x86/boot: Explicitly clean pcpu stacks in debug builds

This reduces confusion when looking at a hexdump of the pcpu stacks and
wondering were on earth some of the junk was coming from.  Also leave some
grep fodder for finding where the BSP switches stack (because it took me
far longer to find than I care to admit to).

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/boot/x86_64.S

@@ -13,6 +13,15 @@
         mov     mmu_cr4_features(%rip),%rcx
         mov     %rcx,%cr4
 
+#ifndef NDEBUG
+        /* Clean stack. */
+        mov     stack_start(%rip),%rdi
+        mov     $(STACK_SIZE / 8),%rcx
+        xor     %eax,%eax
+        rep stosq
+#endif
+
+        /* Switch to cpu0_stack. */
         mov     stack_start(%rip),%rsp
         or      $(STACK_SIZE-CPUINFO_sizeof),%rsp
 

xen/arch/x86/smpboot.c

@@ -659,6 +659,10 @@ static int cpu_smpboot_alloc(unsigned int cpu)
     stack_base[cpu] = alloc_xenheap_pages(STACK_ORDER, 0);
     if ( stack_base[cpu] == NULL )
         goto oom;
+#ifndef NDEBUG
+    for ( order = 0 ; order < (1<<STACK_ORDER); ++order )
+        clear_page(stack_base[cpu] + order * PAGE_SIZE);
+#endif
     memguard_guard_stack(stack_base[cpu]);
 
     order = get_order_from_pages(NR_RESERVED_GDT_PAGES);