ArmPkg/ArmExceptionLib: use EL0 stack for synchronous exceptions

In order to be able to produce meaningful diagnostic output when taking
synchronous exceptions that have been caused by corruption of the stack
pointer, prepare the EL0 stack pointer and switch to it when handling the
'Sync exception using SPx' exception class.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

ArmPkg/Library/ArmExceptionLib/AArch64/AArch64Exception.c

@@ -16,7 +16,7 @@
 #include <Uefi.h>
 
 #include <Chipset/AArch64.h>
-
+#include <Library/MemoryAllocationLib.h>
 #include <Protocol/DebugSupport.h> // for MAX_AARCH64_EXCEPTION
 
 UINTN                   gMaxExceptionNumber = MAX_AARCH64_EXCEPTION;
@@ -25,11 +25,26 @@ EFI_EXCEPTION_CALLBACK  gDebuggerExceptionHandlers[MAX_AARCH64_EXCEPTION + 1] =
 PHYSICAL_ADDRESS        gExceptionVectorAlignmentMask = ARM_VECTOR_TABLE_ALIGNMENT;
 UINTN                   gDebuggerNoHandlerValue = 0; // todo: define for AArch64
 
+#define EL0_STACK_PAGES   2
+
+VOID
+RegisterEl0Stack (
+  IN  VOID    *Stack
+  );
+
 RETURN_STATUS ArchVectorConfig(
   IN  UINTN       VectorBaseAddress
   )
 {
   UINTN             HcrReg;
+  UINT8             *Stack;
+
+  Stack = AllocatePages (EL0_STACK_PAGES);
+  if (Stack == NULL) {
+    return RETURN_OUT_OF_RESOURCES;
+  }
+
+  RegisterEl0Stack ((UINT8 *)Stack + EFI_PAGES_TO_SIZE (EL0_STACK_PAGES));
 
   if (ArmReadCurrentEL() == AARCH64_EL2) {
     HcrReg = ArmReadHcr();