UefiCpuPkg/CpuDxe: Add stackless assembly AP entry points

The AP startup code simply jumps into this code with the CpuDxe driver
without setting up a stack for the processor.

Therefore, this code must setup the stack before calling into C code.

This is the basic flow:
* AP enters CpuDxe driver code (AsmApEntryPoint) without stack
  - AP grabs a lock
  - AP sets up stack
  - AP calls CpuMp.c:ApEntryPointInC
  - If ApEntryPointInC returns, the lock is freed, and another AP may
    run
  - The AP C code may call AsmApDoneWithCommonStack to indicate that
    the AP is no longer using the stack, and another may therefore
    proceed to use the stack and then call ApEntryPointInC

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16347 6f19259b-4bc3-4df7-8a09-765794883524

UefiCpuPkg/CpuDxe/CpuMp.h

@@ -24,5 +24,32 @@ InitializeMpSupport (
   VOID
   );
 
+/**
+  The AP entry point that the Startup-IPI target code will jump to.
+
+  The processor jumps to this code in flat mode, but the processor's
+  stack is not initialized.
+
+**/
+VOID
+EFIAPI
+AsmApEntryPoint (
+  VOID
+  );
+
+/**
+  Releases the lock preventing other APs from using the shared AP
+  stack.
+
+  Once the AP has transitioned to using a new stack, it can call this
+  function to allow another AP to proceed with using the shared stack.
+
+**/
+VOID
+EFIAPI
+AsmApDoneWithCommonStack (
+  VOID
+  );
+
 #endif // _CPU_MP_H_