[PATCH v3 3/3] OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Restore C-bit when SEV is active

[Brijesh Singh <brijesh.singh@amd.com>]

AmdSevDxe maps the flash memory range with C=0, but
SetMemorySpaceAttributes() unconditionally resets the C-bit to '1'. Lets
restore the mapping back to C=0.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien.grall@linaro.org>
Cc: Justen Jordan L <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
 .../FvbServicesRuntimeDxe.inf                           |  1 +
 .../QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c  | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
index d7b4ec06c4e6..6bb5c2093790 100644
--- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
+++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
@@ -54,6 +54,7 @@ [LibraryClasses]
   DevicePathLib
   DxeServicesTableLib
   MemoryAllocationLib
+  MemEncryptSevLib
   PcdLib
   UefiBootServicesTableLib
   UefiDriverEntryPoint
diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c
index 646427bf4e2c..3add4bbad74c 100644
--- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c
+++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c
@@ -24,6 +24,7 @@
 #include <Protocol/FirmwareVolumeBlock.h>
 #include <Library/DxeServicesTableLib.h>
 #include <Library/MemoryAllocationLib.h>
+#include <Library/MemEncryptSevLib.h>
 
 #include "FwBlockService.h"
 #include "QemuFlash.h"
@@ -204,5 +205,21 @@ MarkIoMemoryRangeForRuntimeAccess (
                   );
   ASSERT_EFI_ERROR (Status);
 
+  //
+  // When SEV is active, AmdSevDxe should have mapped the BaseAddress with
+  // C=0 but SetMemorySpaceAttribute() remap the range with C=1. Lets restore
+  // the mapping so that both guest and hyervisor can access the flash
+  // memory range.
+  //
+  if (MemEncryptSevIsEnabled()) {
+    Status = MemEncryptSevClearPageEncMask (
+               0,
+               BaseAddress,
+               EFI_SIZE_TO_PAGES (Length),
+               FALSE
+               );
+    ASSERT_EFI_ERROR (Status);
+  }
+
   return Status;
 }
-- 
2.7.4