OvmfPkg/MemEncryptSevLib: add support to validate > 4GB memory in PEI phase

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

The initial page built during the SEC phase is used by the
MemEncryptSevSnpValidateSystemRam() for the system RAM validation. The
page validation process requires using the PVALIDATE instruction;  the
instruction accepts a virtual address of the memory region that needs
to be validated. If hardware encounters a page table walk failure (due
to page-not-present) then it raises #GP.

The initial page table built in SEC phase address up to 4GB. Add an
internal function to extend the page table to cover > 4GB. The function
builds 1GB entries in the page table for access > 4GB. This will provide
the support to call PVALIDATE instruction for the virtual address >
4GB in PEI phase.

Cc: Michael Roth <michael.roth@amd.com>
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: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>

OvmfPkg/Library/BaseMemEncryptSevLib/X64/VirtualMemory.h

@@ -144,4 +144,28 @@ InternalMemEncryptSevClearMmioPageEncMask (
   IN  UINTN             Length
   );
 
+/**
+  Create 1GB identity mapping for the specified virtual address range.
+
+  The function is preliminary used by the SEV-SNP page state change
+  APIs to build the page table required before issuing the PVALIDATE
+  instruction. The function must be removed after the EDK2 core is
+  enhanced to do the lazy validation.
+
+  @param[in]  Cr3BaseAddress          Cr3 Base Address (if zero then use
+                                      current CR3)
+  @param[in]  VirtualAddress          Virtual address
+  @param[in]  Length                  Length of virtual address range
+
+  @retval RETURN_INVALID_PARAMETER    Number of pages is zero.
+
+**/
+RETURN_STATUS
+EFIAPI
+InternalMemEncryptSevCreateIdentityMap1G (
+  IN    PHYSICAL_ADDRESS  Cr3BaseAddress,
+  IN    PHYSICAL_ADDRESS  PhysicalAddress,
+  IN    UINTN             Length
+  );
+
 #endif