Comment 0 for bug 1649821

Every hundred boots or so when booting Ubuntu 16.04 under Xen, it crashes early on boot with the following stack trace:

  kernel BUG at /build/linux-Ay7j_C/linux-4.4.0/drivers/xen/events/events_base.c:1210!
  invalid opcode: 0000 [#1] SMP
  ...
  RIP: 0010:[<ffffffff814c97c9>] [<ffffffff814c97c9>] xen_send_IPI_one+0x59/0x60
  ...
  Call Trace:
   [<ffffffff8102be9e>] xen_qlock_kick+0xe/0x10
   [<ffffffff810cabc2>] __pv_queued_spin_unlock+0xb2/0xf0
   [<ffffffff810ca6d1>] ? __raw_callee_save___pv_queued_spin_unlock+0x11/0x20
   [<ffffffff81052936>] ? check_tsc_warp+0x76/0x150
   [<ffffffff81052aa6>] check_tsc_sync_source+0x96/0x160
   [<ffffffff81051e28>] native_cpu_up+0x3d8/0x9f0
   [<ffffffff8102b315>] xen_hvm_cpu_up+0x35/0x80
   [<ffffffff8108198c>] _cpu_up+0x13c/0x180
   [<ffffffff81081a4a>] cpu_up+0x7a/0xa0
   [<ffffffff81f80dfc>] smp_init+0x7f/0x81
   [<ffffffff81f5a121>] kernel_init_freeable+0xef/0x212
   [<ffffffff81817f30>] ? rest_init+0x80/0x80
   [<ffffffff81817f3e>] kernel_init+0xe/0xe0
   [<ffffffff8182488f>] ret_from_fork+0x3f/0x70
   [<ffffffff81817f30>] ? rest_init+0x80/0x80

This is fixed by the following commit:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/arch/x86/xen?id=707e59ba494372a90d245f18b0c78982caa88e48

Unfortunately this wasn't backported to Linux 4.4. Can you please include this in the next Ubuntu 16.04 kernel release?

Thanks
Ross