OvmfPkg/PlatformPei: detect SMRAM at default SMBASE (skeleton)

Introduce the Q35SmramAtDefaultSmbaseInitialization() function for
detecting the "SMRAM at default SMBASE" feature.

For now, the function is only a skeleton, so that we can gradually build
upon the result while the result is hard-coded as FALSE. The actual
detection will occur in a later patch.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1512
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Message-Id: <20200129214412.2361-6-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

OvmfPkg/PlatformPei/MemDetect.c

@@ -43,6 +43,8 @@ STATIC UINT32 mS3AcpiReservedMemorySize;
 
 STATIC UINT16 mQ35TsegMbytes;
 
+BOOLEAN mQ35SmramAtDefaultSmbase;
+
 UINT32 mQemuUc32Base;
 
 VOID
@@ -90,6 +92,22 @@ Q35TsegMbytesInitialization (
 }
 
 
+VOID
+Q35SmramAtDefaultSmbaseInitialization (
+  VOID
+  )
+{
+  RETURN_STATUS PcdStatus;
+
+  ASSERT (mHostBridgeDevId == INTEL_Q35_MCH_DEVICE_ID);
+
+  mQ35SmramAtDefaultSmbase = FALSE;
+  PcdStatus = PcdSetBoolS (PcdQ35SmramAtDefaultSmbase,
+                mQ35SmramAtDefaultSmbase);
+  ASSERT_RETURN_ERROR (PcdStatus);
+}
+
+
 VOID
 QemuUc32BaseInitialization (
   VOID