arch: x86_64: Fix idle stack allocation

Summary:
- I noticed that qemu-intel64:nsh does not work
- And I found that commit 6e6eecaa73 affected this issue
- This commit changes idle stack allocation to fix the issue

Impact:
- None

Testing:
- Tested with qemu-intel64:nsh and qemu-intel64:ostest

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

arch/x86_64/src/common/up_allocateheap.c

@@ -50,6 +50,9 @@
  * Public Functions
  ****************************************************************************/
 
+const uintptr_t g_idle_topstack = (uintptr_t)&_ebss +
+  CONFIG_IDLETHREAD_STACKSIZE;
+
 /****************************************************************************
  * Name: up_allocate_heap
  *
@@ -71,7 +74,7 @@ void up_allocate_heap(FAR void **heap_start, size_t *heap_size)
 
   /* Calculate the end of .bss section */
 
-  uintptr_t hstart = (((uintptr_t)&_ebss + PAGE_SIZE - 1) & PAGE_MASK);
+  uintptr_t hstart = (g_idle_topstack + PAGE_SIZE - 1) & PAGE_MASK;
   *heap_start = (void *)hstart;
 
   /* The size is the rest of the RAM */

arch/x86_64/src/common/up_internal.h

@@ -120,7 +120,7 @@ extern volatile uint64_t *g_current_regs;
  * end of the heap is CONFIG_RAM_END
  */
 
-extern uint64_t g_idle_topstack;
+extern const uintptr_t g_idle_topstack;
 
 /* Address of the saved user stack pointer */
 

arch/x86_64/src/intel64/intel64_head.S

@@ -290,8 +290,7 @@ clear_bss:
     jne     clear_bss
 
     // Properly setup RSP to idle stack
-    movabs  $idle_stack,    %rbx
-    add     $CONFIG_IDLETHREAD_STACKSIZE,   %rbx
+    movabs  $g_idle_topstack,    %rbx
     mov     %rbx,   %rsp
 
 
@@ -468,35 +467,3 @@ pt_low:
     .fill X86_NUM_PAGE_ENTRY * X86_NUM_PAGE_ENTRY, X86_PAGE_ENTRY_SIZE, 0
     .fill X86_NUM_PAGE_ENTRY * X86_NUM_PAGE_ENTRY, X86_PAGE_ENTRY_SIZE, 0
 
-/****************************************************************************
- * .bss
- ****************************************************************************/
-
-/* The stack for the IDLE task thread is declared in .bss.  NuttX boots and
- * initializes on the IDLE thread, then at the completion of OS startup, this
- * thread becomes the thread that executes when there is nothing else to
- * do in the system (see up_idle()).
- */
-
-    .section    .bss
-
-    .type    idle_stack, @object
-    .comm    idle_stack, CONFIG_IDLETHREAD_STACKSIZE, 32
-    .size    idle_stack, CONFIG_IDLETHREAD_STACKSIZE
-
-/****************************************************************************
- * .rodata
- ****************************************************************************/
-
-    .section    .rodata, "a"
-
-/* HEAP BASE: _sbss is the start of the BSS region (see ld.script) _ebss is
- * the end of the BSS region (see liker script). The heap continues from there
- * until the end of memory.
- */
-
-    .type    g_idle_topstack, @object
-g_idle_topstack:
-    .quad    _ebss
-    .size    g_idle_topstack, . - g_idle_topstack
-    .end