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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 21:59:24 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Jul 05, 2018 at 08:10:15PM +0300, Michael S. Tsirkin wrote:
> On Thu, Jul 05, 2018 at 05:43:43PM +0100, Richard W.M. Jones wrote:
> > On Thu, Jul 05, 2018 at 04:20:33PM +0200, Laszlo Ersek wrote:
> > > QEMU does the right thing. If other hypervisors don't do this -- while
> > > still taking and displaying the value in UUID / GUID textual format --,
> > > they are wrong. The VMGENID spec from Microsoft
> > > <http://go.microsoft.com/fwlink/?LinkId=260709> specifically mentions
> > > "GUID".
> > 
> > The MSFT spec does mention GUID, but it seems to me that it's only
> > using GUID as an incidental example -- ie. that you might use the VM
> > Generation ID to generate a GUID.  Outside that example it
> > consistently refers to the VM Gen ID as a 128-bit integer.  It also
> > says that it could be used as a "high entropy random data source",
> > which is not in fact true if it's a UUID.
> > 
> > It has to be said that after reading the spec again [the MSFT spec,
> > not qemu's spec] and what other hypervisors are doing, I'm not sure
> > qemu is doing the right thing here.
> > 
> > Rich.
> So right now we have a "GUID" property.
> We could always add an alternative property for people who want
> to treat the ID as an integer. Would that address the issue?

TBH I'm fairly relaxed about this and I don't want to put in any
effort to change anything.  As Dan says later in the thread, no data
is ever lost in the transformation.

I have now written a "128 bit int" -> "GUID" mangler for virt-v2v's
VM Generation ID handling code, so I'm happy enough.


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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