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

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] Byte ordering of VM Generation ID in Windows VMs
Date: Thu, 5 Jul 2018 13:52:31 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Jul 05, 2018 at 01:39:29PM +0100, Richard W.M. Jones wrote:
> 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.

OK after examining util/uuid.c and the qemu_uuid_bswap function,
I sort of see what's going on here.

FWIW other hypervisors seem to store these as two 64 bit integers.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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