OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Clear C-bit when SEV is active

Commit:24e4ad7 (OvmfPkg: Add AmdSevDxe driver) added a driver which runs
early in DXE phase and clears the C-bit from NonExistent entry -- which
is later split and accommodate the flash MMIO. When SMM is enabled, we
build two sets of page tables; first page table is used when executing
code in non SMM mode (SMM-less-pgtable) and second page table is used
when we are executing code in SMM mode (SMM-pgtable).

During boot time, AmdSevDxe driver clears the C-bit from the
SMM-less-pgtable. But when SMM is enabled, Qemu Flash services are used
from SMM mode.

In this patch we explicitly clear the C-bit from Qemu flash MMIO range
before we probe the flash. When OVMF is built with SMM_REQUIRE then
call to initialize the flash services happen after the SMM-pgtable is
created and processor has served the first SMI. At this time we will
have access to the SMM-pgtable.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
[lersek@redhat.com: trivial coding style improvements]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c

@@ -244,6 +244,15 @@ QemuFlashInitialize (
   ASSERT(PcdGet32 (PcdOvmfFirmwareFdSize) % mFdBlockSize == 0);
   mFdBlockCount = PcdGet32 (PcdOvmfFirmwareFdSize) / mFdBlockSize;
 
+  //
+  // execute module specific hooks before probing the flash
+  //
+  QemuFlashBeforeProbe (
+    (EFI_PHYSICAL_ADDRESS)(UINTN) mFlashBase,
+    mFdBlockSize,
+    mFdBlockCount
+    );
+
   if (!QemuFlashDetected ()) {
     ASSERT (!FeaturePcdGet (PcdSmmSmramRequire));
     return EFI_WRITE_PROTECTED;