OvmfPkg/AmdSev: Expose the Sev Secret area using a configuration table

Now that the secret area is protected by a boot time HOB, extract its
location details into a configuration table referenced by
gSevLaunchSecretGuid so the boot loader or OS can locate it before a
call to ExitBootServices().

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3077
Signed-off-by: James Bottomley <jejb@linux.ibm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201130202819.3910-7-jejb@linux.ibm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
[lersek@redhat.com: fix indentation of InstallConfigurationTable() args]

OvmfPkg/AmdSev/SecretDxe/SecretDxe.c

@@ -0,0 +1,27 @@
+/** @file
+  SEV Secret configuration table constructor
+
+  Copyright (C) 2020 James Bottomley, IBM Corporation.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+#include <PiDxe.h>
+#include <Library/UefiBootServicesTableLib.h>
+#include <Guid/SevLaunchSecret.h>
+
+STATIC SEV_LAUNCH_SECRET_LOCATION mSecretDxeTable = {
+  FixedPcdGet32 (PcdSevLaunchSecretBase),
+  FixedPcdGet32 (PcdSevLaunchSecretSize),
+};
+
+EFI_STATUS
+EFIAPI
+InitializeSecretDxe(
+  IN EFI_HANDLE           ImageHandle,
+  IN EFI_SYSTEM_TABLE     *SystemTable
+  )
+{
+  return gBS->InstallConfigurationTable (
+                &gSevLaunchSecretGuid,
+                &mSecretDxeTable
+                );
+}