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Eric,
I had assistance from portdirect in IRC who provided the 'multistring' solution to this problem.
This solution does not require a change on nova side, or a change to nova chart.  I should have replied a day ago.

The nova solution WIP you coded would work. It requires slight documentation change to remove the one-line-per-entry input limitation.

The following helm multistring method works for OSLO.conf compatible with oslo_config.MultiStringOpt().
I get correct nova.conf output if I individually JSON encode each string in the list of values (eg, for PCI alias, PCI passthrough whitelist).

Here is sample YAML for multistring :
conf:
  nova:
    pci:
      alias:
        type: multistring
        values:
          - '{"vendor_id": "8086", "product_id": "0435", "name": "qat-dh895xcc-pf"}'
          - '{"class_id": "030000", "name": "gpu"}'

Here is the resultant nova.conf :
[pci]
alias = {"vendor_id": "8086", "product_id": "0435", "name": "qat-dh895xcc-pf"}
alias = {"class_id": "030000", "name": "gpu"}

This solution does not require a change on nova side, or a change to nova helm chart.
IMO, I did not find the multistring example obvious when I was looking for documentation.

-Jim Gauld

-----Original Message-----
From: Eric Fried [mailto:openstack at fried.cc] 
Sent: February-06-19 10:45 AM
To: openstack-discuss at lists.openstack.org
Subject: Re: [openstack-helm] How to specify nova override for multiple pci alias

Folks-

On 2/6/19 4:13 AM, Jean-Philippe Evrard wrote:
> On Wed, 2019-01-30 at 15:40 +0000, Gauld, James wrote:
>> How can I specify a helm override to configure nova PCI alias when 
>> there are multiple aliases?
>> I haven't been able to come up with a YAML compliant specification 
>> for this.
>>
>> Are there other alternatives to be able to specify this as an 
>> override? I assume that a nova Chart change would be required to 
>> support this custom one-alias-entry-per-line formatting.
>>
>> Any insights on how to achieve this in helm are welcomed.

<snip>

>> The following nova configuration format is desired, but not as yet 
>> supported by nova:
>> [pci]
>> alias = [{dict 1}, {dict 2}]
>>
>> The following snippet of YAML works for PCI passthrough_whitelist, 
>> where the value encoded is a JSON string:
>>
>> conf:
>>   nova:
>>   overrides:
>>     nova_compute:
>>       hosts:
>>       - conf:
>>           nova:
>>             pci:
>>               passthrough_whitelist: '[{"class_id": "030000",
>> "address": "0000:00:02.0"}]'

<snip>

I played around with the code as it stands, and I agree there doesn't seem to be a way around having to specify the alias key multiple times to get multiple aliases. Lacking some fancy way to make YAML understand a dict with repeated keys ((how) do you handle HTTP headers?), I've hacked up a solution on the nova side [1] which should allow you to do what you've described above. Do you have a way to pull it down and try it?

(Caveat: I put this up as a proof of concept, but it (or anything that messes with the existing pci passthrough mechanisms) may not be a mergeable solution.)

-efried

[1] https://review.openstack.org/#/c/635191/