[Qemu-devel] Byte ordering of VM Generation ID in Windows VMs

[Richard W.M. Jones]

I was doing a bit of investigation around how different hypervisors
handle the VM Generation ID feature.  QEMU's behaviour seems quite
strange, I wonder if this is a bug or expected?

(1) I booted a Windows 2016 VM with:

  qemu-system-x86_64 -M pc,accel=kvm -m 2G -hda w2k16-mincore.img \
      -device vmgenid,guid=01020304-0506-0708-090a-0b0c0d0e0f00,id=vmgenid0

(2) Inside the guest I used the VMGENID.EXE program from:

  https://bugzilla.redhat.com/show_bug.cgi?id=1598350#c3
  
https://docs.microsoft.com/en-gb/windows/desktop/HyperV_v2/virtual-machine-generation-identifier

Note this is self-compiled using mingw64-g++ (not using Visual Studio
which I don't have available), but I don't believe that could have
caused the problem.

(3) The program prints:

  VmCounterValue: 708050601020304:f0e0d0c0b0a09

To make it easier to see, this is the same number but zero-extended:

  VmCounterValue: 07 08 05 06 01 02 03 04 : 00 0f 0e 0d 0c 0b 0a 09
                  \________ LOW ________/   \_______ HIGH _______/
                            WORD                     WORD

As you can see it looks like there is no clear relationship between
the order of the bytes in the guid= parameter and the order that they
are seen by Windows.

BTW if you want to try to reproduce this you will need to use
Windows 2012 R2 or above.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/