On Thu, Jul 05, 2018 at 04:54:16PM +0300, Michael S. Tsirkin wrote: > 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. > > What do others do, and what is, in your opinion, the correct behaviour here?
I don't want to claim any particular behaviour is correct. However ... VMware represents these internally as two signed 64 bit integers, eg: vm.genid = "-570734802784577186" vm.genidx = "-5042519231342505152" I am still trying to get verification, but I believe the first is the low 64 bit word and the second is the high 64 bit word. Hyper-V's implementation is completely opaque, but their documentation claims that the VM Generation ID is "a 128-bit, cryptographically random integer value identifier" whatever that means. As described in the link above, Microsoft provides some example code for reading the VM Gen ID from a Windows guest. In this code it is clearly represented as a pair of unsigned 64 bit integers. Windows >= 2012 R2 provides a device driver which has an API which also reads a struct containing two unsigned 64 bit ints. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org
