EmulatorPkg: Move magic page to first allocation

[mdkinney]

Description

Fixes #10652

The magic page is allocated from a fixed address specified by PcdPeiServicesTablePage. This allocation has been observed to sometimes fail. Move the allocation of this buffer to the very beginning of main() to minimize the chance that another allocation is allocated from the PcdPeiServicesTablePage address.

• Breaking change?
• Impacts security?
• Includes tests?

How This Was Tested

The issue was reproduced with a build of EmulatorPkg VS2022 IA32 RELEASE. In order to minimize the time required to boot cycle the EmulatorPkg build, the boot manager front page timeout is set to 0 and the Shell default delay is also set to 0.

build -a IA32 -t VS2022 -p EmulatorPkg\EmulatorPkg.dsc -b RELEASE --pcd gEfiMdePkgTokenSpaceGuid.PcdPlatformBootTimeOut=0 --pcd gEfiShellPkgTokenSpaceGuid.PcdShellDefaultDelay=0

A cmd script was then used to boot the EmulatorPkg in a loop and log the results to a file.

@echo off
:loop
WinHost.exe
goto loop

A startup.nsh file is used to automatically reset the EmulatorPkg when the boot to Shell completes

BOOT SUCCESS !!!
reset

Within 10 to 20 boot cycles the allocation of the magic page fails.

With the fix in this PR that moves the allocation of the magic page to very beginning of main() the issue was not reproduced in over 11000 boot cycles of the EmulatorPkg.

Integration Instructions

N/A

[niruiyu]

Mike, Thanks for fixing this.
I sometimes met this issue and just cannot boot. Then I have to change the PcdPeiServicesTablePage value to workaround it.