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

[Michael S. Tsirkin]

On Thu, Jul 05, 2018 at 01:52:31PM +0100, Richard W.M. Jones wrote:
> 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.
> 
> Rich.

What do others do, and what is, in your opinion, the correct behaviour here?


> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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