x86: slightly improve stack trace on debug builds

As was rather obvious from crashes recently happening in stage testing,
the debug hypervisor, in that special case, has a drawback compared to
the non-debug one: When a call through a bad pointer happens, there's
no frame, and the top level (and frequently most important for
analysis) stack entry would get skipped:

(XEN) ----[ Xen-4.3-unstable  x86_64  debug=y  Not tainted ]----
(XEN) CPU:    1
(XEN) RIP:    e008:[<0000000000000000>] ???
(XEN) RFLAGS: 0000000000010046   CONTEXT: hypervisor
(XEN) rax: 0000000000000008   rbx: 0000000000000001   rcx: 0000000000000003
(XEN) rdx: 0000003db54eb700   rsi: 7fffffffffffffff   rdi: 0000000000000001
(XEN) rbp: ffff8302357e7ee0   rsp: ffff8302357e7e58   r8:  0000000000000000
(XEN) r9:  000000000000003e   r10: ffff8302357e7f18   r11: ffff8302357e7f18
(XEN) r12: ffff8302357ee340   r13: ffff82c480263980   r14: ffff8302357ee3d0
(XEN) r15: 0000000000000001   cr0: 000000008005003b   cr4: 00000000000026f0
(XEN) cr3: 00000000bf473000   cr2: 0000000000000000
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0000   cs: e008
(XEN) Xen stack trace from rsp=ffff8302357e7e58:
(XEN)    ffff82c4801a3d05 ffff8302357eca70 0000000800000020 ffff82c4802ead60
(XEN)    0000000000000001 ffff8302357e7ea0 ffff82c48016bf07 0000000000000000
(XEN)    0000000000000000 ffff8302357e7ee0 fffff830fffff830 0000000000000046
(XEN)    ffff8302357e7f18 ffff82c480263980 ffff8302357e7f18 0000000000000000
(XEN)    0000000000000000 ffff8302357e7f10 ffff82c48015c2be 8302357dc0000fff
...
(XEN) Xen call trace:
(XEN)    [<0000000000000000>] ???
(XEN)    [<ffff82c48015c2be>] idle_loop+0x6c/0x7a
(XEN)    
(XEN) Pagetable walk from 0000000000000000:

Since the bad pointer is being printed anyway (as part of the register
state), replace it with the top of stack value in such a case.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -217,8 +217,18 @@ static void show_trace(struct cpu_user_r
 
     printk("Xen call trace:\n   ");
 
-    printk("[<%p>]", _p(regs->eip));
-    print_symbol(" %s\n   ", regs->eip);
+    addr = regs->eip;
+    while ( !is_kernel_text(addr) &&
+            (system_state > SYS_STATE_boot || !is_kernel_inittext(addr)) )
+    {
+        /* Special case when a bad pointer was called. */
+        addr ^= regs->eip ^ *ESP_BEFORE_EXCEPTION(regs);
+        if ( addr == regs->eip )
+            break;
+    }
+
+    printk("[<%p>]", _p(addr));
+    print_symbol(" %s\n   ", addr);
 
     /* Bounds for range of valid frame pointer. */
     low  = (unsigned long)(ESP_BEFORE_EXCEPTION(regs) - 2);