Ring3: All the same.

ArmPkg/Library/ArmExceptionLib/AArch64/ExceptionSupport.S

@@ -12,6 +12,8 @@
 #include <AsmMacroIoLibV8.h>
 #include <Protocol/DebugSupport.h> // for exception type definitions
 
+.cpu cortex-a76
+
 /*
   This is the stack constructed by the exception handler (low address to high address).
   X0 to FAR makes up the EFI_SYSTEM_CONTEXT for AArch64.
@@ -116,7 +118,24 @@ ASM_PFX(ExceptionHandlersStart):
 VECTOR_BASE(ExceptionHandlersStart)
 #endif
 
-  .macro  ExceptionEntry, val
+  .macro  ExceptionEntry, val, sp=SPx
+  //
+  // Our backtrace and register dump code is written in C and so it requires
+  // a stack. This makes it difficult to produce meaningful diagnostics when
+  // the stack pointer has been corrupted. So in such cases (i.e., when taking
+  // synchronous exceptions), this macro is expanded with \sp set to SP0, in
+  // which case we switch to the SP_EL0 stack pointer, which has been
+  // initialized to point to a buffer that has been set aside for this purpose.
+  //
+  // Since 'sp' may no longer refer to the stack frame that was active when
+  // the exception was taken, we may have to switch back and forth between
+  // SP_EL0 and SP_ELx to record the correct value for SP in the context struct.
+  //
+  .ifnc   \sp, SPx
+  msr     SPsel, xzr
+  msr     pan, #0
+  .endif
+
   // Move the stackpointer so we can reach our structure with the str instruction.
   sub sp, sp, #(FP_CONTEXT_SIZE + SYS_CONTEXT_SIZE)
 
@@ -138,7 +157,13 @@ VECTOR_BASE(ExceptionHandlersStart)
   stp      x28, x29, [sp, #0xe0]
   add      x28, sp, #(GP_CONTEXT_SIZE + FP_CONTEXT_SIZE + SYS_CONTEXT_SIZE)
 
+  .ifnc    \sp, SPx
+  msr      SPsel, #1
+  mov      x7, sp
+  msr      SPsel, xzr
+  .else
   mov      x7, x28
+  .endif
 
   stp      x30,  x7, [sp, #0xf0]
 
@@ -179,7 +204,7 @@ ASM_PFX(SErrorSP0):
 //
 VECTOR_ENTRY(ExceptionHandlersStart, ARM_VECTOR_CUR_SPX_SYNC)
 ASM_PFX(SynchronousExceptionSPx):
-  ExceptionEntry  EXCEPT_AARCH64_SYNCHRONOUS_EXCEPTIONS
+  ExceptionEntry  EXCEPT_AARCH64_SYNCHRONOUS_EXCEPTIONS, SP0
 
 VECTOR_ENTRY(ExceptionHandlersStart, ARM_VECTOR_CUR_SPX_IRQ)
 ASM_PFX(IrqSPx):