[PATCH] 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>

---

I came across this while developing my pcpu stack hexdumping capability into
the crashdump analyser.  While I know it is not neccessary for Xen to function
correctly, I do feel that it is quite important when getting to this level of
debugging.

I guess this patch is partly RFC, but it does successfully boot on my test
box, and fix stack junk seen on all CPUs.
---
 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..a45549b 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
+
+        /* Move 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 3df4e88..eced9ba 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -662,6 +662,10 @@ 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