Public bug reported:

[ Impact ]

* The previous change made the ENA driver built-in to the kernel
  in order to mitigate a cloud-init early boot race condition on
  AWS instances (LP #2144694).

* This change introduced a regression for users who ship custom or
  upstream AWS ENA drivers via DKMS, as the built-in driver cannot 
  be overridden by out-of-tree modules.

* This SRU reverts ENA from built-in back to a loadable kernel
  module, restoring the expected module override behavior.

[ Test Plan ]

 * 1. Launch an AWS instance using a kernel with this revert applied.

 * 2. Verify ENA is built as a loadable module: 
      "grep CONFIG_ENA /boot/config-$(uname -r)" 
      Expected result: "CONFIG_ENA_ETHERNET=m".

[ Where problems could occur ]

 * Reverting ENA to a module may reintroduce or increase exposure to
   a known timing issue where cloud-init local runs before NIC 
   initialization completes. This can affect datasource detection timing
   on some AWS instance types (e.g. notably hpc7a.*), potentially delaying
   metadata availability during first boot (LP #2144694).

[ Other Info ]

 * This change is the result of a regression introduced by LP #2144694.
   https://bugs.launchpad.net/ubuntu/+source/linux-aws/+bug/2144694

** Affects: linux-aws (Ubuntu)
     Importance: High
     Assignee: Leah Goldberg (goldberl)
         Status: New

** Affects: linux-aws (Ubuntu Noble)
     Importance: High
     Assignee: Leah Goldberg (goldberl)
         Status: New

** Affects: linux-aws (Ubuntu Questing)
     Importance: High
     Assignee: Leah Goldberg (goldberl)
         Status: New

** Affects: linux-aws (Ubuntu Resolute)
     Importance: High
     Assignee: Leah Goldberg (goldberl)
         Status: New

** Changed in: linux-aws (Ubuntu)
     Assignee: (unassigned) => Leah Goldberg (goldberl)

** Also affects: linux-aws (Ubuntu Questing)
   Importance: Undecided
       Status: New

** Also affects: linux-aws (Ubuntu Resolute)
   Importance: Undecided
       Status: New

** Also affects: linux-aws (Ubuntu Noble)
   Importance: Undecided
       Status: New

** Changed in: linux-aws (Ubuntu Noble)
     Assignee: (unassigned) => Leah Goldberg (goldberl)

** Changed in: linux-aws (Ubuntu Questing)
     Assignee: (unassigned) => Leah Goldberg (goldberl)

** Changed in: linux-aws (Ubuntu Resolute)
     Assignee: (unassigned) => Leah Goldberg (goldberl)

** Changed in: linux-aws (Ubuntu)
   Importance: Undecided => High

** Changed in: linux-aws (Ubuntu Noble)
   Importance: Undecided => High

** Changed in: linux-aws (Ubuntu Questing)
   Importance: Undecided => High

** Changed in: linux-aws (Ubuntu Resolute)
   Importance: Undecided => High

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https://bugs.launchpad.net/bugs/2156924

Title:
  ENA built-in config prevents DKMS ENA driver override

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