NetworkPkg/SnpDxe: Shutdown SnpDxe on BeforeExitBootServices Event. [Rebase & FF]

[apop5]

Description

SnpDxe registers for an ExitBootServices callback and runs the PXE_OPCODE_SHUTDOWN and PXE_OPCODE_STOP commands for any network controllers that the driver is attached to. If the Bus Master Enable bit is cleared by PciBusDxe before this point, then completions returned by the network cards will be unreadable to their drivers during the shutdown sequence.

Register the SnpDxe callback for gEfiEventBeforeExitBootServicesGuid instead of gEfiEventExitBootServicesGuid to ensure the correct ordering:

1. ExitBootServices event
2. Network card shutdown sequence is completed
3. BME is disabled on PCI bridges (if configured)

Also, close the event to prevent rerunning the shutdown if multiple ExitBootServices events need to be called by the OS.

Fixes https://github.com/microsoft/mu_basecore/issues/520

• [ X] Impacts functionality?
  • Functionality - Does the change ultimately impact how firmware functions?
  • Examples: Add a new library, publish a new PPI, update an algorithm, ...
• [ ] Impacts security?
  • Security - Does the change have a direct security impact on an application, flow, or firmware?
  • Examples: Crypto algorithm change, buffer overflow fix, parameter validation improvement, ...
• [ ] Breaking change?
  • Breaking change - Will anyone consuming this change experience a break in build or boot behavior?
  • Examples: Add a new library class, move a module to a different repo, call a function in a new library class in a pre-existing module, ...
• [ ] Includes tests?
  • Tests - Does the change include any explicit test code?
  • Examples: Unit tests, integration tests, robot tests, ...
• [ ] Includes documentation?
  • Documentation - Does the change contain explicit documentation additions outside direct code modifications (and comments)?
  • Examples: Update readme file, add feature readme file, link to documentation on an a separate Web page, ...

How This Was Tested

Tested on AARCH64 platform with dual network cards.

Integration Instructions

N/A

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Codecov Report

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Additional details and impacted files

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