diff -r c4f3f719d997 xen/arch/x86/sysctl.c
--- a/xen/arch/x86/sysctl.c	Sat Sep 23 13:54:58 2006
+++ b/xen/arch/x86/sysctl.c	Sun Oct  8 01:20:36 2006
@@ -24,6 +24,91 @@
 #include <asm/hvm/hvm.h>
 #include <asm/hvm/support.h>
 #include <asm/processor.h>
+
+/*
+ * Lockup debugging
+ */
+extern char *video;
+int enable_lockup_debug_dump = 0;
+static unsigned int print_counter;
+unsigned long last_page_fault_from_rip = 0;
+
+static int word_to_hex(char* dest, uint64_t v, int bytes) {
+    static const char* hexdigits = "0123456789abcdef";
+    int i;
+    int n = bytes * 2;
+ 
+    for (i = 0; i < n; i++) {
+      dest[(n-1) - i] = hexdigits[v & 0xf];
+      v >>= 4;
+    }
+
+    return n;
+}
+
+void print_direct_to_screen(const char* buf, int len, int line) {
+    int i;
+    char* dest;
+
+    if (!enable_lockup_debug_dump) return;
+
+    dest = video + (line * 80 * 2);
+
+    for (i = 0; i < len; i++) {
+        dest[i*2 + 0] = buf[i];
+        dest[i*2 + 1] = 7; // white on black
+    }
+}
+
+void print_lockup_debug_dump(struct cpu_user_regs *regs) {
+    char buf[128];
+    char* p;
+    unsigned long cr2;
+    unsigned long cr3;
+    unsigned int flags;
+
+    if (!enable_lockup_debug_dump)
+        return;
+
+    if (!video)
+       return;
+
+    if (!dom0)
+        return;
+
+    print_counter++;
+
+    if ((print_counter & enable_lockup_debug_dump) != 0)
+	return;
+
+    __asm__ __volatile__("mov %%cr2,%0" : "=r" (cr2) : );
+    cr3 = read_cr3();
+    __save_flags(flags);
+
+    memset(buf, ' ', sizeof(buf));
+
+    p = buf;
+    p += word_to_hex(p, regs->rip, 8) + 1;
+    p += word_to_hex(p, cr2, 8) + 1;
+    p += word_to_hex(p, last_page_fault_from_rip, 8) + 1;
+    p += word_to_hex(p, cr3 >> 12, 4) + 1;
+    p += word_to_hex(p, flags, 4) + 1;
+    if (dom0->shared_info) {
+        *p++ = (dom0->shared_info->vcpu_info[0].evtchn_upcall_mask) ? 'M' : '-';
+        *p++ = ' ';
+        *p++ = (dom0->shared_info->vcpu_info[0].evtchn_upcall_pending) ? 'P' : '-';
+        *p++ = ' ';
+        //p += word_to_hex(p, dom0->shared_info->evtchn_mask[0], 2) + 1;
+        //p += word_to_hex(p, dom0->shared_info->evtchn_pending[0], 2) + 1;
+    } else {
+        *p++ = 'X';
+        p++;
+    }
+
+    p += word_to_hex(p, print_counter, 2) + 1;
+
+    print_direct_to_screen(buf, 79, 0);
+}
 
 long arch_do_sysctl(
     struct xen_sysctl *sysctl, XEN_GUEST_HANDLE(xen_sysctl_t) u_sysctl)
@@ -57,6 +142,19 @@
     }
     break;
     
+    case XEN_SYSCTL_set_lockup_debug:
+    {
+        unsigned int enable = sysctl->u.set_lockup_debug.enable;
+        if (enable) {
+            printk("Enable lockup debugging printout at level %d (video %p)\n", enable, video);
+            enable_lockup_debug_dump = enable;
+        } else {
+            printk("Disable lockup debugging printout\n");
+            enable_lockup_debug_dump = 0;
+        }
+        ret = 0;
+    }
+    break;
 
     default:
         ret = -ENOSYS;
diff -r c4f3f719d997 xen/arch/x86/time.c
--- a/xen/arch/x86/time.c	Sat Sep 23 13:54:58 2006
+++ b/xen/arch/x86/time.c	Sun Oct  8 01:20:36 2006
@@ -143,12 +154,18 @@
     return product;
 }
 
+extern int enable_lockup_debug_dump;
+extern void print_lockup_debug_dump(struct cpu_user_regs *regs);
+
 void timer_interrupt(int irq, void *dev_id, struct cpu_user_regs *regs)
 {
     ASSERT(local_irq_is_enabled());
 
     /* Update jiffies counter. */
     (*(unsigned long *)&jiffies)++;
+
+    if (unlikely (enable_lockup_debug_dump))
+        print_lockup_debug_dump(regs);
 
     /* Rough hack to allow accurate timers to sort-of-work with no APIC. */
     if ( !cpu_has_apic )
diff -r c4f3f719d997 xen/include/public/sysctl.h
--- a/xen/include/public/sysctl.h	Sat Sep 23 13:54:58 2006
+++ b/xen/include/public/sysctl.h	Sun Oct  8 01:20:36 2006
@@ -122,6 +122,13 @@
 typedef struct xen_sysctl_getdomaininfolist xen_sysctl_getdomaininfolist_t;
 DEFINE_XEN_GUEST_HANDLE(xen_sysctl_getdomaininfolist_t);
 
+#define XEN_SYSCTL_set_lockup_debug 7
+struct xen_sysctl_set_lockup_debug {
+    unsigned int enable;
+};
+typedef struct xen_sysctl_set_lockup_debug xen_sysctl_set_lockup_debug_t;
+DEFINE_XEN_GUEST_HANDLE(xen_sysctl_set_lockup_debug_t);
+
 struct xen_sysctl {
     uint32_t cmd;
     uint32_t interface_version; /* XEN_SYSCTL_INTERFACE_VERSION */
@@ -132,6 +139,7 @@
         struct xen_sysctl_sched_id          sched_id;
         struct xen_sysctl_perfc_op          perfc_op;
         struct xen_sysctl_getdomaininfolist getdomaininfolist;
+        struct xen_sysctl_set_lockup_debug  set_lockup_debug;
         uint8_t                             pad[128];
     } u;
 };
diff -r c4f3f719d997 xen/drivers/video/vga.c
--- a/xen/drivers/video/vga.c	Sat Sep 23 13:54:58 2006
+++ b/xen/drivers/video/vga.c	Sun Oct  8 01:20:36 2006
@@ -561,7 +561,7 @@
 static const struct font_desc *font;
 
 static int xpos, ypos;
-static unsigned char *video;
+unsigned char *video;
 
 /* vga: comma-separated options. */
 static char opt_vga[30] = "";