kernel 2.6.29.6-xen-dom0:
wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.29.6.tar.bz2
wget http://gentoo-xen-kernel.googlecode.com/files/xen-patches-2.6.29-6.tar.bz2

patch (adapted from 2.6.31-r7 one):
--- linux-2.6.29.6-xen-dom0/arch/x86/kernel/microcode_core-xen.c
2009-12-01 11:01:23.000000000 +0100
+++ linux-2.6.29.6-xen-dom0/arch/x86/kernel/microcode_core-xen.c
2009-12-01 11:04:52.000000000 +0100
@@ -30,6 +30,7 @@
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
+#include <linux/swap.h>
 #include <linux/miscdevice.h>
 #include <linux/spinlock.h>
 #include <linux/mm.h>

This patch:
diff --git a/arch/x86/kernel/time_32-xen.c b/arch/x86/kernel/time_32-xen.c
index aa7e6a8..d61b20f 100644
--- a/arch/x86/kernel/time_32-xen.c
+++ b/arch/x86/kernel/time_32-xen.c
@@ -519,6 +519,7 @@ irqreturn_t timer_interrupt(int irq, void *dev_id)
 		do_div(delta, NS_PER_TICK);
 		processed_system_time += delta * NS_PER_TICK;
 		while (delta > HZ) {
+			clobber_induction_variable(delta) ;
 			do_timer(HZ);
 			delta -= HZ;
 		}

solves a problem on debian:
http://mulps.wordpress.com/2009/05/29/compiling-xen-kernel-2-6-29-2/

Link to patch:
http://xenbits.xen.org/gitweb?p=xenclient/linux-2.6.27-pq.git;a=blob_plain;f=master/clobber-induction-variable.patch;hb=HEAD

Patch and compile.

It fails on rebbot when done on a 2.6.31-r7.

What the fuck is going on???