OvmfPkg/AmdSevDxe: Clear encryption bit on PCIe MMCONFIG range

OvmfPkg/AmdSevDxe: Clear encryption bit on PCIe MMCONFIG range

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3108

The PCIe MMCONFIG range should be treated as an MMIO range. However,
there is a comment in the code explaining why AddIoMemoryBaseSizeHob()
is not called. The AmdSevDxe walks the GCD map looking for MemoryMappedIo
or NonExistent type memory and will clear the encryption bit for these
ranges.

Since the MMCONFIG range does not have one of these types, the encryption
bit is not cleared for this range. Add support to detect the presence of
the MMCONFIG range and clear the encryption bit. This will be needed for
follow-on support that will validate that MMIO is not being performed to
an encrypted address range under SEV-ES.

Even though the encryption bit was set for this range, this still worked
under both SEV and SEV-ES because the address range is marked by the
hypervisor as MMIO in the nested page tables:
- For SEV, access to this address range triggers a nested page fault (NPF)
  and the hardware supplies the guest physical address (GPA) in the VMCB's
  EXITINFO2 field as part of the exit information. However, the encryption
  bit is not set in the GPA, so the hypervisor can process the request
  without any issues.
- For SEV-ES, access to this address range triggers a #VC. Since OVMF runs
  identity mapped (VA == PA), the virtual address is used to avoid the
  lookup of the physical address. The virtual address does not have the
  encryption bit set, so the hypervisor can process the request without
  any issues.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <711ae2dcb6cb29e4c60862c18330cff627269b81.1610045305.git.thomas.lendacky@amd.com>