ArmVirtPkg: Define variables for emulating runtime variables

Kvmtool allows guest VMs to be launched with or without a
CFI flash device.

When the kvmtool option '--flash <flash filename>' is used to
launch a guest VM a CFI flash device maps the flash file that
was specified at the command line. The NorFlash driver uses
this flash as the variable storage backend.

However, when the above option is not specified, a CFI flash
device is not present. In such cases, the firmware can fallback
to use emulated runtime variables (which uses the VMs DRAM as
the storage backend).

Therefore, define the PCD PcdEmuVariableNvModeEnable required
to enable the emulated runtime variable support, but do not
enable it by default.

The firmware is expected to dynamically discover if the CFI
flash is present and subsequently enable NorFlash or emulate
the runtime variables.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

ArmVirtPkg/ArmVirtKvmTool.dsc

@@ -1,7 +1,7 @@
 #  @file
 #  Workspace file for KVMTool virtual platform.
 #
-#  Copyright (c) 2018 - 2022, ARM Limited. All rights reserved.
+#  Copyright (c) 2018 - 2023, Arm Limited. All rights reserved.
 #
 #  SPDX-License-Identifier: BSD-2-Clause-Patent
 #
@@ -219,6 +219,10 @@
   gEfiMdeModulePkgTokenSpaceGuid.PcdFlashNvStorageFtwSpareBase|0
   gEfiMdeModulePkgTokenSpaceGuid.PcdFlashNvStorageFtwSpareSize|0x40000
 
+  # Define PCD for emulating Runtime Variable storage when
+  # CFI flash is absent.
+  gEfiMdeModulePkgTokenSpaceGuid.PcdEmuVariableNvModeEnable|FALSE
+
   ## RTC Register address in MMIO space.
   gPcAtChipsetPkgTokenSpaceGuid.PcdRtcIndexRegister64|0x0
   gPcAtChipsetPkgTokenSpaceGuid.PcdRtcTargetRegister64|0x0