MdePkg/DxeServicesLib: introduce AllocatePeiAccessiblePages routine

Add a routine to DxeServicesLib that abstracts the allocation of memory
that should be accessible by PEI after resuming from S3. We will use it
to replace open coded implementations that limit the address to < 4 GB,
which may not be possible on non-Intel systems that have no 32-bit
addressable memory at all.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Include/Library/DxeServicesLib.h

@@ -305,5 +305,26 @@ GetFileDevicePathFromAnyFv (
   OUT       EFI_DEVICE_PATH_PROTOCOL  **FvFileDevicePath
   );
 
-#endif
+/**
+  Allocates one or more 4KB pages of a given type from a memory region that is
+  accessible to PEI.
 
+  Allocates the number of 4KB pages of type 'MemoryType' and returns a
+  pointer to the allocated buffer.  The buffer returned is aligned on a 4KB
+  boundary.  If Pages is 0, then NULL is returned.  If there is not enough
+  memory remaining to satisfy the request, then NULL is returned.
+
+  @param[in]  MemoryType            The memory type to allocate
+  @param[in]  Pages                 The number of 4 KB pages to allocate.
+
+  @return A pointer to the allocated buffer or NULL if allocation fails.
+
+**/
+VOID *
+EFIAPI
+AllocatePeiAccessiblePages (
+  IN EFI_MEMORY_TYPE  MemoryType,
+  IN UINTN            Pages
+  );
+
+#endif