[Patch v2] x86/boot: Explicitly clean pcpu stacks in debug builds

[Andrew Cooper <andrew.cooper3@citrix.com>]

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>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>

---

Changes since v1: Brackets around shift operation, and correct the calculation
for cpu0_stack, both noticed by Jan Beulich.
---
 xen/arch/x86/boot/x86_64.S |    9 +++++++++
 xen/arch/x86/smpboot.c     |    4 ++++
 2 files changed, 13 insertions(+)

diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index ed3888d..399541d 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/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
 
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 9f81c7b..f6b42f5 100644
--- a/xen/arch/x86/smpboot.c
+++ b/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);
-- 
1.7.10.4