PCD Migration issue

["Andrew Fish" <afish@apple.com>]

We recently hit an issue when updating our UDK version in our common code base. Our recovery update path started failing for older platforms since the PEI was the old UDK version and the DXE from the Capsule was the new UDK version. Basically the version of the PCD changed. It looks like there are some checks, but the seem to be more about making sure the build systems matches the code vs. dealing with an update case like we hit. 

It does not look like the PCD database is designed to deal with this. We are fixing this by extracting the PEI PCD database from the capsule and then having some platform specific code to patch any PCD entries that got set in PEI that are needed by DXE. 

In the future it would be helpful if the PCD database would change in some what of a backward compatible way, and have the headers needed to parse the old and new version. 

It also seems like adding a Dynamic PCD could potentially change the token layout and thus break compatibility? Is there any scheme to keep a previous token layout? 

Thanks,

Andrew Fish