[PATCH 2/2] UefiCpuPkg/PiSmmCpuDxeSmm: Place AP to 32bit protected mode on S3 path

[Jeff Fan <jeff.fan@intel.com>]

On S3 path, we may transfer to long mode (if DXE is long mode) to restore CPU
contexts with CR3 = SmmS3Cr3 (in SMM). AP will execute hlt-loop after CPU
contexts restoration. Once one NMI or SMI happens, APs may exit from hlt state
and execute the instruction after HLT instruction. If APs are running on long
mode, page table is required to fetch the instruction. However, CR3 pointer to
page table in SMM. APs will crash.

This fix is to disable long mode on APs and transfer to 32bit protected mode to
execute hlt-loop. Then CR3 and page table will no longer be required.

https://bugzilla.tianocore.org/show_bug.cgi?id=216

Reported-by: Laszlo Ersek <lersek@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
---
 UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c | 43 ++++++++++++++++++++++++----
 1 file changed, 38 insertions(+), 5 deletions(-)

diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c b/UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c
index c05dec7..1db0a6e 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c
@@ -68,6 +68,38 @@ InitGdt (
   *GdtStepSize = GdtTableStepSize;
   return GdtTssTables;
 }
+
+/**
+  Get Protected mode code segment from current GDT table.
+
+  @return  Protected mode code segment value.
+**/
+UINT16
+GetProtectedModeCS (
+  VOID
+  )
+{
+  IA32_DESCRIPTOR          GdtrDesc;
+  IA32_SEGMENT_DESCRIPTOR  *GdtEntry;
+  UINTN                    GdtEntryCount;
+  UINT16                   Index;
+
+  Index = (UINT16) -1;
+  AsmReadGdtr (&GdtrDesc);
+  GdtEntryCount = (GdtrDesc.Limit + 1) / sizeof (IA32_SEGMENT_DESCRIPTOR);
+  GdtEntry = (IA32_SEGMENT_DESCRIPTOR *) GdtrDesc.Base;
+  for (Index = 0; Index < GdtEntryCount; Index++) {
+    if (GdtEntry->Bits.L == 0) {
+      if (GdtEntry->Bits.Type > 8 && GdtEntry->Bits.L == 0) {
+        break;
+      }
+    }
+    GdtEntry++;
+  }
+  ASSERT (Index != -1);
+  return Index * 8;
+}
+
 /**
   Transfer AP to safe hlt-loop after it finished restore CPU features on S3 patch.
   
@@ -81,11 +113,12 @@ TransferApToSafeState (
   IN UINT32             TopOfStack
   )
 {
-  SwitchStack (
-    (SWITCH_STACK_ENTRY_POINT) (UINTN) ApHltLoopCode,
-    NULL,
-    NULL,
-    (VOID *) (UINTN) TopOfStack
+  AsmDisablePaging64 (
+    GetProtectedModeCS (),
+    (UINT32) (UINTN) ApHltLoopCode,
+    0,
+    0,
+    TopOfStack
     );
   //
   // It should never reach here
-- 
2.9.3.windows.2