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.
Register the SnpDxe callback for gEfiEventBeforeExitBootServicesGuid
instead of gEfiEventExitBootServicesGuid to ensure the correct ordering:
1. ExitBootServices event
2. Network card shutdown sequence is completed
Also, close the event to prevent rerunning the shutdown if multiple
ExitBootServices events need to be called by the OS.
Signed-off-by: Aaron Pop <aaronpop@microsoft.com>
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