[PATCH 1/2] OvmfPkg: Introduce NULL class library to inhibit driver load

["Ard Biesheuvel" <ardb@kernel.org>]

Add a new library that can be incorporated into any driver built from
source, and which permits loading of the driver to be inhibited based on
the value of a QEMU fw_cfg boolean variable. This will be used in a
subsequent patch to allow dispatch of the IPv6 and IPv6 network protocol
driver to be controlled from the QEMU command line.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 OvmfPkg/Library/DriverLoadInhibitorLib/DriverLoadInhibitorLib.c   | 30 ++++++++++++++++++++
 OvmfPkg/Library/DriverLoadInhibitorLib/DriverLoadInhibitorLib.inf | 28 ++++++++++++++++++
 OvmfPkg/OvmfPkg.dec                                               |  4 +++
 3 files changed, 62 insertions(+)

diff --git a/OvmfPkg/Library/DriverLoadInhibitorLib/DriverLoadInhibitorLib.c b/OvmfPkg/Library/DriverLoadInhibitorLib/DriverLoadInhibitorLib.c
new file mode 100644
index 000000000000..dc8544bc38be
--- /dev/null
+++ b/OvmfPkg/Library/DriverLoadInhibitorLib/DriverLoadInhibitorLib.c
@@ -0,0 +1,30 @@
+// @file
+// Copyright (c) 2022, Google LLC. All rights reserved.<BR>
+// SPDX-License-Identifier: BSD-2-Clause-Patent
+//
+
+#include <PiDxe.h>
+
+#include <Library/QemuFwCfgSimpleParserLib.h>
+#include <Library/UefiBootServicesTableLib.h>
+
+STATIC CHAR16 mExitData[] = L"Driver dispatch inhibited by QEMU fw_cfg variable.";
+
+EFI_STATUS
+EFIAPI
+DriverLoadInhibitorLibConstructor (
+  IN  EFI_HANDLE        Handle,
+  IN  EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  RETURN_STATUS     Status;
+  BOOLEAN           Enabled;
+
+  Status = QemuFwCfgParseBool (FixedPcdGetPtr (PcdDriverInhibitorFwCfgVarName),
+             &Enabled);
+  if (!RETURN_ERROR (Status) && !Enabled) {
+    return gBS->Exit (Handle, EFI_REQUEST_UNLOAD_IMAGE, sizeof mExitData,
+                  mExitData);
+  }
+  return EFI_SUCCESS;
+}
diff --git a/OvmfPkg/Library/DriverLoadInhibitorLib/DriverLoadInhibitorLib.inf b/OvmfPkg/Library/DriverLoadInhibitorLib/DriverLoadInhibitorLib.inf
new file mode 100644
index 000000000000..ed521d12d335
--- /dev/null
+++ b/OvmfPkg/Library/DriverLoadInhibitorLib/DriverLoadInhibitorLib.inf
@@ -0,0 +1,28 @@
+## @file
+#  Copyright (c) 2022, Google LLC. All rights reserved.<BR>
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 1.29
+  BASE_NAME                      = DriverLoadInhibitorLib
+  FILE_GUID                      = af4c2c0b-f7ed-4d61-ad97-5953982c3531
+  MODULE_TYPE                    = DXE_DRIVER
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = NULL
+  CONSTRUCTOR                    = DriverLoadInhibitorLibConstructor
+
+[Sources]
+  DriverLoadInhibitorLib.c
+
+[LibraryClasses]
+  QemuFwCfgSimpleParserLib
+  UefiBootServicesTableLib
+
+[Packages]
+  MdePkg/MdePkg.dec
+  OvmfPkg/OvmfPkg.dec
+
+[FixedPcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdDriverInhibitorFwCfgVarName
diff --git a/OvmfPkg/OvmfPkg.dec b/OvmfPkg/OvmfPkg.dec
index 5af76a540529..e9a22cab088c 100644
--- a/OvmfPkg/OvmfPkg.dec
+++ b/OvmfPkg/OvmfPkg.dec
@@ -399,6 +399,10 @@ [PcdsFixedAtBuild]
   ## The Tdx accept page size. 0x1000(4k),0x200000(2M)
   gUefiOvmfPkgTokenSpaceGuid.PcdTdxAcceptPageSize|0x200000|UINT32|0x65
 
+  ## The QEMU fw_cfg variable that DriverLoadInhibitorLib will check to
+  #  decide whether to abort dispatch of the driver it is linked into.
+  gUefiOvmfPkgTokenSpaceGuid.PcdDriverInhibitorFwCfgVarName|""|VOID*|0x68
+
 [PcdsDynamic, PcdsDynamicEx]
   gUefiOvmfPkgTokenSpaceGuid.PcdEmuVariableEvent|0|UINT64|2
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfFlashVariablesEnable|FALSE|BOOLEAN|0x10
-- 
2.35.1