[vbox-dev] [PATCH] Fix for Guest "OS type" future-compatibility

[Klaus Espenlaub]

Hi Alexey,

On 08.05.2015 00:08, Alexey Eromenko wrote:
> Hi,
>
> Today if I revert to older VirtualBox (to 4.3.20) from newer (v5.0
> BETA), I lose access to certain VMs, such as "Windows 10", because it
> has a new guest OS type ID.
>
> Those VMs become "inaccessible" for no good reason. I can understand a
> VM becoming "inaccessible" if hard disk new format is coming or a disk
> image is lacking, but Guest OS type ?

Yes, it's intentional. The reason is that VirtualBox uses it for certain 
small detail "tweaks" - like for Windows 8.1 it enables the CMPXCHG16B 
instruction support.

> My patch allows to recognize and start VMs with future guest OS type
> IDs. They will revert to "unknown" OS type.

See above. Possible loss of significant information, which might make 
some VMs fail to run in the new version after upgrading again. That's 
why we opted for making the VMs inaccessible.

We're running into such issues ourselves, and we keep re-evaluating out 
options, but so far we decided that the risk outweighs the benefits of 
being more generous with new configs in old versions.

Another way to look at it: predicting the effects of such "config 
adjustments" would need a time machine: this code needs to be in the 
"old" version, which means that it would be a promise that we will never 
change the underlying assumption.

> This is particularly useful in combination with "vbox-unattended"
> patch, that adds new OS types every now and then, but also for Oracle
> VirtualBox itself.
>
> I have tested it together with VirtualBox GUI and with VBoxManage commands.
> Plus tested OVF, Import / Export appliance.

Klaus