Bug 727896

Reproduced on qemu-kvm-0.12.1.2-2.160.el6_1.6 and verified pass on
qemu-kvm-0.12.1.2-2.160.el6_1.8.
Steps:
1. Boot guest A with
-chardev tty,id=serial0,path=/dev/ttyS0 -device isa-serial,chardev=serial0
Inside guest:
#udevadm info --query path --name /dev/ttyS0
#!! --attribute-walk | grep looking

2. Boot guest B with
-serial /dev/ttyS0
Inside guest:
#udevadm info --query path --name /dev/ttyS0
#!! --attribute-walk | grep looking

Results:
For old qemu-kvm version:
Guest A:
[root@virtlab-66-85-138 ~]# udevadm info --query path --name /dev/ttyS0
/devices/platform/serial8250/tty/ttyS0
[root@virtlab-66-85-138 ~]# !! --attribute-walk | grep looking
udevadm info --query path --name /dev/ttyS0 --attribute-walk | grep looking
  looking at device '/devices/platform/serial8250/tty/ttyS0':
  looking at parent device '/devices/platform/serial8250':
  looking at parent device '/devices/platform':

Guest B:
[root@virtlab-66-85-105 ~]#  udevadm info --query path --name /dev/ttyS0
/devices/pnp0/00:06/tty/ttyS0
[root@virtlab-66-85-105 ~]#  !! --attribute-walk | grep looking
  udevadm info --query path --name /dev/ttyS0 --attribute-walk | grep looking
  looking at device '/devices/pnp0/00:06/tty/ttyS0':
  looking at parent device '/devices/pnp0/00:06':
  looking at parent device '/devices/pnp0':

For fixed version:
Guest A and guest B are the same:
[root@virtlab-66-85-138 ~]# !! --attribute-walk | grep looking
udevadm info --query path --name /dev/ttyS0 --attribute-walk | grep looking
  looking at device '/devices/pnp0/00:06/tty/ttyS0':
  looking at parent device '/devices/pnp0/00:06':
  looking at parent device '/devices/pnp0':

So, this bug is fixed.

    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:
When KVM guests were configured with the "-serial" option or the "-device isa-serial" option, serial devices created were displayed incorrectly by the system and were not visible by Windows guests. This issue arose because such devices were described incorrectly in the Advanced Configuration and Power Interface (ACPI) Differentiated System Description Table (DSDT). To fix this issue, the code has been modified to report such serial devices in the PCI IDE ISA Xcelerator 4 (PIIX4) configuration space. The system now shows these serial devices correctly and such devices are now registered by ACPI DSDT so Windows systems can now see these devices properly.

Fixed and cleaned up the description; the bug only occurs with "-serial", not "-device isa-serial".  I omitted the expansion of ACPI, and avoided mentioning PIIX4 which is just an implementation detail of current versions of QEMU.  In general, if somebody really cares about how the fix works, they should know at least what ACPI is, even though perhaps not that the fix involves the DSDT.

    Technical note updated. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    Diffed Contents:
@@ -1 +1 @@
-When KVM guests were configured with the "-serial" option or the "-device isa-serial" option, serial devices created were displayed incorrectly by the system and were not visible by Windows guests. This issue arose because such devices were described incorrectly in the Advanced Configuration and Power Interface (ACPI) Differentiated System Description Table (DSDT). To fix this issue, the code has been modified to report such serial devices in the PCI IDE ISA Xcelerator 4 (PIIX4) configuration space. The system now shows these serial devices correctly and such devices are now registered by ACPI DSDT so Windows systems can now see these devices properly.+When KVM guests were launched with the "-device isa-serial" option instead of the "-serial" option (as libvirt does in Red Hat Enterprise Linux version 6), serial devices created were not visible by Windows guests. This issue arose because QEMU did not expose these devices in the guest's ACPI tables.  With the fix, the ACPI Differentiated System Description Table (DSDT) will properly sense the presence of serial devices and Windows guests will see them properly.

    Technical note updated. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    Diffed Contents:
@@ -1 +1 @@
-When KVM guests were launched with the "-device isa-serial" option instead of the "-serial" option (as libvirt does in Red Hat Enterprise Linux version 6), serial devices created were not visible by Windows guests. This issue arose because QEMU did not expose these devices in the guest's ACPI tables.  With the fix, the ACPI Differentiated System Description Table (DSDT) will properly sense the presence of serial devices and Windows guests will see them properly.+When KVM guests were launched with the "-device isa-serial" option instead of the "-serial" option, serial devices created were not visible by Windows guests. This issue arose because QEMU did not expose these devices in guests' ACPI tables.  With this fix, the guest's ACPI Differentiated System Description Table (DSDT) now properly senses the presence of serial devices and Windows guests can now see them properly.

I've done several style corrections and reflected the description to the related advisory.

    Technical note updated. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    Diffed Contents:
@@ -1 +1 @@
-When KVM guests were launched with the "-device isa-serial" option instead of the "-serial" option, serial devices created were not visible by Windows guests. This issue arose because QEMU did not expose these devices in guests' ACPI tables.  With this fix, the guest's ACPI Differentiated System Description Table (DSDT) now properly senses the presence of serial devices and Windows guests can now see them properly.+When KVM guests were launched with the "-device isa-serial" option instead of the "-serial" option, serial devices created were not visible by Windows guests. This issue arose because QEMU did not expose these devices in the guests' ACPI tables.  With this fix, the guest's ACPI Differentiated System Description Table (DSDT) now properly senses the presence of serial devices and Windows guests can now see them properly.

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/RHBA-2011-1211.html