Create ARM platform library function for customizing PEI memory (#63)

## Description

Creates a new function in the ARM platform library to allow the platform
to customize the location of the initial PEI memory installation region.

- [ ] 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, ...
- [x] 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 against the SBSA platform with custom memory initialization
logic.

## Integration Instructions

Platforms with their own arm platform package will need to add
implementation of ArmPlatformGetPeiMemory that returns FALSE to be
unaffected.

ArmPlatformPkg/Include/Library/ArmPlatformLib.h

@@ -136,4 +136,24 @@ ArmPlatformGetPlatformPpiList (
   OUT EFI_PEI_PPI_DESCRIPTOR  **PpiList
   );
 
+// MU_CHANGE START: Allow platform to customize initial memory region.
+
+/**
+  Checks if the platform requires a special initial EFI memory region.
+
+  @param[out]  EfiMemoryBase  The custom memory base, will be unchanged if FALSE is returned.
+  @param[out]  EfiMemorySize  The custom memory size, will be unchanged if FALSE is returned.
+
+  @retval   TRUE    A custom memory region was set.
+  @retval   FALSE   A custom memory region was not set.
+**/
+BOOLEAN
+EFIAPI
+ArmPlatformGetPeiMemory (
+  OUT UINTN   *EfiMemoryBase,
+  OUT UINT32  *EfiMemorySize
+  );
+
+// MU_CHANGE END
+
 #endif

ArmPlatformPkg/Library/ArmPlatformLibNull/ArmPlatformLibNullMem.c

@@ -26,3 +26,26 @@ ArmPlatformGetVirtualMemoryMap (
 {
   ASSERT (0);
 }
+
+// MU_CHANGE START
+
+/**
+  Checks if the platform requires a special initial EFI memory region.
+
+  @param[out]  EfiMemoryBase  The custom memory base, will be unchanged if FALSE is returned.
+  @param[out]  EfiMemorySize  The custom memory size, will be unchanged if FALSE is returned.
+
+  @retval   TRUE    A custom memory region was set.
+  @retval   FALSE   A custom memory region was not set.
+**/
+BOOLEAN
+EFIAPI
+ArmPlatformGetPeiMemory (
+  OUT UINTN   *EfiMemoryBase,
+  OUT UINT32  *EfiMemorySize
+  )
+{
+  return FALSE;
+}
+
+// MU_CHANGE END

ArmPlatformPkg/MemoryInitPei/MemoryInitPeim.c

@@ -94,6 +94,7 @@ InitializeMemory (
   UINTN       FdBase;
   UINTN       FdTop;
   UINTN       UefiMemoryBase;
+  UINT32      UefiMemorySize; // MU_CHANGE
 
   DEBUG ((DEBUG_LOAD | DEBUG_INFO, "Memory Init PEIM Loaded\n"));
 
@@ -114,8 +115,16 @@ InitializeMemory (
   // Declare the UEFI memory to PEI
   //
 
-  // In case the firmware has been shadowed in the System Memory
-  if ((FdBase >= SystemMemoryBase) && (FdTop <= SystemMemoryTop)) {
+  // MU_CHANGE START: Allow platform to customize initial memory region.
+  UefiMemorySize = FixedPcdGet32 (PcdSystemMemoryUefiRegionSize);
+  if (ArmPlatformGetPeiMemory (&UefiMemoryBase, &UefiMemorySize)) {
+    // Check the Firmware does not intersect with the provided memory region.
+    ASSERT ((FdBase < UefiMemoryBase) || (FdBase >= (UefiMemoryBase + UefiMemorySize)));
+    ASSERT ((FdTop <= UefiMemoryBase) || (FdTop > (UefiMemoryBase + UefiMemorySize)));
+  } else if ((FdBase >= SystemMemoryBase) && (FdTop <= SystemMemoryTop)) {
+    // In case the firmware has been shadowed in the System Memory
+    // MU_CHANGE END
+
     // Check if there is enough space between the top of the system memory and the top of the
     // firmware to place the UEFI memory (for PEI & DXE phases)
     if (SystemMemoryTop - FdTop >= FixedPcdGet32 (PcdSystemMemoryUefiRegionSize)) {
@@ -134,7 +143,7 @@ InitializeMemory (
     UefiMemoryBase = SystemMemoryTop - FixedPcdGet32 (PcdSystemMemoryUefiRegionSize);
   }
 
-  Status = PeiServicesInstallPeiMemory (UefiMemoryBase, FixedPcdGet32 (PcdSystemMemoryUefiRegionSize));
+  Status = PeiServicesInstallPeiMemory (UefiMemoryBase, UefiMemorySize); // MU_CHANGE: Allow platform to customize initial memory region.
   ASSERT_EFI_ERROR (Status);
 
   // Initialize MMU and Memory HOBs (Resource Descriptor HOBs)