[PATCH 0/1] vmx: Fix <genid/> mapping

[Laszlo Ersek]

On 09/30/21 09:33, Richard W.M. Jones wrote:
> 
> More data: I found a colleague who has a Hyper-V instance with a
> Windows guest and he helped me to understand how Hyper-V represents
> generation IDs.  Hyper-V has had support for generation IDs since long
> before Microsoft proposed the feature for standardization.  Originally
> (I think pre-2013) Hyper-V used an XML description which included:
> 
> <generation_id type="string">4a07df4361fdf4c883f97bc30e524b9d</generation_id>
> 
> Note this is a pure hex string, not a GUID.
> 
> In current Hyper-V, the same representation is used but it's embedded
> in a binary file.
> 
> My colleague ran two Windows VMs on Hyper-V and examined the
> generation_id on the hypervisor and compared it to the output of
> VMGENID.EXE inside the guest.
> 
> The first guest had this in the binary hypervisor metadata:
> 
> 000056b0  00 00 0e 67 65 6e 65 72  61 74 69 6f 6e 5f 69 64  |...generation_id|
> 000056c0  00 40 00 00 00 38 00 30  00 35 00 61 00 32 00 38  |. at ...8.0.5.a.2.8|
> 000056d0  00 37 00 61 00 32 00 35  00 30 00 39 00 38 00 39  |.7.a.2.5.0.9.8.9|
> 000056e0  00 65 00 34 00 66 00 65  00 36 00 66 00 36 00 39  |.e.4.f.e.6.f.6.9|
> 000056f0  00 39 00 32 00 62 00 65  00 33 00 33 00 34 00 61  |.9.2.b.e.3.3.4.a|
> 00005700  00 34 00 33 00 00 00 00  00 00 00 00 00 00 00 00  |.4.3............|
> 
> and reported the output of VMGENID in this guest as:
> 
> VmCounterValue: fe6f6992be334a43:805a287a250989e4
> 
> The second guest had:
> 
> 00005360  c5 06 00 00 00 0e 67 65  6e 65 72 61 74 69 6f 6e  |......generation|
> 00005370  5f 69 64 00 40 00 00 00  65 00 62 00 66 00 62 00  |_id. at ...e.b.f.b.|
> 00005380  62 00 37 00 39 00 37 00  33 00 36 00 35 00 37 00  |b.7.9.7.3.6.5.7.|
> 00005390  32 00 38 00 65 00 37 00  30 00 62 00 33 00 66 00  |2.8.e.7.0.b.3.f.|
> 000053a0  64 00 33 00 63 00 37 00  31 00 33 00 65 00 63 00  |d.3.c.7.1.3.e.c.|
> 000053b0  65 00 38 00 34 00 32 00  00 00 00 00 00 00 00 00  |e.8.4.2.........|
> 
> and VMGENID was:
> 
> VmCounterValue: b3fd3c713ece842:ebfbb797365728e7
> 
> Note:
> 
>  - In both cases, the generation ID is clearly not a GUID.
> 
>  - The two 64 bit words are swapped over somewhere, but individual
>    bytes are not byte-swapped, and there is again no evidence of
>    UUID-aware byte swapping.
> 
> ----------------------------------------------------------------------
> 
> So how do we get from where we are to a more sane vmgenid in qemu?
> 
> I propose we deprecate the guid parameter in:
> 
>   -device vmgenid,guid=8987940a-0951-2cc5-e815-10634ff550b9,id=vmgenid0
> 
> Instead it will be replaced by bytes= which will simply write
> the bytes, in the order they appear, into guest memory with no
> attempt to interpret or byte-swap.  Something like:
> 
>   -device vmgenid,bytes=112233445566778899aabbccddeeff00,id=vmgenid0
> 
> (guid although deprecated will need to be kept around for a while,
> along with its weird byte-swapping behaviour).
> 
> We will then have a plain and simple method to emulate the behaviour
> of other hypervisors.  We will look at exactly what bytes they write
> to guest memory and copy that behaviour when v2v converting from those
> hypervisors.

I don't have anything against this, I just don't know who's going to
implement the QEMU change.

Thanks
Laszlo