[PATCH v2 5/7] OvmfPkg/XenPlatformPei: Map extra physical address

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

Some information available in a Xen guest can be mapped anywhere in
the physical address space and they don't need to be backed by RAM.
For example, the shared info page.

While it's easier to put those pages anywhere, it is better to avoid
mapping it where the RAM is. It might split a nice 1G guest page table
into 4k pages and thus reducing performance of the guest when it
access its memory. Also mapping a page like the shared info page and
then unmapping it or mapping it somewhere else would live a hole in
the RAM that the guest would propably not been able to use anymore.

So the patch introduce a new function which can be used to 1:1
mapping of guest physical memory above 4G during the PEI phase so we
can map the Xen shared pages outside of memory that can be used by
guest, and as high as possible.

Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
---

Notes:
    v2:
    - new patch

 OvmfPkg/XenPlatformPei/XenPlatformPei.inf |  1 +
 OvmfPkg/XenPlatformPei/Platform.h         |  5 ++
 OvmfPkg/XenPlatformPei/Xen.c              | 71 +++++++++++++++++++++++
 3 files changed, 77 insertions(+)

diff --git a/OvmfPkg/XenPlatformPei/XenPlatformPei.inf 
b/OvmfPkg/XenPlatformPei/XenPlatformPei.inf
index 0ef77db92c03..8790d907d3ec 100644
--- a/OvmfPkg/XenPlatformPei/XenPlatformPei.inf
+++ b/OvmfPkg/XenPlatformPei/XenPlatformPei.inf
@@ -66,6 +66,7 @@ [Pcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfPeiMemFvSize
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfDxeMemFvBase
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfDxeMemFvSize
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPageTablesBase
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfLockBoxStorageBase
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfLockBoxStorageSize
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfHostBridgePciDevId
diff --git a/OvmfPkg/XenPlatformPei/Platform.h 
b/OvmfPkg/XenPlatformPei/Platform.h
index 7661f4a8de0a..e70ca58078eb 100644
--- a/OvmfPkg/XenPlatformPei/Platform.h
+++ b/OvmfPkg/XenPlatformPei/Platform.h
@@ -127,6 +127,11 @@ XenGetE820Map (
   UINT32 *Count
   );
 
+EFI_STATUS
+PhysicalAddressIdentityMapping (
+  IN EFI_PHYSICAL_ADDRESS AddressToMap
+  );
+
 extern EFI_BOOT_MODE mBootMode;
 
 extern UINT8 mPhysMemAddressWidth;
diff --git a/OvmfPkg/XenPlatformPei/Xen.c b/OvmfPkg/XenPlatformPei/Xen.c
index c41fecdc486e..b2a7d1c21dac 100644
--- a/OvmfPkg/XenPlatformPei/Xen.c
+++ b/OvmfPkg/XenPlatformPei/Xen.c
@@ -17,6 +17,8 @@
 //
 // The Library classes this module consumes
 //
+#include <Library/BaseMemoryLib.h>
+#include <Library/CpuLib.h>
 #include <Library/DebugLib.h>
 #include <Library/HobLib.h>
 #include <Library/MemoryAllocationLib.h>
@@ -25,6 +27,7 @@
 #include <IndustryStandard/E820.h>
 #include <Library/ResourcePublicationLib.h>
 #include <Library/MtrrLib.h>
+#include <IndustryStandard/PageTable.h>
 #include <IndustryStandard/Xen/arch-x86/hvm/start_info.h>
 #include <Library/XenHypercallLib.h>
 #include <IndustryStandard/Xen/memory.h>
@@ -386,3 +389,71 @@ InitializeXen (
 
   return EFI_SUCCESS;
 }
+
+EFI_STATUS
+PhysicalAddressIdentityMapping (
+  IN EFI_PHYSICAL_ADDRESS   AddressToMap
+  )
+{
+  INTN                            Index;
+  PAGE_MAP_AND_DIRECTORY_POINTER  *L4, *L3;
+  PAGE_TABLE_ENTRY                *PageTable;
+
+  DEBUG ((DEBUG_INFO, "Mapping 1:1 of address 0x%lx\n", (UINT64)AddressToMap));
+
+  // L4 / Top level Page Directory Pointers
+
+  L4 = (VOID*)(UINTN)PcdGet32 (PcdOvmfSecPageTablesBase);
+  Index = PML4_OFFSET (AddressToMap);
+
+  if (!L4[Index].Bits.Present) {
+    L3 = AllocatePages (1);
+    if (L3 == NULL) {
+      return EFI_OUT_OF_RESOURCES;
+    }
+
+    ZeroMem (L3, EFI_PAGE_SIZE);
+
+    L4[Index].Bits.ReadWrite = 1;
+    L4[Index].Bits.Accessed = 1;
+    L4[Index].Bits.PageTableBaseAddress = (EFI_PHYSICAL_ADDRESS)L3 >> 12;
+    L4[Index].Bits.Present = 1;
+  }
+
+  // L3 / Next level Page Directory Pointers
+
+  L3 = (VOID*)(EFI_PHYSICAL_ADDRESS)(L4[Index].Bits.PageTableBaseAddress << 
12);
+  Index = PDP_OFFSET (AddressToMap);
+
+  if (!L3[Index].Bits.Present) {
+    PageTable = AllocatePages (1);
+    if (PageTable == NULL) {
+      return EFI_OUT_OF_RESOURCES;
+    }
+
+    ZeroMem (PageTable, EFI_PAGE_SIZE);
+
+    L3[Index].Bits.ReadWrite = 1;
+    L3[Index].Bits.Accessed = 1;
+    L3[Index].Bits.PageTableBaseAddress = (EFI_PHYSICAL_ADDRESS)PageTable >> 
12;
+    L3[Index].Bits.Present = 1;
+  }
+
+  // L2 / Page Table Entries
+
+  PageTable = 
(VOID*)(EFI_PHYSICAL_ADDRESS)(L3[Index].Bits.PageTableBaseAddress << 12);
+  Index = PDE_OFFSET (AddressToMap);
+
+  if (!PageTable[Index].Bits.Present) {
+    PageTable[Index].Bits.ReadWrite = 1;
+    PageTable[Index].Bits.Accessed = 1;
+    PageTable[Index].Bits.Dirty = 1;
+    PageTable[Index].Bits.MustBe1 = 1;
+    PageTable[Index].Bits.PageTableBaseAddress = AddressToMap >> 21;
+    PageTable[Index].Bits.Present = 1;
+  }
+
+  CpuFlushTlb ();
+
+  return EFI_SUCCESS;
+}
-- 
Anthony PERARD