.6.32-391.el6.x86_64, qemu-kvm-0.12.1.2-2.355.el6.x86_64. info qtree didn't tell the serial specified in -drive option.
(qemu) info qtree 
bus: main-system-bus
 ...
      dev: usb-ehci, id "ehci_1"
        dev-prop: freq = 1000
        dev-prop: maxframes = 128
        bus-prop: addr = 03.0
        bus-prop: romfile = <null>
        bus-prop: rombar = 1
        bus-prop: multifunction = off
        class USB controller, addr 00:03.0, pci id 8086:24cd (sub 1af4:1100)
        bar 0: mem at 0xf4020000 [0xf4020fff]
        bus: ehci_1.0
          type USB
          dev: usb-storage, id "virtio-disk0"
            dev-prop: drive = <null>
            dev-prop: logical_block_size = 512
            dev-prop: physical_block_size = 512
            dev-prop: min_io_size = 0
            dev-prop: opt_io_size = 0
            dev-prop: bootindex = 1
            dev-prop: discard_granularity = 0
            dev-prop: serial = "abcdefgh-520c-469b-90c2-663277e90438"
            dev-prop: removable = off
            bus-prop: port = <null>
            bus-prop: create_unique_serial = 1
            addr 0.1, port 1, speed 480, name QEMU USB MSD, attached
            bus: virtio-disk0.0
              type SCSI
              dev: scsi-disk, id ""
                dev-prop: drive = drive-virtio-disk0
                dev-prop: logical_block_size = 512
                dev-prop: physical_block_size = 512
                dev-prop: min_io_size = 0
                dev-prop: opt_io_size = 0
                dev-prop: bootindex = 1
                dev-prop: discard_granularity = 0
                dev-prop: ver = "0.12.1"
                dev-prop: serial = <null> 
                          ^^^^^^^^^^^^^^^ shouldn't be null
                dev-prop: wwn = 0x0
                dev-prop: removable = off
                bus-prop: channel = 0
                bus-prop: scsi-id = 0
                bus-prop: lun = 0


In guest:
scsi_id --whitelisted --device=/dev/sdb
0QEMU    QEMU HARDDISK   drive-virtio-disk0  
                         ^^^^^^^^^^^^^^^^^^ shouldn't be this
scsi_id --whitelisted --device=/dev/sda
0QEMU    QEMU HARDDISK   qwertui-520c-469b-90


Verification:
------------
Verified with 2.6.32-391.el6.x86_64, qemu-kvm-0.12.1.2-2.376.el6.x86_64. 
After step 2:
      dev: usb-ehci, id "ehci_1"
        dev-prop: freq = 1000
        dev-prop: maxframes = 128
        bus-prop: addr = 03.0
        bus-prop: romfile = <null>
        bus-prop: rombar = 1
        bus-prop: multifunction = off
        class USB controller, addr 00:03.0, pci id 8086:24cd (sub 1af4:1100)
        bar 0: mem at 0xf4020000 [0xf4020fff]
        bus: ehci_1.0
          type USB
          dev: usb-storage, id "virtio-disk0"
            dev-prop: drive = <null>
            dev-prop: logical_block_size = 512
            dev-prop: physical_block_size = 512
            dev-prop: min_io_size = 0
            dev-prop: opt_io_size = 0
            dev-prop: bootindex = 1
            dev-prop: discard_granularity = 0
            dev-prop: serial = "abcdefgh-520c-469b-90c2-663277e90438"
            dev-prop: removable = off
            bus-prop: port = <null>
            bus-prop: create_unique_serial = 1
            addr 0.1, port 1, speed 480, name QEMU USB MSD, attached
            bus: virtio-disk0.0
              type SCSI
              dev: scsi-disk, id ""
                dev-prop: drive = drive-virtio-disk0
                dev-prop: logical_block_size = 512
                dev-prop: physical_block_size = 512
                dev-prop: min_io_size = 0
                dev-prop: opt_io_size = 0
                dev-prop: bootindex = 1
                dev-prop: discard_granularity = 0
                dev-prop: ver = "0.12.1"
                dev-prop: serial = "abcdefgh-520c-469b-90c2-663277e90438" 
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                dev-prop: wwn = 0x0
                dev-prop: removable = off
                bus-prop: channel = 0
                bus-prop: scsi-id = 0
                bus-prop: lun = 0
In guest:
# scsi_id --whitelisted --device=/dev/sdb
0QEMU    QEMU HARDDISK   abcdefgh-520c-469b-9


