linux-3.6 with BFS 425 seems not to boot on kvm.
I use this command to start the VM
qemu-kvm -smp 3 -m 1G ../virtual/Ubuntu.12.04.qcow2 -kernel
../tmp-linux-3.6/arch/x86_64/boot/bzImage -append
"root=/dev/sda1 console=ttyS0" -s

Using gdb to see where the kernel hangs shows following:

(gdb) target remote :1234
Remote debugging using :1234
0xffffffff816d2321 in _raw_spin_lock_irq 
(lock=lock@entry=0xffff88003fc8e440)
at /home/findus/kernel/linux-3.6-bfs-not-booting/kernel/spinlock.c:152
152	{
(gdb) backtrace
#0  0xffffffff816d2321 in _raw_spin_lock_irq 
(lock=lock@entry=0xffff88003fc8e440)
     at /home/findus/kernel/linux-3.6-bfs-not-booting/kernel/spinlock.c:152
#1  0xffffffff81070d1e in spin_lock_irq (lock=0xffff88003fc8e440)
     at 
/home/findus/kernel/linux-3.6-bfs-not-booting/include/linux/spinlock.h:310
#2  worker_maybe_bind_and_lock (worker=0xffff88003e1c9240, 
worker=0xffff88003e1c9240)
     at 
/home/findus/kernel/linux-3.6-bfs-not-booting/kernel/workqueue.c:1288
#3  0xffffffff810740fc in idle_worker_rebind (worker=0xffff88003e1c9240)
     at 
/home/findus/kernel/linux-3.6-bfs-not-booting/kernel/workqueue.c:1323
#4  worker_thread (__worker=0xffff88003e1c9240)
     at 
/home/findus/kernel/linux-3.6-bfs-not-booting/kernel/workqueue.c:2166
#5  0xffffffff8107913e in kthread (_create=0xffff88003e1dfc18)
     at /home/findus/kernel/linux-3.6-bfs-not-booting/kernel/kthread.c:121
#6  0xffffffff816dc104 in kernel_thread_helper ()
     at 
/home/findus/kernel/linux-3.6-bfs-not-booting/arch/x86/kernel/entry_64.S:1216
#7  0x0000000000000000 in ??


This patch fixes the issue:


diff -uNr linux-3.6-bfs-not-booting/kernel/sched/bfs.c 
linux-3.6-bfs/kernel/sched/bfs.c
--- linux-3.6-bfs-not-booting/kernel/sched/bfs.c	2012-10-17 
11:33:07.717384503 +0200
+++ linux-3.6-bfs/kernel/sched/bfs.c	2012-10-17 11:25:05.499245945 +0200
@@ -4654,10 +4654,6 @@

  static void __cond_resched(void)
  {
-	/* NOT a real fix but will make voluntary preempt work. 馬鹿な事 */
-	if (unlikely(system_state != SYSTEM_RUNNING))
-		return;
-
  	add_preempt_count(PREEMPT_ACTIVE);
  	schedule();
  	sub_preempt_count(PREEMPT_ACTIVE);