PlatformPei calls GetSystemMemorySizeBelow4gb() in three locations:

- PublishPeiMemory(): on normal boot, the permanent PEI RAM is installed
  so that it ends with the RAM below 4GB,

- QemuInitializeRam(): on normal boot, memory resource descriptor HOBs are
  created for the RAM below 4GB; plus MTRR attributes are set
  (independently of S3 vs. normal boot)

- MemMapInitialization(): an MMIO resource descriptor HOB is created for
  PCI resource allocation, on normal boot, starting at max(RAM below 4GB,
  2GB).

The first two of these is adjusted for the configured TSEG size, if
PcdSmmSmramRequire is set:

- In PublishPeiMemory(), the permanent PEI RAM is kept under TSEG.

- In QemuInitializeRam(), we must keep the DXE out of TSEG.

  One idea would be to simply trim the [1MB .. LowerMemorySize] memory
  resource descriptor HOB, leaving a hole for TSEG in the memory space
  map.

  The SMM IPL will however want to massage the caching attributes of the
  SMRAM range that it loads the SMM core into, with
  gDS->SetMemorySpaceAttributes(), and that won't work on a hole. So,
  instead of trimming this range, split the TSEG area off, and report it
  as a cacheable reserved memory resource.

  Finally, since reserved memory can be allocated too, pre-allocate TSEG
  in InitializeRamRegions(), after QemuInitializeRam() returns. (Note that
  this step alone does not suffice without the resource descriptor HOB
  trickery: if we omit that, then the DXE IPL PEIM fails to load and start
  the DXE core.)

- In MemMapInitialization(), the start of the PCI MMIO range is not
  affected.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <ler...@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.jus...@intel.com>
---
 OvmfPkg/OvmfPkg.dec                 |  7 ++++
 OvmfPkg/PlatformPei/PlatformPei.inf |  1 +
 OvmfPkg/PlatformPei/MemDetect.c     | 34 +++++++++++++++++++-
 3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/OvmfPkg/OvmfPkg.dec b/OvmfPkg/OvmfPkg.dec
index 3fab2af..da6967f 100644
--- a/OvmfPkg/OvmfPkg.dec
+++ b/OvmfPkg/OvmfPkg.dec
@@ -93,6 +93,13 @@ [PcdsFixedAtBuild]
   gUefiOvmfPkgTokenSpaceGuid.PcdVirtioScsiMaxTargetLimit|31|UINT16|6
   gUefiOvmfPkgTokenSpaceGuid.PcdVirtioScsiMaxLunLimit|7|UINT32|7
 
+  ## The following setting controls how many megabytes we configure as TSEG on
+  #  Q35, for SMRAM purposes. Permitted values are: 1, 2, 8. Other values cause
+  #  undefined behavior.
+  #
+  #  This PCD is only consulted if PcdSmmSmramRequire is TRUE (see below).
+  gUefiOvmfPkgTokenSpaceGuid.PcdQ35TsegMbytes|1|UINT8|0x20
+
 [PcdsFixedAtBuild]
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfFlashNvStorageEventLogBase|0x0|UINT32|0x8
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfFlashNvStorageEventLogSize|0x0|UINT32|0x9
diff --git a/OvmfPkg/PlatformPei/PlatformPei.inf 
b/OvmfPkg/PlatformPei/PlatformPei.inf
index 4b1e68d..dc77293 100644
--- a/OvmfPkg/PlatformPei/PlatformPei.inf
+++ b/OvmfPkg/PlatformPei/PlatformPei.inf
@@ -75,6 +75,7 @@ [Pcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdGuidedExtractHandlerTableSize
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfHostBridgePciDevId
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfDecompressionScratchEnd
+  gUefiOvmfPkgTokenSpaceGuid.PcdQ35TsegMbytes
   gEfiIntelFrameworkModulePkgTokenSpaceGuid.PcdS3AcpiReservedMemorySize
   gEfiMdePkgTokenSpaceGuid.PcdGuidedExtractHandlerTableAddress
   gEfiMdeModulePkgTokenSpaceGuid.PcdVariableStoreSize
diff --git a/OvmfPkg/PlatformPei/MemDetect.c b/OvmfPkg/PlatformPei/MemDetect.c
index 5fe8b28..1bdc2df 100644
--- a/OvmfPkg/PlatformPei/MemDetect.c
+++ b/OvmfPkg/PlatformPei/MemDetect.c
@@ -214,6 +214,12 @@ PublishPeiMemory (
     MemorySize = PcdGet32 (PcdS3AcpiReservedMemorySize);
   } else {
     LowerMemorySize = GetSystemMemorySizeBelow4gb ();
+    if (FeaturePcdGet (PcdSmmSmramRequire)) {
+      //
+      // TSEG is chipped from the end of low RAM
+      //
+      LowerMemorySize -= FixedPcdGet8 (PcdQ35TsegMbytes) * SIZE_1MB;
+    }
 
     PeiMemoryCap = GetPeiMemoryCap ();
     DEBUG ((EFI_D_INFO, "%a: mPhysMemAddressWidth=%d PeiMemoryCap=%u KB\n",
@@ -277,7 +283,18 @@ QemuInitializeRam (
     // Create memory HOBs
     //
     AddMemoryRangeHob (0, BASE_512KB + BASE_128KB);
-    AddMemoryRangeHob (BASE_1MB, LowerMemorySize);
+
+    if (FeaturePcdGet (PcdSmmSmramRequire)) {
+      UINT32 TsegSize;
+
+      TsegSize = FixedPcdGet8 (PcdQ35TsegMbytes) * SIZE_1MB;
+      AddMemoryRangeHob (BASE_1MB, LowerMemorySize - TsegSize);
+      AddReservedMemoryBaseSizeHob (LowerMemorySize - TsegSize, TsegSize,
+        TRUE);
+    } else {
+      AddMemoryRangeHob (BASE_1MB, LowerMemorySize);
+    }
+
     if (UpperMemorySize != 0) {
       AddUntestedMemoryBaseSizeHob (BASE_4GB, UpperMemorySize);
     }
@@ -409,5 +426,20 @@ InitializeRamRegions (
       (UINT64)(UINTN) PcdGet32 (PcdOvmfLockBoxStorageSize),
       mS3Supported ? EfiACPIMemoryNVS : EfiBootServicesData
       );
+
+    if (FeaturePcdGet (PcdSmmSmramRequire)) {
+      UINT32 TsegSize;
+
+      //
+      // Make sure the TSEG area that we reported as a reserved memory resource
+      // cannot be used for reserved memory allocations.
+      //
+      TsegSize = FixedPcdGet8 (PcdQ35TsegMbytes) * SIZE_1MB;
+      BuildMemoryAllocationHob (
+        GetSystemMemorySizeBelow4gb() - TsegSize,
+        TsegSize,
+        EfiReservedMemoryType
+        );
+    }
   }
 }
-- 
1.8.3.1


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