After step 3:
(qemu) __com.redhat_drive_add file=/home/usb_data.qcow2,id=drive-virtio-disk2,format=qcow2,cache=none,werror=stop,rerror=stop,aio=native
(qemu) device_add usb-storage,bus=ehci_1.0,drive=drive-virtio-disk2,id=virtio-disk2,serial=zxcvbnm1-520c-469b-90c2-663277e90438

...
 bus: virtio-disk2.0
              type SCSI
              dev: scsi-disk, id ""
                dev-prop: drive = drive-virtio-disk2
                dev-prop: logical_block_size = 512
                dev-prop: physical_block_size = 512
                dev-prop: min_io_size = 0
                dev-prop: opt_io_size = 0
                dev-prop: bootindex = -1
                dev-prop: discard_granularity = 0
                dev-prop: ver = "0.12.1"
                dev-prop: serial = "zxcvbnm1-520c-469b-90c2-663277e90438"
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                dev-prop: wwn = 0x0
                dev-prop: removable = off
                bus-prop: channel = 0
                bus-prop: scsi-id = 0
                bus-prop: lun = 0
In guest:
# scsi_id --whitelisted --device=/dev/sdc 
0QEMU    QEMU HARDDISK   zxcvbnm1-520c-469b-9


Conclusion:
----------
This issue has been fixed as per above.

Hi Gerd,
  The behaviour of serial is different between -drive option and -device option. The difference is that serial is truncated to 20 bytes from output of 'info qtree' if it gets specified in -drive, but this truncation doesn't happen in -device. Is this expected? 
  BTW, the common behaviour is both got truncated to 20 bytes in guest from output of scsi_id and hdparm.

(In reply to chayang from comment #9)
> Hi Gerd,
>   The behaviour of serial is different between -drive option and -device
> option. The difference is that serial is truncated to 20 bytes from output
> of 'info qtree' if it gets specified in -drive, but this truncation doesn't
> happen in -device. Is this expected? 
>   BTW, the common behaviour is both got truncated to 20 bytes in guest from
> output of scsi_id and hdparm.

I guess the scsi guest interface is limited to 20 bytes, and therefore -drive will limit the serial number to that length (or simply have 20 bytes storage and ignores anything longer).  Kevin?

The serial property for -device is simply declared as string property which is stored as-is, nobody cuts it down in length.  And specifically in case of usb-storage it also shows up in 'lsusb -v -s $device' in the guest, in full length, so I don't think we should apply limits here.

(In reply to Gerd Hoffmann from comment #10)
> I guess the scsi guest interface is limited to 20 bytes, and therefore
> -drive will limit the serial number to that length (or simply have 20 bytes
> storage and ignores anything longer).  Kevin?

The RHEL 6 SCSI code indeed limits to 20 characters when passing the
information, but I think the real limit for 'info qtree' may be
DriveInfo.serial, which is char[21].

Maybe scsi_initfn() should truncate the string so that the 'info qtree' output
is consistent with what the guest sees, but then, probably nobody cares and
I've already wasted more time with this than it's worth.


Upstream is doing different, but not less weird things with the serial number.
I don't really understand that code:

    const char *str = s->serial ?: bdrv_get_device_name(s->qdev.conf.bs);
    int max_len = s->serial ? 20 : 255 - 8;

So it seems to be explicitly limited to 20 bytes for the -device property, but
not if it gets the value from -drive (which is now unlimited in length in
upstream).

> Upstream is doing different, but not less weird things with the serial
> number.
> I don't really understand that code:
> 
>     const char *str = s->serial ?: bdrv_get_device_name(s->qdev.conf.bs);
>     int max_len = s->serial ? 20 : 255 - 8;
> 
> So it seems to be explicitly limited to 20 bytes for the -device property,
> but
> not if it gets the value from -drive (which is now unlimited in length in
> upstream).

I guess someone was overly careful when killing the 20 bytes limit for -drive.

Simply going for "max_len = 255 - 8" should be correct I think:  255 bytes == max mode page size, minus some space for fixed fields ...

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.

http://rhn.redhat.com/errata/RHSA-2013-1553.html