OvmfPkg: add PcdQemuVarsRequire

Add PcdQemuVarsRequire FeaturePcd, so firmware code can figure whenever
the given build is supposed to use the qemu uefi variable service.

Skip the emulated variable store setup in case PcdQemuVarsRequire is
true.  This is needed to make secure boot work.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

OvmfPkg/OvmfPkg.dec

@@ -464,3 +464,6 @@
   #  framebuffer. This might be required on platforms that do not tolerate
   #  misaligned accesses otherwise.
   gUefiOvmfPkgTokenSpaceGuid.PcdRemapFrameBufferWriteCombine|FALSE|BOOLEAN|0x75
+
+  ## This feature flag indicates the firmware build needs the qemu variable service.
+  gUefiOvmfPkgTokenSpaceGuid.PcdQemuVarsRequire|FALSE|BOOLEAN|0x77

OvmfPkg/OvmfPkgX64.dsc

@@ -502,6 +502,7 @@
   gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache|FALSE
 !endif
 !if $(QEMU_PV_VARS) == TRUE
+  gUefiOvmfPkgTokenSpaceGuid.PcdQemuVarsRequire|TRUE
   gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache|FALSE
 !endif
 !if $(SECURE_BOOT_ENABLE) == TRUE

OvmfPkg/PlatformPei/Platform.c

@@ -221,6 +221,10 @@ ReserveEmuVariableNvStore (
   EFI_PHYSICAL_ADDRESS  VariableStore;
   RETURN_STATUS         PcdStatus;
 
+  if (FeaturePcdGet (PcdQemuVarsRequire)) {
+    return;
+  }
+
   VariableStore = (EFI_PHYSICAL_ADDRESS)(UINTN)PlatformReserveEmuVariableNvStore ();
   PcdStatus     = PcdSet64S (PcdEmuVariableNvStoreReserved, VariableStore);
 

OvmfPkg/PlatformPei/PlatformPei.inf

@@ -143,6 +143,7 @@
 
 [FeaturePcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
+  gUefiOvmfPkgTokenSpaceGuid.PcdQemuVarsRequire
 
 [Ppis]
   gEfiPeiMasterBootModePpiGuid