[PATCH] OvmfPkg: Create additional PML4 entries for large SEV-SNP VMs

[lisik@google.com]

Edk2 was failing, rather than creating more PML4 entries, when they
weren't present in the initial memory acceptance flow. Because of that
VMs with more than 512G memory were crashing. This code fixes that.

This change affects only SEV-SNP VMs.

The code was tested by successfully booting a 512G SEV-SNP VM.

Signed-off-by: Mikolaj Lisik <lisik@google.com>
---
 .../X64/PeiDxeVirtualMemory.c                  | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c
index b9c0a5b25a..3dbff51ac2 100644
--- a/OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c
+++ b/OvmfPkg/Library/BaseMemEncryptSevLib/X64/PeiDxeVirtualMemory.c
@@ -548,6 +548,7 @@ InternalMemEncryptSevCreateIdentityMap1G (
   PAGE_MAP_AND_DIRECTORY_POINTER  *PageMapLevel4Entry;
   PAGE_TABLE_1G_ENTRY             *PageDirectory1GEntry;
   UINT64                          PgTableMask;
+  UINT64                          *NewPageTable;
   UINT64                          AddressEncMask;
   BOOLEAN                         IsWpEnabled;
   RETURN_STATUS                   Status;
@@ -602,15 +603,13 @@ InternalMemEncryptSevCreateIdentityMap1G (
     PageMapLevel4Entry  = (VOID *)(Cr3BaseAddress & ~PgTableMask);
     PageMapLevel4Entry += PML4_OFFSET (PhysicalAddress);
     if (!PageMapLevel4Entry->Bits.Present) {
-      DEBUG ((
-        DEBUG_ERROR,
-        "%a:%a: bad PML4 for Physical=0x%Lx\n",
-        gEfiCallerBaseName,
-        __FUNCTION__,
-        PhysicalAddress
-        ));
-      Status = RETURN_NO_MAPPING;
-      goto Done;
+      NewPageTable = AllocatePageTableMemory(1);
+      ASSERT(NewPageTable != NULL);
+      SetMem (NewPageTable, EFI_PAGE_SIZE, 0);
+      PageMapLevel4Entry->Uint64 = (UINT64)(UINTN)NewPageTable | AddressEncMask;
+      PageMapLevel4Entry->Bits.Present    = 1;
+      PageMapLevel4Entry->Bits.MustBeZero = 0;
+      PageMapLevel4Entry->Bits.ReadWrite  = 1;
     }
 
     PageDirectory1GEntry = (VOID *)(
@@ -640,7 +639,6 @@ InternalMemEncryptSevCreateIdentityMap1G (
   //
   CpuFlushTlb ();
 
-Done:
   //
   // Restore page table write protection, if any.
   //
-- 
2.39.0.314.g84b9a713c41-goog