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The proposal from the spec for this feature was to add an image property 
(hw_unique_serial), flavor extra spec (hw:unique_serial) and new 
"unique" choice to the [libvirt]/sysinfo_serial config option. The image 
property and extra spec would be booleans but really only True values 
make sense and False would be more or less ignored. There were no plans 
to enforce strict checking of a boolean value, e.g. if the image 
property was True but the flavor extra spec was False, we would not 
raise an exception for incompatible values, we would just use OR logic 
and take the image property True value.

The boolean usage proposed is a bit confusing, as can be seen from 
comments in the spec [1] and the proposed code change [2].

After thinking about this a bit, I'm now thinking maybe we should just 
use a single-value enum for the image property and flavor extra spec:

image: hw_guest_serial=unique
flavor: hw:guest_serial=unique

If either are set, then we use a unique serial number for the guest. If 
neither are set, then the serial number is based on the host 
configuration as it is today.

I think that's more clear usage, do others agree? Alex does. I can't 
think of any cases where users would want hw_unique_serial=False, so 
this removes that ability and confusion over whether or not to enforce 
mismatching booleans.

[1] 
https://review.openstack.org/#/c/612531/2/specs/stein/approved/per-instance-libvirt-sysinfo-serial.rst@43
[2] 
https://review.openstack.org/#/c/619953/7/nova/virt/libvirt/driver.py@4894

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Thanks,

Matt