OvmfPkg/BaseMemEncryptSevLib: introduce MemEncryptSevClearMmioPageEncMask()

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275

The MemEncryptSevClearMmioPageEncMask() helper can be used for clearing
the memory encryption mask for the Mmio region.

The MemEncryptSevClearMmioPageEncMask() is a simplified version of
MemEncryptSevClearPageEncMask() -- it does not flush the caches after
clearing the page encryption mask.

Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Message-Id: <20210519181949.6574-10-brijesh.singh@amd.com>

OvmfPkg/Library/BaseMemEncryptSevLib/X64/MemEncryptSevLib.c

@@ -118,3 +118,36 @@ MemEncryptSevGetAddressRangeState (
            Length
            );
 }
+
+/**
+  This function clears memory encryption bit for the mmio region specified by
+  BaseAddress and NumPages.
+
+  @param[in]  Cr3BaseAddress          Cr3 Base Address (if zero then use
+                                      current CR3)
+  @param[in]  BaseAddress             The physical address that is the start
+                                      address of a mmio region.
+  @param[in]  NumPages                The number of pages from start memory
+                                      region.
+
+  @retval RETURN_SUCCESS              The attributes were cleared for the
+                                      memory region.
+  @retval RETURN_INVALID_PARAMETER    Number of pages is zero.
+  @retval RETURN_UNSUPPORTED          Clearing the memory encryption attribute
+                                      is not supported
+**/
+RETURN_STATUS
+EFIAPI
+MemEncryptSevClearMmioPageEncMask (
+  IN PHYSICAL_ADDRESS         Cr3BaseAddress,
+  IN PHYSICAL_ADDRESS         BaseAddress,
+  IN UINTN                    NumPages
+  )
+{
+  return InternalMemEncryptSevClearMmioPageEncMask (
+           Cr3BaseAddress,
+           BaseAddress,
+           EFI_PAGES_TO_SIZE (NumPages)
+           );
+
+}